L6258EX_07

L6258EX PWM controlled high current DMOS universal motor driver Features ■ ■ ■ ■ ■ ■ ■ ■ ■ Able to drive both windings of a bipolar stepper motor or two DC motors Output current up to 1.5A each winding Wide voltage range: 12V to 40V Four quadrant current control, ideal for microstepping and DC motor control Precision PWM control No need for recirculation diodes TTL/CMOS compatible inputs Cross conduction protection Thermal shutdow The power stage is a dual DMOS full bridge capable of sustaining up to 40V, and includes the diodes for current recirculation.The output current capability is 1.5A per winding in continuous mode, with peak start-up current up to 2A. A thermal protection circuitry disables the outputs if the chip temperature exceeds the safe limits. PowerSO36 Description L6258EX is a dual full bridge for motor control applications realized in BCD technology, with the capability of driving both windings of a bipolar stepper motor or bidirectionally control two DC motors. L6258EX and a few external components form a complete control and drive circuit. It has high efficiency phase shift chopping that allows a very low current ripple at the lowest current control levels, and makes this device ideal for steppers as well as for DC motors. Table 1. Device summary Order code E-L6258EX E-L6258EXTR Package PowerSO36 PowerSO36 Packing Tube Tape&Reel December 2007 Rev 6 1/32 www.st.com 1 Contents L6258EX Contents 1 2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Reference voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Input logic (I0 - I1 - I2 - I3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Phase input ( PH ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Triangular generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Charge pump circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Current control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Current control loop compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3 PWM current control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1 3.2 3.3 3.4 3.5 Open loop transfer function analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Power amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Load attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Error amplifier and sense amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Effect of the Bemf on the current control loop stability . . . . . . . . . . . . . . . 22 4 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.1 4.2 4.3 4.4 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Motor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Unused inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Notes on PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5 6 7 Operation mode time diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2/32 L6258EX List of tables List of tables Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Device summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Absolute maximum rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Pin functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Current levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Charge pump capacitor's values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3/32 List of figures L6258EX List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Pin connection (top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Thermal characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Power bridge configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Current control loop block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Output comparator waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Ax bode plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Aloop bode plot (uncompensated) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Aloop bode plot (compensated) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Electrical model of the load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Full step operation mode timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Half step operation mode timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4 bit microstep operation mode timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 PowerSO36 mechanical data & package dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4/32 L6258EX Block diagram 1 Figure 1. Block diagram Block diagram R1 1M RC1 CP VCP1 VCP2 EA_IN1 CC1 EA_OUT1 TRI_0 CHARGE PUMP VR VREF1 I3_1 I2_1 I1_1 I0_1 PH_1 SENSE1A VR GEN VR (VDD/2) THERMAL PROT. DISABLE VS VREF1 I3_2 I2_2 I1_2 I0_2 PH_2 TRI_CAP CFREF SENSE2A TRIANGLE GENERATOR TRI_0 TRI_180 DAC VR INPUT & SENSE AMP + ERROR AMP TRI_0 DAC + INPUT & SENSE AMP ERROR AMP TRI_180 VS CBOOT VBOOT + + - C POWER BRIDGE 1 OUT1A C OUT1B SENSE1B Rs VDD(5V) + + C POWER BRIDGE 2 OUT2A TRI_180 - C OUT2B SENSE2B Rs GND EA_IN2 RC2 R2 1M EA_OUT2 D96IN430D CC2 Table 2. Absolute maximum rating Description Supply voltage Logic supply voltage Reference voltage Output current (peak)(1) Value 45 7 2.5 2 1.5 -0.3 to 7 60 15 150 -55 to 150 Unit V V V A A V V V °C °C Parameter Vs VDD Vref1/Vref2 IO IO Vin Vboot Vboot - Vs Tj Tstg Output current (continuous) Logic input voltage range Bootstrap supply Maximum Vgate applicable Junction temperature Storage temperature range 1. This current is intended as not repetitive current for max. 1 second. 5/32 Block diagram Figure 2. Pin connection (top view) L6258EX PWR_GND PH_1 I1_1 I0_1 OUT1A DISABLE TRI_CAP VCC GND VCP1 VCP2 VBOOT VS OUT2A I0_2 I1_2 PH_2 PWR_GND 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 D96IN432E 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 PWR_GND SENSE1 OUT1B I3_1 I2_1 VS EA_OUT1 EA_IN1 VREF1 SIG_GND VREF2 EA_IN2 EA_OUT2 I2_2 I3_2 OUT2B SENSE2 PWR_GND Table 3. Pin # 1, 36 Pin functions Name PWR_GND Description Ground connection (1). They also conduct heat from die to printed circuit copper. These TTL compatible logic inputs set the direction of current flow through the load. A high level causes current to flow from OUTPUT A to OUTPUT B. Logic input of the internal DAC (1). The output voltage of the DAC is a percentage of the Vref voltage applied according to the thruth Table 5 on page 12. See pin 3 Bridge output connection (1) Disables the bridges for additional safety during switching. When not connected the bridges are enabled Triangular wave generation circuit capacitor. The value of this capacitor defines the output switching frequency 2, 17 PH_1, PH_2 3 4 5 6 7 I1_1 I0_1 OUT1A DISABLE TRI_cap 6/32 L6258EX Table 3. Pin # 8 9 10 11 12 13, 31 14 15 16 18, 19 20, 35 21 22 23 24 25 26, 28 27 29 30 32 33 34 Block diagram Pin functions (continued) Name VDD (5V) GND VCP1 VCP2 VBOOT VS OUT2A I0_2 I1_2 PWR_GND Description Supply voltage input for logic circuitry Power ground connection of the internal charge pump circuit Charge pump oscillator output Input for external charge pump capacitor Overvoltage input for driving of the upper DMOS Supply voltage input for output stage. They are shorted internally Bridge output connection (2) Logic input of the internal DAC (2). The output voltage of the DAC is a percentage of the VRef voltage applied according to the truth Table 5 on page 12. See pin 15 Ground connection. They also conduct heat from die to printed circuit copper SENSE2, SENSE1 Negative input of the transconductance input amplifier (2, 1) OUT2B I3_2 I2_2 EA_OUT_2 EA_IN_2 VREF2, VREF1 SIG_GND EA_IN_1 EA_OUT_1 I2_1 I3_1 OUT1B Bridge output connection and positive input of the tranconductance (2) See pin 15 See pin 15 Error amplifier output (2) Negative input of error amplifier (2) Reference voltages for the internal DACs, determining the output current value. Output current also depends on the logic inputs of the DAC and on the sensing resistor value Signal ground connection Negative input of error amplifier (1) Error amplifier output (1) See pin 3 See pin 3 Bridge output connection and positive input of the tranconductance (1) Note: The number in parenthesis shows the relevant Power Bridge of the circuit. Pins 18, 19, 1 and 36 are connected together. 7/32 Block diagram Figure 3. Thermal characteristics L6258EX Conditions Power Dissipated T Ambient Thermal J-A resistance (W) (˚C) (˚C/W) 5.3 70 15 pad layout + ground layers + 16 via hol PCB ref.: 4 LAYER cm 12 x 12 4.0 pad layout + ground layers PCB ref.: 4 LAYER cm 12 x 12 70 20 2.3 pad layout + 6cm2 on board heat sink PCB ref.: 2 LAYER cm 12 x 12 70 35 D02IN1370 12 10 Power Dissipated (W) 8 20˚C/W 6 4 2 0 35˚C/W 15˚C/W 0 20 40 60 80 100 120 Ambient Temperature (˚C) 140 160 D02IN1371 Table 4. Parameter VS VDD VBOOT VSense VS(off) VDD(off) IS(on) IS(off) IDD Electrical characteristics (VS = 40V; VDD = 5V; Tj = 25°; unless otherwise specified.) Description Supply voltage Logic supply voltage Storage voltage Max drop across sense resistor Power off reset Power off reset VS quiescent current VS quiescent current VDD operative current Off threshold Off threshold Both bridges ON, no load Both bridges OFF 6 3.3 VS = 12 to 40V Test condition Min. 12 4.75 VS+6 Typ. Max. 40 5.25 VS+12 1.25 7.2 4.1 15 7 15 Unit V V V V V V mA mA mA 8/32 L6258EX Table 4. Parameter ΔTSD-H TSD fosc Block diagram Electrical characteristics (continued) (VS = 40V; VDD = 5V; Tj = 25°; unless otherwise specified.) Description Shut down hysteresis Thermal shutdown Triangular oscillator frequency(1) CFREF = 1nF 12.5 Test condition Min. Typ. 25 150 15 18.5 Max. Unit °C °C KHz TRANSISTORS IDSS Rds(on) Vf Leakage current On resistance Flywheel diode voltage OFF State ON state If =1.0A 0.6 1 500 0.75 1.4 μA W V CONTROL LOGIC Vin(H) Vin(L) Iin Idis Vref1/ref2 Iref FI = Vref/Vsense VFS Voffset lnput voltage Input voltage Input current (2) Disable pin input current Reference voltage Vref terminal input current PWM loop transfer ratio DAC full scale precision Current loop offset DAC factor ratio SENSE AMPLIFIER Vcm Iinp lnput common mode voltage range Input bias sense1/sense2 -0.7 -200 VS+0.7 0 V μA Vref = 2.5V I0/I1/I2/I3 = L Vref = 2.5V I0/I1/I2/I3 = H Normalized @ full scale value 1.23 -40 -2 0 < Vin < 5V Operating Vref = 1.25 All Inputs All inputs 2 0 -150 -10 0 -2 2 1.34 +40 +2 V mV % VDD 0.8 +10 +150 2.5 5 V V μA μA V μA ERROR AMPLIFIER GV SR GBW Open loop voltage gain Output slew rate Gain bandwidth product Open loop 70 0.2 400 dB V/μs kHz 1. Chopping frequency is twice fosc value. 2. This is true for all the logic inputs except the disable input. 9/32 Functional description L6258EX 2 Functional description The circuit is intended to drive both windings of a bipolar stepper motor or two DC motors. The current control is generated through a switch mode regulation. With this system the direction and the amplitude of the load current are depending on the relation of phase and duty cycle between the two outputs of the current control loop. The L6258EX power stage is composed by power DMOS in bridge configuration as it is shown in Figure 4, where the bridge outputs OUT_A and OUT_B are driven to Vs with an high level at the inputs IN_A and IN_B while are driven to ground with a low level at the same inputs. The zero current condition is obtained by driving the two half bridge using signals IN_A and IN_B with the same phase and 50% of duty cycle. In this case the outputs of the two half bridges are continuously switched between power supply (Vs) and ground, but keeping the differential voltage across the load equal to zero. In Figure 4 is shown the timing diagram of the two outputs and the load current for this working condition. Following we consider positive the current flowing into the load with a direction from OUT_A to OUT_B, while we consider negative the current flowing into load with a direction from OUT_B to OUT_A. Now just increasing the duty cycle of the IN_A signal and decreasing the duty cycle of IN_B signal we drive positive current into the load. In this way the two outputs are not in phase, and the current can flow into the load trough the diagonal bridge formed by T1 and T4 when the output OUT_A is driven to Vs and the output OUT_B is driven to ground, while there will be a current recirculation into the higher side of the bridge, through T1 and T2, when both the outputs are at Vs and a current recirculation into the lower side of the bridge, through T3 and T4, when both the outputs are connected to ground. Since the voltage applied to the load for recirculation is low, the resulting current discharge time constant is higher than the current charging time constant during the period in which the current flows into the load through the diagonal bridge formed by T1 and T4. In this way the load current will be positive with an average amplitude depending on the difference in duty cycle of the two driving signals. In Figure 4 is shown the timing diagram in the case of positive load current On the contrary, if we want to drive negative current into the load is necessary to decrease the duty cycle of the IN_A signal and increase the duty cycle of the IN_B signal. In this way we obtain a phase shift between the two outputs such to have current flowing into the diagonal bridge formed by T2 and T3 when the output OUT_A is driven to ground and output OUT_B is driven to Vs, while we will have the same current recirculation conditions of the previous case when both the outputs are driven to Vs or to ground. So, in this case the load current will be negative with an average amplitude always depending by the difference in duty cycle of the two driving signals. In Figure 4 is shown the timing diagram in the case of negative load current. Figure 5 shows the device block diagram of the complete current control loop. 10/32 L6258EX Functional description 2.1 Reference voltage The voltage applied to VREF pin is the reference for the internal DAC and, together with the sense resistor value, defines the maximum current into the motor winding according to the following relation: 0,5 ⋅ V REF 1 V REF I MAX = ----------------------------- = ---- ⋅ ------------FI R R S S where Rs = sense resistor value Figure 4. Power bridge configuration VS IN_A T1 OUT_A LOAD T2 OUT_B IN_B T3 T4 OUTA OUTB Fig. 4A Iload 0 OUTA OUTB Fig. 4B Iload 0 OUTA OUTB Fig. 4C 0 Iload D97IN624 11/32 Functional description Figure 5. Current control loop block diagram L6258EX POWER AMPL. VS OUTA Tri_0 INPUT TRANSCONDUCTANCE ERROR AMPL. AMPL. VR VREF I0 I1 I2 I3 PH Gin=1/Ra ib DAC VDAC Rc Cc + ia Tri_180 + ic + - LOAD RL VSENSE + D97IN625 Gs=1/Rb SENSE TRANSCONDUCTANCE AMPL. 2.2 Input logic (I0 - I1 - I2 - I3) The current level in the motor winding is selected according to this table: Table 5. I3 H H H H H H H H L L L Current levels I2 H H H H L L L L H H H I1 H H L L H H L L H H L I0 H L H L H L H L H L H Current level % of IMAX No Current 9.5 19.1 28.6 38.1 47.6 55.6 63.5 71.4 77.8 82.5 12/32 + LL VS RS OUTB L6258EX Table 5. I3 L L L L L Functional description Current levels (continued) I2 H L L L L I1 L H H L L I0 L H L H L Current level % of IMAX 88.9 92.1 95.2 98.4 100 2.3 Phase input ( PH ) The logic level applied to this input determines the direction of the current flowing in the winding of the motor. High level on the phase input causes the motor current flowing from OUT_A to OUT_B through the load. 2.4 Triangular generator This circuit generates the two triangular waves TRI_0 and TRI_180 internally used to generate the duty cycle variation of the signals driving the output stage in bridge configuration. The frequency of the triangular wave defines the switching frequency of the output, and can be adjusted by changing the capacitor connected at TR1_CAP pin: K F ref = --C where: K = 1.5 x 10-5 2.5 Charge pump circuit To ensure the correct driving of the high side drivers a voltage higher than Vs is supplied on the Vboot pin. This boostrap voltage is not needed for the low side power DMOS transistors because their sources terminals are grounded. To produce this voltage a charge pump method is used. It is made by using two external capacitors; one connected to the internal oscillator (CP) and the other (Cboot) to storage the overvoltage needed for the driving the gates of the high side DMOS. The value suggested for the capacitors are: Table 6. Charge pump capacitor's values Component's function Storage capacitor Pump capacitor Value 100 10 Unit nF nF Component name Cboot CP 13/32 Functional description L6258EX 2.6 Current control loop The current control loop is a transconductance amplifier working in PWM mode. The motor current is a function of the programmed DAC voltage. To keep under control the output current, the current control modulates the duty cycle of the two outputs OUT_A and OUT_B, and a sensing resistor Rs is connected in series with the motor winding in order to produce a voltage feedback compared with the programmed voltage of the DAC. The duty cycle modulation of the two outputs is generated comparing the voltage at the outputs of the error amplifier, with the two triangular wave references. In order to drive the output bridge with the duty cycle modulation explained before, the signals driving each output (OUTA & OUTB) are generated by the use of the two comparators having as reference two triangular wave signals Tri_0 and Tri_180 of the same amplitude, the same average value (in our case Vr), but with a 180° of phase shift each other. The two triangular wave references are respectively applied to the inverting input of the first comparator and to the non inverting input of the second comparator. The other two inputs of the comparators are connected together to the error amplifier output voltage resulting by the difference between the programmed DAC. The reset of the comparison between the mentioned signals is shown in Figure 6. Figure 6. Output comparator waveforms Tri_0 Error Ampl. Output Tri_180 First Comp. Output Second Comp. Output In the case of VDAC equal to zero, the transconductance loop is balanced at the value of Vr, so the outputs of the two comparators are signals having the same phase and 50% of duty cycle. As we have already mentioned, in this situation, the two outputs OUT_A and OUT_B are simultaneously driven from Vs to ground; and the differential voltage across the load in this case is zero and no current flows in the motor winding. 14/32 L6258EX Functional description With a positive differential voltage on VDAC (see Figure 5, the transconductance loop will be positively unbalanced respected Vr. In this case being the error amplifier output voltage greater than Vr, the output of the first comparator is a square wave with a duty cycle higher than 50%, while the output of the second comparator is a square wave with a duty cycle lower than 50%. The variation in duty cycle obtained at the outputs of the two comparators is the same, but one is positive and the other is negative with respect to the 50% level. The two driving signals, generated in this case, drive the two outputs in such a way to have switched current flowing from OUT_A through the motor winding to OUT_B. With a negative differential voltage VDAC, the transconductance loop will be negatively unbalanced respected Vr. In this case the output of the first comparator is a square wave with a duty cycle lower than 50%, while the output of the second comparator is a square wave with a duty cycle higher than 50%. The variation in the duty cycle obtained at the outputs of the two comparators is always of the same. The two driving signals, generated in this case, drive the the two outputs in order to have the switched current flowing from OUT_B through the motor winding to OUT_A. 2.7 Current control loop compensation In order to have a flexible system able to drive motors with different electrical characteristics, the non inverting input and the output of the error amplifier ( EA_OUT ) are available. Connecting at these pins an external RC compensation network it is possible to adjust the gain and the bandwidth of the current control loop. 15/32 PWM current control loop L6258EX 3 3.1 PWM current control loop Open loop transfer function analysis Block diagram: refer to Figure 5. Input parameters: ● ● ● ● ● ● ● ● ● ● ● ● VS = 24V LL = 12mH RL = 12Ω RS = 0.33Ω RC = to be calculated CC = to be calculated Gs transconductance gain = 1/Rb Gin transconductance gain = 1/Ra Ampl. of the Tria_0_180 ref. = 1.6V (peak to peak) Ra = 40KΩ Rb = 20KΩ Vr = Internal reference equal to VDD/2 (Typ. 2.5V) these data refer to a typical application, and will be used as an example during the analysis of the stability of the current control loop. The block diagram shows the schematics of the L6258EX internal current control loop working in PWM mode; the current into the load is a function of the input control voltage VDAC , and the relation between the two variables is given by the following formula: ILOAD · RS · GS = VDAC · Gin 1 1 I LOAD ⋅ R S ⋅ ------- = V DAC ⋅ ------Ra Rb Rb V DAC I LOAD = V DAC ⋅ --------------------- = 0,5 ⋅ -------------- ( A ) Ra ⋅ Rs RS where: VDAC Gin Gs Rs is the control voltage defining the load current value is the gain of the input transconductance amplifier ( 1/Ra ) is the gain of the sense transconductance amplifier ( 1/Rb ) is the resistor connected in series to the output to sense the load current In this configuration the input voltage is compared with the feedback voltage coming from the sense resistor, then the difference between this two signals is amplified by the error amplifier in order to have an error signal controlling the duty cycle of the output stage keeping the load current under control. It is clear that to have a good performance of the current control loop, the error amplifier must have an high DC gain and a large bandwidth. 16/32 L6258EX PWM current control loop Gain and bandwidth must be chosen depending on many parameters of the application, like the characteristics of the load, power supply etc..., and most important is the stability of the system that must always be guaranteed. To have a very flexible system and to have the possibility to adapt the system to any application, the error amplifier must be compensated using an RC network connected between the output and the negative input of the same. For the evaluation of the stability of the system, we have to consider the open loop gain of the current control loop: Aloop = ACerr · ACpw · ACload · ACsense where AC... is the gain of the blocks that refers to the error, power and sense amplifier plus the attenuation of the load block. The same formula in dB can be written in this way: AloopdB = ACerrdB + ACpwdB + ACloaddB + ACsensedB So now we can start to analyse the dynamic characteristics of each single block, with particular attention to the error amplifier. 3.2 Power amplifier The power amplifier is not a linear amplifier, but is a circuit driving in PWM mode the output stage in full bridge configuration. The output duty cycle variation is given by the comparison between the voltage of the error amplifier and two triangular wave references Tri_0 and Tri_180. Because all the current control loop is referred to the Vr reference, the result is that when the output voltage of the error amplifier is equal to the Vr voltage the two output Out_A and Out_B have the same phase and duty cycle at 50%; increasing the output voltage of the error amplifier above the Vr voltage, the duty cycle of the Out_A increases and the duty cycle of the Out_B decreases of the same percentage; on the contrary decreasing the voltage of the error amplifier below the Vr voltage, the duty cycle of the Out_A decreases and the duty cycle of the Out_B increases of the same percentage. The gain of this block is defined by the amplitude of the two triangular wave references; more precisely the gain of the power amplifier block is a reversed proportion of the amplitude of the two references. In fact a variation of the error amplifier output voltage produces a larger variation in duty cycle of the two outputs Out_A and Out_B in case of low amplitude of the two triangular wave references. The duty cycle has the max value of 100% when the input voltage is equal to the amplitude of the two triangular references. The transfer function of this block consist in the relation between the output duty cycle and the amplitude of the triangular references. Vout = 2 · VS · (0.5 - DutyCycle) ΔV out 2 ⋅ VS ACpw dB = 20 ⋅ log ---------------- = -----------------------------------------------------ΔV in Triangular Amplitude 17/32 PWM current control loop ACpw 2 ⋅ 24 = 20 ⋅ log ---------------- = 29,5dB 1,6 L6258EX dB Moreover, having the two references Tri_0 and Tri_180 a triangular shape it is clear that the transfer function of this block is a linear constant gain without poles and zeros. 3.3 Load attenuation The load block is composed by the equivalent circuit of the motor winding (resistance and inductance) plus the sense resistor. We will considered the effect of the Bemf voltage of the motor in the next chapter. The input of this block is the PWM voltage of the power amplifier and as output we have the voltage across the sense resistor produced by the current flowing into the motor winding. The relation between the two variable is: V out V sense = ---------------------- ⋅ R S RL + RS so the gain of this block is: V sense RS ACload = -------------------- = ---------------------v out RL + RS RS ACload dB = 20 ⋅ log ---------------------RL + RS 0,33 Aload dB = 20 ⋅ log ----------------------- = – 31,4dB 12 + 0,33 where: RL = equivalent resistance of the motor winding RS = sense resistor Because of the inductance of the motor LL, the load has a pole at the frequency: 1 Fpole = ---------------------------------LL 2 π ⋅ -------------------RL + RS 1 Fpole = --------------------------------------------- = 163Hz –3 12 ⋅ 10 6,28 ⋅ -------------------------12 + 0,33 18/32 L6258EX PWM current control loop Before analysing the error amplifier block and the sense transconductance block, we have to do this consideration: AloopdB = AxdB + BxdB Ax|dB = ACpw|dB + ACload|dB and Bx|dB = ACerr|dB + ACsense|dB this means that Ax|dB is the sum of the power amplifier and load blocks; Ax|dB = (29,5) + (-31.4) = -1.9dB The BODE analysis of the transfer function of Ax is: Figure 7. Ax bode plot The Bode plot of the Ax|dB function shows a DC gain of -1.9dB and a pole at 163Hz. It is clear now that (because of the negative gain of the Ax function), Bx function must have an high DC gain in order to increment the total open loop gain increasing the bandwidth too. 3.4 Error amplifier and sense amplifier As explained before the gain of these two blocks is: BxdB = ACerrdB + ACsensedB Being the voltage across the sense resistor the input of the Bx block and the error amplifier voltage the output of the same, the voltage gain is given by: 1 ib = Vsense ⋅ Gs = Vsense ⋅ ------Rb 19/32 PWM current control loop 1 Verr_out = -(ic · Zc) so ic = -(Verr_out · ------ ) Zc because ib = icwe have: Vsense · L6258EX 11------- = -(Verr_out · ------ ) Rb Zc Verr_out Zc Bx = – ----------------------- = – ------Vsense Rb In the case of no external RC network is used to compensate the error amplifier, the typical open loop transfer function of the error plus the sense amplifier is something with a gain around 80dB and a unity gain bandwidth at 400kHz. In this case the situation of the total transfer function Aloop, given by the sum of the AxdB and BxdB is: Figure 8. Aloop bode plot (uncompensated) The BODE diagram shows together the error amplifier open loop transfer function, the Ax function and the resultant total Aloop given by the following equation: AloopdB = AxdB + BxdB The total Aloop has an high DC gain of 78.1dB with a bandwidth of 15KHz, but the problem in this case is the stability of the system; in fact the total Aloop cross the zero dB axis with a slope of -40dB/decade. Now it is necessary to compensate the error amplifier in order to obtain a total Aloop with an high DC gain and a large bandwidth. Aloop must have enough phase margin to guarantee the stability of the system. A method to reach the stability of the system, using the RC network showed in the block diagram, is to cancel the load pole with the zero given by the compensation of the error amplifier. The transfer function of the Bx block with the compensation on the error amplifier is: 20/32 L6258EX 1 Rc – j ----------------------------2 π ⋅ f ⋅ Cc Zc Bx = – ------- = – --------------------------------------------Rb Rb PWM current control loop In this case the Bx block has a DC gain equal to the open loop and equal to zero at a frequency given by the following formula: 1 Fzero = ----------------------------------2 π ⋅ Rc ⋅ Cc In order to cancel the pole of the load, the zero of the Bx block must be located at the same frequency of 163Hz; so now we have to find a compromise between the resistor and the capacitor of the compensation network. Considering that the resistor value defines the gain of the Bx block at the zero frequency, it is clear that this parameter will influence the total bandwidth of the system because, annulling the load pole with the error amplifier zero, the slope of the total transfer function is -20dB/decade. So the resistor value must be chosen in order to have an error amplifier gain enough to guarantee a desired total bandwidth. In our example we fix at 35dB the gain of the Bx block at zero frequency, so from the formula: where: Rb = 20kΩ we have: Rc = 1.1MΩ Therefore we have the zero with a 163Hz the capacitor value: 1 1 Cc = --------------------------------------------- = ---------------------------------------------------------------- = 880pF 2 π ⋅ Fzero ⋅ Rc –6 6,28 ⋅ 163 ⋅ 1,1 ⋅ 10 Now we have to analyse how the new Aloop transfer function with a compensation network on the error amplifier is. The following bode diagram shows: – – – – the Ax function showing the position of the load pole the open loop transfer function of the Bx block the transfer function of the Bx with the RC compensation network on the error amplifier the total Aloop transfer function that is the sum of the Ax function plus the transfer function of the compensated Bx block. Rc Bx_gain @ zero freq. = 20 ⋅ log ------Rb 21/32 PWM current control loop Figure 9. Aloop bode plot (compensated) L6258EX We can see that the effect of the load pole is cancelled by the zero of the Bx block ; the total Aloop cross a the 0dB axis with a slope of -20dB/decade, having in this way a stable system with an high gain at low frequency and a bandwidth of around 8KHz. To increase the bandwidth of the system, we should increase the gain of the Bx block, keeping the zero in the same position. In this way the result is a shift of the total Aloop transfer function up to a greater value. 3.5 Effect of the Bemf on the current control loop stability In order to evaluate what is the effect of the Bemf voltage of the stepper motor we have to look at the load block: 22/32 L6258EX Figure 10. Electrical model of the load OUT+ PWM current control loop Bemf R L L L R to Sense S Amplifier OUT- The schematic now shows the equivalent circuit of the stepper motor including a sine wave voltage generator of the Bemf. The Bemf voltage of the motor is not constant, its value changes depending on the speed of the motor. Increasing the motor speed the Bemf voltage increases: Bemf = Kt · ω where: Kt is the motor constant ω is the motor speed in radiant per second The formula defining the gain of the load considering the Bemf of the stepper motor becomes: RS ( V S – Bemf ) ⋅ ---------------------RL + RS Vsense ACload = -------------------- = --------------------------------------------------------------Vout VS V S – Bemf RS Acload = ---------------------------- ⋅ ---------------------VS RL + RS RS ⎞ ⎛ V S – Bemf ACload dB = 20 ⋅ log ⎜ ---------------------------- ⋅ ----------------------⎟ VS R L + R S⎠ ⎝ we can see that the Bemf influences only the gain of the load block and does not introduce any other additional pole or zero, so from the stability point of view the effect of the Bemf of the motor is not critical because the phase margin remains the same. Practically the only effect of the Bemf is to limit the gain of the total Aloop with a consequent variation of the bandwidth of the system. 23/32 Application information L6258EX 4 Application information A typical application circuit is shown in Figure 11. Note: For avoid current spikes on falling edge of DISABLE a "DC feedback" would be added to the ERROR Amplifier. (R1-R2 on Figure 11). 4.1 Interference Due to the fact that the circuit operates with switch mode current regulation, to reduce the effect of the wiring inductance a good capacitor (100nF) can be placed on the board near the package, between the power supply line (pin 13,31) and the power ground (pin 1,36,18,19) to absorb the small amount of inductive energy. It should be noted that this capacitor is usually required in addition to an electrolytic capacitor, that has poor performance at the high frequencies, always located near the package, between power supply voltage (pin 13,31) and power ground (pin 1,36,18,19), just to have a current recirculation path during the fast current decay or during the phase change. The range value of this capacitor is between few µF and 100µF, and it must be chosen depending on application parameters like the motor inductance and load current amplitude. A decoupling capacitor of 100nF is suggested also between the logic supply and ground. The EA_IN1 and EA_IN2 pins carry out high impedance lines and care must be taken to avoid coupled noise on this signals. The suggestion is to put the components connected to this pins close to the L6258EX, to surround them with ground tracks and to keep as far as possible fast switching outputs of the device. Remember also an 1 Mohm resistor between EA_INx and EA_OUTx to avoid output current spike during supply startup/shutdown. A non inductive resistor is the best way to implement the sensing. Whether this is not possible, some metal film resistor of the same value can be paralleled. The two inputs for the sensing of the winding motor current (SENSE_A & SENSE_B) should be connected directly on the sensing resistor Rs terminals, and the path lead between the Rs and the two sensing inputs should be as short as possible. Note: Connect the DISABLE pin to a low impedance (< 300 Ω ) voltage source to reduce at minimum the interference on the output current due to capacitive coupling of OUT1A (pin5) and DISABLE (pin 6). 24/32 L6258EX Figure 11. Typical application circuit Application information VCP1 10nF 0.33 10 21 20 OUT2B SENSE2 M 14 13,31 7 35 34 OUT2A SENSE1 0.33 OUT1B OUT1A GND PWR_GND 12mH 10Ω VCP2 VBOOT 11 12 STEPPER MOTOR 100nF VS VS TRI_CAP 1nF PH1 I0_1 I1_1 I2_1 I3_1 PH2 I0_2 I1_2 I2_2 I3_2 DISABLE L6258EX 2 4 3 32 33 17 15 16 23 22 6 29 EA_OUT1 1M 30 EA_IN2 25 EA_OUT2 1M 5 9 PWSSO36 PACKAGE 1,36 18,19 8 27 VDD SIG_GND VDD(5V) 28 26 24 VREF1 VREF2 VREF EA_IN1 820pF 820pF D97IN626EX R1 1M R2 1M 4.2 Motor selection Some stepper motor have such high core losses that they are not suitable for switch mode current regulation. Furthermore, some stepper motors are not designed for continuous operating at maximum current. Since the circuit can drive a constant current through the motor, its temperature might exceed, both at low and high speed operation. 4.3 Unused inputs Unused inputs should be connected to the proper voltage levels in order to get the highest noise immunity. 25/32 Application information L6258EX 4.4 Notes on PCB design We recommend to observe the following layout rules to avoid application problems with ground and anomalous recirculation current. The by-pass capacitors for the power and logic supply must be kept as near as possible to the IC. It's important to separate on the PCB board the logic and power grounds and the internal charge pump circuit ground avoiding that ground traces of the logic signals cross the ground traces of the power signals. Because the IC uses the board as a heat sink, the dissipating copper area must be sized in accordance with the required value of Rthj-amb. 26/32 L6258EX Operation mode time diagrams 5 Operation mode time diagrams Figure 12. Full step operation mode timing diagram (Phase - DAC input and motor current) Position 5V 0 1 2 3 0 1 2 3 0 FULL Step Vector Ph1 Phase 1 Phase 2 I0_1 0 5V 0 5V 0 5V 1 0 Ph2 Ph2 I1_1 DAC 1 Inputs I2_1 0 5V 0 5V 2 3 Ph1 I3_1 0 5V I3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 I2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 I1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 I0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 I0_2 Current level% of IMAX 100 98.4 95.2 92.1 88.9 82.5 77.8 71.4 63.5 55.6 47.6 38.1 28.6 19.1 9.5 No Current DAC 2 Inputs I1_2 0 5V 0 5V I2_2 0 I3_2 0 95.2% Motor drive Current 1 19.1% 0 95.2% Motor drive Current 2 19.1% 0 D97IN629A 27/32 Operation mode time diagrams Figure 13. Half step operation mode timing diagram (Phase - DAC input and motor current) L6258EX 0 1 2 3 4 5 6 7 Phase 1 Phase 2 I0_1 5V 0 5V 0 5V 0 5V 0 5V 0 5V 0 5V 0 5V 0 5V 0 5V 0 Half Step Vector Ph1 2 3 1 DAC 1 Inputs I1_1 I2_1 I3_1 I0_2 Ph2 4 0 Ph2 DAC 2 Inputs I1_2 I2_2 I3_2 5 6 7 Ph1 100% 71.4% I3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 I2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 I1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 I0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Current level% of IMAX 100 98.4 95.2 92.1 88.9 82.5 77.8 71.4 63.5 55.6 47.6 38.1 28.6 19.1 9.5 No Current 0 Motor drive Current 1 -71.4% -100% 100% 71.4% Motor drive Current 2 0 -71.4% -100% D97IN627C 28/32 L6258EX Operation mode time diagrams Figure 14. 4 bit microstep operation mode timing diagram (Phase - DAC input and motor current) Position 5V 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 Micro Step Vector Ph1 16 24 8 Phase 1 0 5V 0 Phase 5V 2 I0_1 0 5V Ph2 32 0 Ph2 I1_1 DAC 1 Inputs I2_1 0 5V 0 5V 40 48 56 Ph1 I3_1 0 5V I0_2 DAC 2 Inputs I1_2 0 5V 0 5V I3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 I2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 I1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 I0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Current level% of IMAX 100 98.4 95.2 92.1 88.9 82.5 77.8 71.4 63.5 55.6 47.6 38.1 28.6 19.1 9.5 No Current I2_2 0 I3_2 0 100% 95.2% 82.5% 63.5% 47.6% 38.1% 19.1% 0% Motor drive Current 1 0 Motor drive 0 Current 2 D97IN628A 29/32 Package information L6258EX 6 Package information In order to meet environmental requirements, ST offers these devices in ECOPACK® packages. These packages have a Lead-free second level interconnect. The category of second Level Interconnect is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an ST trademark. ECOPACK specifications are available at: www.st.com. Figure 15. PowerSO36 mechanical data & package dimensions DIM. A a1 a2 a3 b c D D1 E E1 E2 E3 e e3 G H h L N s 0.8 0 15.50 5.80 0.65 11.05 0.10 15.90 0.6102 1.10 1.10 0.0315 10˚ (max) 8˚ (max) 0 0.22 0.23 15.80 9.40 13.90 10.90 0.10 mm MIN. TYP. MAX. 3.60 0.30 3.30 0.10 0.38 0.32 9.80 14.5 2.90 6.20 0.2283 0.0256 0.4350 0.0039 0.6260 0.0433 0.0433 0.0087 0.0091 0.3701 0.5472 0.0039 MIN. inch TYP. MAX. 0.1417 0.0118 0.1299 0.0039 0.0150 0.0126 0.6299 0.3858 0.5709 0.4370 0.1142 0.2441 OUTLINE AND MECHANICAL DATA 16.00 0.6220 11.10 0.4291 PowerSO-36 Note: “D and E1” do not include mold flash or protusions. - Mold flash or protusions shall not exceed 0.15mm (0.006”) - Critical dimensions are "a3", "E" and "G". 0096119 C 30/32 L6258EX Revision history 7 Revision history Table 7. Date 15-Sep-2003 11-May-2004 24-Sep-2004 28-Feb-2005 23-Mar-2005 03-Dec-2007 Document revision history Revision 1 2 3 4 5 6 Changes First Issue in the EDOCS DMS. Restyling of the graphic form, changed all VCC with VDD; delete TSD parameter in the Electrical characteristic on Table 4. Changed on the page 5 the fosc parameter max. value from 17.5 to 18.5kHz Add. note at the bottom of Table 2: Absolute maximum rating. Modified the note “(1)” of the Table 2. Document reformatted. Modified the ACpw formula in Section 3.2 on page 17. Added the disable note in Section 4.1 on page 24. 31/32 L6258EX Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. 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