ST1S31
3 A DC step-down switching regulator
Datasheet - production data
Applications
µP/ASIC/DSP/FPGA core and I/O supplies
Point of load for: STB, TVs, DVDs
Optical storage, hard disk drive, printers,
audio/graphic cards
VFDFPN 3 x 3 - 8L
SO8
Description
The ST1S31 device is an internally compensated
1.5 MHz fixed-frequency PWM synchronous stepdown regulator. The ST1S31 operates from 2.8 V
to 5.5 V input, while it regulates an output voltage
as low as 0.8 V and up to VIN.
Features
3 A DC output current
2.8 V to 5.5 V input voltage
Output voltage adjustable from 0.8 V
The ST1S31 integrates a 60 m high-side switch
and a 45 m synchronous rectifier allowing very
high efficiency with very low output voltages.
1.5 MHz switching frequency
Internal soft-start and enable
Integrated 60 m and 45 m power MOSFETs
All ceramic capacitor
Power Good (POR)
The peak current mode control with internal
compensation delivers a very compact solution
with a minimum component count.
The ST1S31 device is available in 3 mm x 3 mm,
8 lead VFDFPN and SO8 packages.
Cycle-by-cycle current limiting
Current foldback short-circuit protection
VFDFPN 3 x 3 - 8L, SO8 packages
Figure 1. Application circuit
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August 2018
This is information on a product in full production.
DocID022998 Rev 5
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www.st.com
Contents
ST1S31
Contents
1
2
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Thermal data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4
Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
4.1
Output voltage adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3
Error amplifier and control loop stability . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4
Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5
Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6
Light load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.7
Hysteretic thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1
Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3
Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.4
Thermal dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.5
Layout consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6
Demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7
Typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8
Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2/36
8.1
VFDFPN 3 x 3 - 8L package information . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.2
SO8 package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
DocID022998 Rev 5
ST1S31
Contents
9
Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
10
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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Pin settings
ST1S31
1
Pin settings
1.1
Pin connection
Figure 2. Pin connection (top view)
1.2
Pin description
Table 1. Pin description
4/36
No.
Type
Description
1
VINA
2
EN
Enable input. With EN higher than 1.5 V the device is ON and with EN
lower than 0.5 V the device is OFF.
3
FB
Feedback input. Connecting the output voltage directly to this pin the
output voltage is regulated at 0.8 V. To have higher regulated voltages an
external resistor divider is required from VOUT to the FB pin.
4
AGND
Unregulated DC input voltage
Ground
Open drain Power Good (POR) pin. It is released (open drain) when the
output voltage is higher than 0.92 * VOUT with a delay of 170 s. If the
output voltage is below 0.92 * VOUT, the POR pin goes to low impedance
immediately. If not used, it can be left floating or to GND.
5
PG
6
VINSW
7
SW
8
PGND
Power ground
ePAD
(VFDFPN package only) exposed pad connected to ground assuring
electrical contact and heat conduction.
Power input voltage
Regulator output switching pin
DocID022998 Rev 5
ST1S31
2
Maximum ratings
Maximum ratings
Table 2. Absolute maximum ratings
Symbol
Parameter
Value
VIN
Input voltage
VEN
Enable voltage
VSW
Output switching voltage
VPG
Power-on reset voltage (Power Good)
-0.3 to VIN
VFB
Feedback voltage
-0.3 to 1.5
PTOT
Power dissipation at TA < 60 °C
TOP
Tstg
Unit
-0.3 to 7
-0.3 to VIN
-1 to VIN
V
1.5 (VFDFPN)
0.9 (SO8)
W
Operating junction temperature range
-40 to 150
°C
Storage temperature range
-55 to 150
°C
Thermal data
Table 3. Thermal data
Symbol
RthJA
Parameter
Maximum thermal resistance
junction ambient(1)
Value
VFDFPN
50
SO8
100
Unit
°C/W
1. Package mounted on demonstration board.
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36
Electrical characteristics
3
ST1S31
Electrical characteristics
TJ = 25 °C, VIN = 5 V, unless otherwise specified.
Table 4. Electrical characteristics
Values
Symbol
Parameter
Test condition
Unit
Min.
Typ.
Operating input voltage range
(1)
VINON
Turn-on VCC threshold
(1)
2.4
VINOFF
Turn-off VCC threshold
(1)
2.0
VIN
2.8
Max.
5.5
V
RDSON-P
High-side switch ON-resistance ISW = 300 mA
60
m
RDSON-N
Low-side switch ON-resistance
ISW = 300 mA
45
m
Maximum limiting current
(2)
ILIM
4.0
A
Oscillator
FSW
DMAX
Switching frequency
Maximum duty cycle
1.2
(2)
1.5
95
1.9
MHz
100
%
Dynamic characteristics
VFB
Feedback voltage
(1)
Io = 10 mA to 4 A
0.792
0.8
0.808
0.776
0.8
0.824
V
%VOUT/
IOUT
Reference load regulation
Io = 10 mA to 4 A(2)
0.2
0.6
%
%VOUT/
VIN
Reference line regulation
VIN = 2.8 V to 5.5 V(2)
0.2
0.3
%
Quiescent current
Duty cycle = 0, no load VFB = 1.2 V
630
1200
µA
Total standby quiescent current
OFF
1
µA
DC characteristics
IQ
IQST-BY
Enable
Device ON level
VEN
EN threshold voltage
IEN
EN current
1.5
V
Device OFF level
0.5
0.1
µA
Power Good
PG threshold
92
PG hystereris
30
%VFB
50
PG
mV
PG output voltage low
Isink = 6 mA open drain
PG rise delay
6/36
400
170
DocID022998 Rev 5
µs
ST1S31
Electrical characteristics
Table 4. Electrical characteristics (continued)
Values
Symbol
Parameter
Test condition
Unit
Min.
Typ.
Max.
Soft-start
TSS
Soft-start duration
400
Thermal shutdown
150
Hystereris
20
µs
Protection
TSHDN
°C
1. Specification referred to TJ from -40 to +125 °C. Specifications in the -40 to +125 °C temperature range are assured by
design, characterization and statistical correlation.
2. Guaranteed by design.
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Functional description
4
ST1S31
Functional description
The ST1S31 device is based on a “peak current mode”, constant frequency control. The
output voltage VOUT is sensed by the feedback pin (FB) compared to an internal reference
(0.8 V) providing an error signal that, compared to the output of the current sense amplifier,
controls the ON and OFF time of the power switch.
The main internal blocks are shown in the block diagram in Figure 3. They are:
A fully integrated oscillator that provides the internal clock and the ramp for the slope
compensation avoiding sub-harmonic instability
The soft-start circuitry to limit inrush current during the startup phase
The transconductance error amplifier
The pulse width modulator and the relative logic circuitry necessary to drive the internal
power switches
The drivers for embedded P-channel and N-channel power MOSFET switches
The high-side current sensing block
The low-side current sense to implement diode emulation
A voltage monitor circuitry (UVLO) that checks the input and internal voltages
A thermal shutdown block, to prevent thermal runaway.
Figure 3. Block diagram
AM11417v1
8/36
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ST1S31
4.1
Functional description
Output voltage adjustment
The error amplifier reference voltage is 0.8 V typical. The output voltage is adjusted
according to the following formula (see Figure 1 on page 1):
Equation 1
R1
V OUT = 0.8 1 + -------
R2
The internal architecture of the device requires a minimum off time, cycle-by-cycle, for the
output voltage regulation. The minimum off time is typically equal to 94 ns.
The control loop compensates for conversion losses with duty cycle control. Since the
power losses are proportional to the delivered output power, the duty cycle increases with
the load current request.
Figure 4 shows the maximum regulated output voltage over the input voltage range at
different loading conditions.
Figure 4. Maximum output voltage over loading conditions
4.2
Soft-start
The soft-start is essential to assure the correct and safe startup of the step-down converter.
It avoids inrush current surge and makes the output voltage rise monotonically.
The soft-start is managed by ramping the reference of the error amplifier from 0 V to 0.8 V.
The internal soft-start capacitor is charged with a resistor to 0.8 V, then the FB pin follows
the reference so that the output voltage is regulated to rise to the set value monotonically.
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36
Functional description
4.3
ST1S31
Error amplifier and control loop stability
The error amplifier provides the error signal to be compared with the high-side switch
current through the current sense circuitry. The non inverting input is connected with the
internal 0.8 V reference, while the inverting input is the FB pin. The compensation network is
internal and connected between the E/A output and GND.
The error amplifier of the ST1S31 device is a transconductance operational amplifier, with
high bandwidth and high output impedance.
Table 5. Characteristics of the uncompensated error amplifier
Description
Value
DC gain
94 dB
gm
238 µA/V
Ro
96 M
The ST1S31 device embeds the compensation network that assures the stability of the loop
in the whole operating range. All the tools needed to check the loop stability are shown on
the next pages of this section.
In Figure 5 the simple small signal model for the peak current mode control loop is shown.
Figure 5. Block diagram of the loop for the small signal analysis
VIN
GCO(s)
Slope
Compensation
High side
Switch
L
Current sense
Logic
And
Driver
VOUT
GDIV (s)
Cout
Low side
Switch
PWM comparator
0.8V
R1
VC
Rc
VFB
Error Amp
R2
Cc
G EA(s)
AM11418v1
Three main terms can be identified to obtain the loop transfer function:
1.
From control (output of E/A) to output, GCO(s)
2.
From output (VOUT) to the FB pin, GDIV(s)
3.
From the FB pin to control (output of E/A), GEA(s).
The transfer function from control to output GCO(s) results:
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ST1S31
Functional description
Equation 2
s
1 + ----
z
R LOAD
1
G CO s = ------------------ --------------------------------------------------------------------------------------------- ---------------------- F H s
Ri
R out T SW
s
------
1 + ---------------------------- m C 1 – D – 0.5 1 +
L
p
where RLOAD represents the load resistance, Ri the equivalent sensing resistor of the
current sense circuitry (0.369 ), p the single pole introduced by the LC filter and z the
zero given by the ESR of the output capacitor.
FH(s) accounts for the sampling effect performed by the PWM comparator on the output of
the error amplifier that introduces a double pole at one half of the switching frequency.
Equation 3
1
Z = ------------------------------ESR C OUT
Equation 4
m C 1 – D – 0.5
1
p = -------------------------------------- + --------------------------------------------R LOAD C OUT
L C OUT f SW
where:
Equation 5
Se
m C = 1 + -----Sn
S = V f
pp SW
e
V
IN – V OUT
S = ----------------------------- Ri
n
L
Sn represents the ON-time slope of the sensed inductor current, Se the slope of the external
ramp (VPP peak-to-peak amplitude - 0.535 V) that implements the slope compensation to
avoid sub-harmonic oscillations at duty cycle over 50%.
The sampling effect contribution FH(s) is:
Equation 6
1
F H s = -----------------------------------------2
s
s
1 + ------------------- + ------2
n QP
n
where:
Equation 7
1
Q P = --------------------------------------------------------- m C 1 – D – 0.5
and
Equation 8
n = f SW
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36
Functional description
ST1S31
The resistor to adjust the output voltage that gives the term from output voltage to the FB
pin. GDIV(s) is:
R2
G DIV s = -------------------R1 + R2
The transfer function from FB to Vc (output of E/A) introduces the singularities (poles and
zeroes) to stabilize the loop. The small signal model of the error amplifier with the internal
compensation network can be seen in Figure 6.
Figure 6. Small signal model for the error amplifier
V FB
Ro
Vd
Gm*Vd
Co
Rc
Cc
Cp
Cc
VREF
AM11419v1
RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect
system stability and can be neglected.
So GEA(s) results:
Equation 9
G EA0 1 + s R c C c
G EA s = ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2
s R0 C0 + Cp Rc Cc + s R0 Cc + R0 C0 + Cp + Rc Cc + 1
where GEA = Gm · Ro.
The poles of this transfer function are (if Cc >> C0+CP):
Equation 10
1
f P LF = ---------------------------------2 R0 Cc
Equation 11
1
f P HF = ---------------------------------------------------2 Rc C0 + Cp
whereas the zero is defined as:
Equation 12
1
f Z = --------------------------------2 Rc Cc
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ST1S31
Functional description
The embedded compensation network is RC = 80 k, CC = 55 pF while CP and CO can be
considered as negligible. The error amplifier output resistance is 96 Mso the relevant
singularities are:
Equation 13
f Z = 36 2 kHz
f P LF = 30 Hz
So closing the loop, the loop gain GLOOP(s) is:
Equation 14
G LOOP s = G CO s G DIV s G EA s
Example 1:
VIN = 5 V, VOUT = 1.2 V, Iomax = 3 A, L = 1.0 H, Cout = 47 µF (MLCC), R1 = 10 k,
R2 = 20 k(see Section 5.2 and Section 5.3 for inductor and output capacitor selection
guidelines).
The module and phase bode plot are reported in Figure 7 and Figure 8.
The bandwidth is 117 kHz and the phase margin is 63 degrees.
Figure 7. Module bode plot
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36
Functional description
ST1S31
Figure 8. Phase bode plot
4.4
Overcurrent protection
The ST1S31 device implements overcurrent protection sensing the current flowing through
the high-side current switch.
If the current exceeds the overcurrent threshold the high-side is turned off, implementing
a cycle-by-cycle current limitation. Since the regulation loop is no longer fixing the duty
cycle, the output voltage is unregulated and the FB pin falls accordingly to the new duty
cycle.
If the FB falls below 0.2 V, the peak current limit is reduced to around 2.3 A and the
switching frequency is reduced to assure that the inductor current is properly limited below
the above mentioned value and above 1.2 A. This strategy is called “current foldback”.
The mechanism to adjust the switching frequency during the current foldback condition
exploits the low-side current sense circuitry. If FB is lower than 0.2 V, the high-side power
MOSFET is turned off when the current reaches the current foldback threshold (2.3 A), then,
after a proper deadtime that avoids the cross conduction, the low-side is turned on until the
low-side current is lower than a valley threshold (1.2 A). Once the low-side is turned off, the
high-side is immediately turned on. In this way the frequency is adjusted to keep the
inductor current ripple between the current foldback value (2.3 A) and valley threshold
(1.2 A), so properly limiting the output current in case of overcurrent or short-circuit.
It should be noted that in some cases, mainly with very low output voltages, the hard
overcurrent can cause the FB to find the new equilibrium just over the current foldback
threshold (0.2 V). In this case no frequency reduction is enabled, then the inductor current
may diverge. This means that the ripple current during the minimum ON-time is higher than
the ripple current during the OFF-time (the switching period minus the minimum ON-time),
so pulse-by-pulse, the average current is rising, exceeding the current limit.
In order to avoid too high current, a further protection is activated when the high-side current
exceeds a further current threshold (OCP2) slightly over the current limit (OCP1). If the
current triggers the second threshold, the converter stops switching, the reference of the
error amplifier is pulled down and then it restarts with a soft-start procedure. If the
overcurrent condition is still active, the current foldback with frequency reduction properly
limits the output current to 2.3 A.
14/36
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ST1S31
4.5
Functional description
Enable function
The enable feature allows the device to be put into standby mode. With the EN pin is lower
than 0.4 V, the device is disabled and the power consumption is reduced to less than 10 A.
With the EN pin higher than 1.2 V, the device is enabled. If the EN pin is left floating, an
internal pull-down ensures that the voltage at the pin reaches the inhibit threshold and the
device is disabled. The pin is also VIN compatible.
4.6
Light load operation
With peak current mode control loop the output of the error amplifier is proportional to the
load current. In the ST1S31 device, to increase light load efficiency when the output of the
error amplifier falls below a certain threshold, the high-side turn-on is prevented.
This mechanism reduces the switching frequency at light load in order to save the switching
losses.
4.7
Hysteretic thermal shutdown
The thermal shutdown block generates a signal that turns off the power stage if the junction
temperature goes above 150 °C. Once the junction temperature goes back to about 130 °C,
the device restarts in normal operation.
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36
Application information
ST1S31
5
Application information
5.1
Input capacitor selection
The capacitor connected to the input must be capable of supporting the maximum input
operating voltage and the maximum RMS input current required by the device. The input
capacitor is subject to a pulsed current, the RMS value of which is dissipated over its ESR,
affecting the overall system efficiency.
So the input capacitor must have an RMS current rating higher than the maximum RMS
input current and an ESR value compliant with the expected efficiency.
The maximum RMS input current flowing through the capacitor can be calculated as:
Equation 15
2
2
2D
D
I RMS = I O D – --------------- + ------2
where Io is the maximum DC output current, D is the duty cycle, and is the efficiency.
Considering = 1, this function has a maximum at D = 0.5 and is equal to Io/2.
The peak-to-peak voltage across the input capacitor can be calculated as:
Equation 16
IO
D
D
V PP = ------------------------- 1 – ---- D + ---- 1 – D + ESR I O
C IN F SW
where ESR is the equivalent series resistance of the capacitor.
Given the physical dimension, ceramic capacitors can well meet the requirements of the
input filter sustaining a higher input RMS current than electrolytic / tantalum types. In this
case the equation of CIN as a function of the target peak-to-peak voltage ripple (VPP) can be
written as follows:
Equation 17
IO
D
D
C IN = --------------------------- 1 – ---- D + ---- 1 – D
V PP F SW
neglecting the small ESR of ceramic capacitors.
Considering = 1, this function has its maximum in D = 0.5, therefore, given the maximum
peak-to-peak input voltage (VPP_MAX), the minimum input capacitor (CIN_MIN) value is:
Equation 18
IO
C IN_MIN = -----------------------------------------------2 V PP_MAX F SW
Typically, CIN is dimensioned to keep the maximum peak-to-peak voltage ripple in the order
of 1% of VINMAX.
16/36
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ST1S31
Application information
The placement of the input capacitor is very important to avoid noise injection and voltage
spikes on the input voltage pin. So the CIN must be placed as close as possible to the
VIN_SW pin. In Table 6 some multilayer ceramic capacitors suitable for this device are
given.
Table 6. Input MLCC capacitors
Manufacturer
Series
Cap value (µF)
Rated voltage (V)
Murata
GRM21
10
10
C3225
10
25
C3216
10
16
LMK212
22
10
TDK
TAIYO YUDEN
A ceramic bypass capacitor, as close as possible to the VINA pin so that additional parasitic
ESR and ESL are minimized, is suggested in order to prevent instability on the output
voltage due to noise. The value of the bypass capacitor can go from 330 nF to 1 µF.
5.2
Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor. So the
minimum inductance value to have the expected current ripple must be selected. The rule to
fix the current ripple value is to have a ripple at 20% - 40% of the output current.
In continuous current mode (CCM), the inductance value can be calculated by Equation 19:
Equation 19
V IN – V OUT
V OUT
I L = ------------------------------ T ON = -------------- T OFF
L
L
where TON is the conduction time of the high-side switch and TOFF is the conduction time of
the low-side switch (in CCM, FSW = 1/(TON + TOFF)). The maximum current ripple, given the
VOUT, is obtained at maximum TOFF, that is, at minimum duty cycle (see previous section to
calculate minimum duty). So by fixing IL = 20% to 30% of the maximum output current, the
minimum inductance value can be calculated:
Equation 20
V OUT 1 – D MIN
L MIN = ---------------- ----------------------I MAX F SWMIN
where FSWMIN is the minimum switching frequency, according to Table 4. The slope
compensation, to prevent the sub-harmonic instability in the peak current control loop, is
internally managed and so fixed. This implies a further lower limit for the inductor value. To
assure sub-harmonic stability:
Equation 21
L V out 2 V pp f sw
where VPP is the peak-to-peak value of the slope compensation ramp. The inductor value
selected based on Equation 20 must satisfy Equation 21. The peak current through the
inductor is given by Equation 22:
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36
Application information
ST1S31
Equation 22
I L
I L PK = I O + -------2
So if the inductor value decreases, the peak current (which must be lower than the current
limit of the device) increases. The higher the inductor value, the higher the average output
current that can be delivered, without reaching the current limit.
In Table 7 some inductor part numbers are listed.
Table 7. Inductors
Manufacturer
Coilcraft
Series
Inductor value (µH)
Saturation current (A)
XAL50xx
1.2 to 3.3
6.3 to 9
XAL60xx
2.2 to 5.6
7.4 to 11
MSS1048
1.0 to 3.8
6.5 to 11
WE-HCI 7030
1.5 to 4.7
7 to 14
WE-PD type L
1.5 to 3.5
6.4 to 10
DR73
1.0 to 2.2
5.5 to7.9
DR74
1.5 to 3.3
5.4 to 8.35
Würth
Coiltronics
5.3
Output capacitor selection
The current in the output capacitor has a triangular waveform which generates a voltage
ripple across it. This ripple is due to the capacitive component (charge or discharge of the
output capacitor) and the resistive component (due to the voltage drop across its ESR). So
the output capacitor must be selected in order to have a voltage ripple compliant with the
application requirements.
The amount of the voltage ripple can be calculated starting from the current ripple obtained
by the inductor selection.
Equation 23
I MAX
V OUT = ESR I MAX + ------------------------------------8 C OUT f SW
For a ceramic (MLCC) capacitor, the capacitive component of the ripple dominates the
resistive one. While for an electrolytic capacitor the opposite is true.
As the compensation network is internal, the output capacitor should be selected in order to
have a proper phase margin and then a stable control loop.
The equations of Section 5.2 help to check loop stability given the application conditions,
the value of the inductor and of the output capacitor.
18/36
DocID022998 Rev 5
ST1S31
Application information
In Table 8 some capacitor series are listed.
Table 8. Output capacitors
Series
Cap value (µF)
Rated voltage (V)
ESR (m)
GRM32
22 to 100
6.3 to 25
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