A8739
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
Discontinued Product
This device is no longer in production. The device should not be
purchased for new design applications. Samples are no longer available.
Date of status change: December 3, 2013
Recommended Substitutions:
For existing customer transition, and for new customers or new applications, contact Allegro Sales.
NOTE: For detailed information on purchasing options, contact your
local Allegro field applications engineer or sales representative.
Allegro MicroSystems, LLC reserves the right to make, from time to time, revisions to the anticipated product life cycle plan
for a product to accommodate changes in production capabilities, alternative product availabilities, or market demand. The
information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its use; nor for any infringements of patents or other rights of third parties which may result from its use.
A8739
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
Features and Benefits
Description
▪ Primary-side output voltage sensing; no resistor divider required
▪ Adjustable switch peak current limit up to 1.5 A with
single-wire programming through the CHARGE pin
▪ Option to fine-tune target voltage
▪ Ultra-small 2 × 2 DFN/MLP-8 package
▪ Integrated IGBT driver with internal gate resistors
▪ Low quiescent current draw (0.5 μA max. in shutdown mode)
▪ 1V logic (VHI(min)) compatibility
▪ Zero-voltage switching for lower loss
▪ >75% efficiency
▪ Charge complete indication
▪ Integrated 50 V DMOS switch
The Allegro™ A8739 is a Xenon photoflash charger IC designed
to provide an ultra-small solution with flexibility to adjust
both charging current limit and output voltage target. By using
primary-side voltage sensing, the need for a secondary-side
resistive voltage divider is eliminated. This has the additional
benefit of reducing leakage currents on the secondary side of
the transformer. Target output voltage is primarily determined
by the transformer turns ratio, but it can be further adjusted (in
steps of –5 V each) using an external resistor. This enables better
output accuracy, as well as greater flexibility in the selection of
transformers.
Package: 8-pin DFN/MLP (suffix EE)
To extend battery life, the A8739 features very low supply
current draw (0.5 μA max in shutdown mode and 10 μA in
standby mode). The switch current limit can be programmed
from 0.45 to 1.5 A in 16 steps with single wire interface,
through the CHARGE pin.
The IGBT driver also has internal gate resistors for minimum
external component count. The charge and trigger voltage
logic thresholds are set at 1 VHI(min) to support applications
implementing low-voltage control logic.
The A8739 is available in an 8-contact 2 mm × 2 mm
DFN/MLP package with a 0.60 maximum overall package
height, and an exposed pad for enhanced thermal performance.
It is lead (Pb) free with 100% matte tin leadframe plating.
2 mm × 2 mm, 0.60 mm height
Not to scale
Typical Applications
Battery Input
2.3 to 5.5 V
Battery Input
2.0 to 5.5 V
+
+
C1
RBAT
100F
315 V
VBAT
C1
RBAT
COUT
VOUT Detect
VOUT Detect
SW
VIN_VDRV
SW
VIN_VDRV
Control
Block
C2
COUT
100F
315 V
VBAT
2.3 to 5.5 V
C2
ISW sense
Control
Block
ISW sense
VPULLUP
CHARGE
100 kΩ
DONE
VPULLUP
CHARGE
DONE
DONE
VIN_VDRV
TRIG
VIN_VDRV
IGBT Driver
IGBT Gate
TRIG
IGBT Driver
IGBT Gate
GATE
GND
(A)
Figure 1. Typical applications: (A) with single battery supply and (B) with separate bias supply
8739-DS
100 kΩ
DONE
GATE
GND
(B)
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Selection Guide
Part Number
A8739EEETR-T
Packing
Package
1500 pieces per reel
8-contact DFN/MLP with exposed thermal pad
Absolute Maximum Ratings
Characteristic
Symbol
SW Pin
VSW
VIN_DRV, VBAT Pins
VIN
Notes
DC voltage.
(VSW is self-clamped by internal active clamp
and is allowed to exceed 50 V during flyback
spike durations. Maximum repetitive energy
during flyback spike: 0.5 μJ at frequency
≤ 400 kHz.)
Care should be taken to limit the current when
–0.6 V is applied to these pins.
¯¯N¯¯E¯ Pins
CHARGE, TRIG, ¯D¯¯O
Remaining Pins
Operating Ambient Temperature
Maximum Junction
Storage Temperature
TA
Range E
Rating
Units
–0.3 to 50
V
–0.3 to 6.0
V
–0.6 to VIN + 0.3 V
V
–0.3 to VIN + 0.3 V
V
–40 to 85
ºC
TJ(max)
150
ºC
Tstg
–55 to 150
ºC
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Functional Block Diagram
SW
VBAT
VSW – VBAT
DCM
Detector
ILIM
Reference
toff(max)
Control Logic
DMOS
18 μs
VDSref
HmL
Triggered Timer
OCP
VIN_DRV
S
Q
R
Q
ton(max)
18 μs
Enable
Decoder
S
Q
R
Q
DONE
CHARGE
VIN_DRV
IGBT Driver
GATE
TRIG
GND
Pin-out Diagram
Terminal List
Number
Name
1
¯D¯¯O
¯¯N¯¯E¯
Function
Open collector output, pulls low when output reaches target value and CHARGE is
high. Goes high during charging or whenever CHARGE is low.
DONE 1
8
CHARGE
TRIG 2
7
VIN_DRV
2
TRIG
6
VBAT
3
GATE
IGBT gate drive output.
5
SW
4
GND
Ground connection.
5
SW
Drain connection of internal DMOS switch. Connect to transformer primary winding.
6
VBAT
Connect to battery voltage; directly to select the default output voltage, or through a
series resistor to select a lower output voltage step.
7
VIN_DRV
Input voltage. Connect to 3 to 5.5 V bias supply. Decouple VIN voltage with 0.1 μF
ceramic capacitor placed close to this pin.
8
CHARGE
Charge enable and current limit serial programming pin. Set this pin low to
shut down the chip.
–
PAD
Exposed pad for enhanced thermal dissipation. Connect to ground plane.
GATE 3
PAD
GND 4
(Top View)
IGBT trigger input.
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115 Northeast Cutoff
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3
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
ELECTRICAL CHARACTERISTICS Typical values are valid at VIN = VBAT = 3.6 V; TA = 25°C, except
guaranteed from −40°C to 85°C ambient, unless otherwise noted
Characteristics
VBAT Voltage Range
VIN_DRV Voltage Range
UVLO Enable Threshold
UVLO Hysteresis
VIN Supply Current
Symbol
Min.
Typ.
Max.
Unit
VBAT
2.0
–
5.5
V
VIN
2.3
–
5.5
V
–
2.05
2.2
V
–
150
–
mV
Shutdown (CHARGE = 0 V, TRIG = 0 V)
–
0.02
0.5
μA
Charging complete
–
50
100
μA
VINUV
Test Conditions
indicates specifications
VIN rising
VINUV(hys)
IIN
VBAT Pin Supply Current
IBAT
Charging (CHARGE = VIN, TRIG = 0 V)
–
2
–
mA
Shutdown (CHARGE = 0 V, TRIG = 0 V)
–
0.01
1
μA
Charging done (CHARGE = VIN,
¯D¯¯O
¯¯N¯¯E¯ = 0 V)
–
–
5
μA
Charging (CHARGE = VIN, TRIG = 0 V)
–
50
–
μA
Current Limit
Primary-Side Current Limit1
Switch Current Limit (ILIM Programming Input
on CHARGE Pin)
Switch On-Resistance
Switch Leakage
Current2
ISWLIM
100% setting
1.35
1.50
1.65
A
ISWLIM1
Default setting
–
100
–
%
ISWLIM2
One pulse applied to CHARGE pin
–
95
–
%
ISWLIM3
Two pulses applied to CHARGE pin
–
90
–
%
ISWLIM4
Three pulses applied to CHARGE pin
–
86
–
%
ISWLIM5
Four pulses applied to CHARGE pin
–
81
–
%
ISWLIM6
Five pulses applied to CHARGE pin
–
76
–
%
ISWLIM7
Six pulses applied to CHARGE pin
–
71
–
%
ISWLIM8
Seven pulses applied to CHARGE pin
–
67
–
%
ISWLIM9
Eight pulses applied to CHARGE pin
–
62
–
%
ISWLIM10
Nine pulses applied to CHARGE pin
–
57
–
%
ISWLIM11
Ten pulses applied to CHARGE pin
–
52
–
%
ISWLIM12
Eleven pulses applied to CHARGE pin
–
48
–
%
ISWLIM13
Twelve pulses applied to CHARGE pin
–
43
–
%
ISWLIM14
Thirteen pulses applied to CHARGE pin
–
38
–
%
ISWLIM15
Fourteen pulses applied to CHARGE pin
–
33
–
%
ISWLIM16
Fifteen pulses applied to CHARGE pin
–
29
–
%
VIN_DRV = 3.6 V, ID = 800 mA, TA = 25°C
–
0.4
–
Ω
VSW = 5.5, over full temperature range
–
–
2
μA
–
100
–
kΩ
High, over input supply range
1.0
–
–
V
Low, over input supply range
–
–
0.4
V
RSWDS(on)
ISWLK
CHARGE Pull-down Resistance
RCHGPD
CHARGE Input Voltage2
VCHARGE
Continued on the next page…
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4
A8739
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
ELECTRICAL CHARACTERISTICS (Continued) Typical values are valid at VIN = VBAT = 3.6 V; TA = 25°C, except
indicates
specifications guaranteed from −40°C to 85°C ambient, unless otherwise noted
Charge Pin Programming
ILIM Programming High at CHARGE pin 2
ILIM Programming Low at CHARGE pin 2
tILIM(H)init
tILIM(H)
Initial Pulse
15
–
–
μs
Subsequent Pulses
0.2
–
–
μs
tILIM(L)
0.2
–
–
μs
tILIM(SU)
–
200
–
μs
Switch-Off Timeout
toff(max)
–
18
–
μs
Switch-On Timeout
ton(max)
–
18
–
μs
Total ILIM Setup Time at CHARGE pin 2
Output Comparator Trip Voltage
¯D¯¯O
¯¯N¯¯E¯ Leakage Current2
Output Low
Voltage2
Output Voltage Overdrive
dV/dt Threshold for ZVS Comparator
VOUTTRIP1
Measured as VSW – VBAT (RBAT = 0 Ω)
31
31.5
32
V
VOUTTRIP2
Measured as VSW – VBAT (RBAT = 845 Ω)
–
31.0
–
V
VOUTTRIP3
Measured as VSW – VBAT (RBAT = 2.32 kΩ)
–
30.5
–
V
VOUTTRIP4
Measured as VSW – VBAT (RBAT = 4.87 kΩ)
–
30.0
–
V
VOUTTRIP5
Measured as VSW – VBAT (RBAT = 9.09 kΩ)
–
29.5
–
V
–
–
1
μA
IDONELK
VDONEL
¯¯N¯¯E¯ pin
32 μA into ¯D¯¯O
–
–
100
mV
VOUTOV
Pulse width = 200 ns (90% to 90%)
–
200
400
mV
Measured at SW pin
–
20
–
V/μs
VTRIG(H)
Input = logic high, over input supply range
1
–
–
V
VTRIG(L)
Input = logic low, over input supply range
–
–
0.4
V
dV/dt
IGBT Driver
TRIG Input Voltage2
TRIG Pull-Down Resistor
–
100
–
kΩ
GATE Resistance to VIN_DRV
RSrcDS(on)
RTRIGPD
VGATE = 1.8 V
–
21
–
Ω
GATE Resistance to GND
RSnkDS(on)
VGATE = 1.8 V
–
27
–
Ω
Measurement taken at GATE pin,
CL= 6500 pF
–
25
–
ns
–
ns
Propagation Delay (Rising)3
tDr
Propagation Delay (Falling)3
tDf
–
60
tr
–
290
–
ns
tf
–
380
–
ns
RGTPD
–
20
–
kΩ
Output Rise
Time3
Output Fall Time3
GATE Pull-Down Resistor
1Current
limit guaranteed by design and correlation to static test.
throughout the range TA = –40°C to 85°C guaranteed by design and characterization.
3See IGBT Drive Timing Definition diagram for further information.
2Specifications
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5
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
IGBT Drive Timing Definition
50%
TRIG
50%
tDr
tr
tDf
90%
GATE
tf
90%
10%
10%
Operation Timing Diagram
VIN
UVLO
CHARGE
SW
Target VOUT
VOUT
DONE
T2
T1
T3
TRIG
GATE
A
B
C
D
E
F
Explanation of Events
A: Start charging by pulling CHARGE to high, provided that VIN is above UVLO level.
B: Charging stops when VOUT reaches the target voltage.
C: Start a new charging process with a low-to-high transition at the CHARGE pin.
D: Pull CHARGE to low to put the controller in low-power standby mode.
E: Charging does not start, because VIN is below UVLO level when CHARGE goes high.
F: After VIN goes above UVLO, another low-to-high transition at the CHARGE pin is required to
start the charging.
T1, T2, T3 (Trigger instances): IGBT driver output pulled high whenever the TRIG pin is at logic
high. It is recommended to avoid applying any trigger pulses during charging.
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115 Northeast Cutoff
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6
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Characteristic Performance
IGBT Drive Performance
IGBT drive waveforms are measured at pin, with capacitive load of 6800 pF
tr
Rising Signal
VIN
Symbol
C1
C2
C3
t
Conditions
Parameter
VTRIGGER
VGATE
VIN
time
Parameter
tDr
tr
CLOAD
Units/Division
1V
1V
1V
100 ns
Value
23 ns
320 ns
6.8 nF
C2,C3
VGATE
C1
VTRIGGER
t
tf
Falling Signal
VIN
Symbol
C1
C2
C3
t
Conditions
Parameter
VTRIGGER
VGATE
VIN
time
Parameter
tDr
tr
CLOAD
Units/Division
1V
1V
1V
100 ns
Value
58 ns
402 ns
6.8 nF
VGATE
C2,C3
VTRIGGER
C1
t
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7
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Characteristic Performance
Charge Time versus Battery Voltage at Various ILIM
RBAT = 0 Ω, Transformer LPRIMARY = 12.8 μH, N =10.25, VIN =3.6 V, COUT = 100 μF / 330 V UCC, at room
temperature
20
Time (Sec)
18
16
ILIM13 (0.65A)
14
ILIM11 (0.79A)
12
ILIM9 (0.93A)
10
ILIM7 (1.07A)
8
ILIM5 (1.22A)
6
ILIM3 (1.36A)
4
ILIM1 (1.50A)
2
0
1.5
2.0
2.5
3.0
3.5
4.0
Battery Voltage (V)
4.5
5.0
5.5
Efficiency versus Battery Voltage at Various ILIM
RBAT = 0 Ω, Transformer LPRIMARY = 12.8 μH, N =10.25, VIN =3.6 V, at room temperature
86%
84%
82%
80%
ILIM13 (0.65A)
78%
Efficiency (%)
76%
ILIM11 (0.79A)
74%
ILIM9 (0.93A)
72%
70%
ILIM7 (1.07A)
68%
ILIM5 (1.22A)
66%
64%
ILIM3 (1.36A)
62%
60%
ILIM1 (1.50A)
58%
56%
54%
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
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8
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Final Output Voltage versus Battery Voltage at Various ILIM
RBAT = 0 Ω, Transformer LPRIMARY = 12.8 μH, N =10.25, VIN =3.6 V, at room temperature
328
327
ILIM15 (0.51A)
326
ILIM13 (0.65A)
VOUT (V)
325
ILIM11 (0.79A)
ILIM9 (0.93A)
324
ILIM7 (1.07A)
323
ILIM5 (1.22A)
322
ILIM3 (1.36A)
ILIM1 (1.50A)
321
320
1.5
2.0
2.5
3.0
3.5
4.0
Battery Voltage (V)
4.5
5.0
5.5
Note: Output voltage is sensed from the primary side winding when the switch turns off. This duration, toff , has
to be long enough (>200 ns) in order to obtain an accurate measurement. The value of toff depends on ISWlim,
primary inductance, LPrimary , and the turns ratio, N, as given by: toff = (ISWlim × LPRIMARY × N) / VOUT .
Final Output Voltage versus Secondary Side Conduction Time
at Various Battery Voltages
RBAT = 0 Ω, Transformer LPRIMARY = 12.8 μH, N =10.25, VIN =3.6 V, at room temperature
328
327
326
VBAT (V)
5.0
VOUT (V)
325
4.2
324
3.6
323
2.2
322
321
320
150
200
250
300
350
400
450
500
550
600
650
toff (ns)
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9
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Average Input Current versus Battery Voltage at Various ILIM
RBAT = 0 Ω, Transformer LPRIMARY = 12.8 μH, N =10.25, VIN =3.6 V, at room temperature
0.7
0.6
ILIM1 (1.50A)
Current (A)
0.5
ILIM3 (1.36A)
ILIM5 (1.22A)
0.4
ILIM7 (1.07A)
ILIM9 (0.93A)
0.3
ILIM11 (0.79A)
0.2
ILIM13 (0.65A)
ILIM15 (0.51A)
0.1
0.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Battery Voltage (V)
Note: Peak switch current is limited by the maximum on-time and di/dt of the transformer primary
current; therefore, average input current drops at very low battery voltage.
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10
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Charging Waveforms
Output Capacitor Charging at Various Peak Current Limits
Test conditions: VIN = 3.0 V, VBAT = 3.7 V, COUT = 100 μF / 330 V UCC, transformer = T-16-024A (LPRIMARY =12.8 μH, N = 10.25), at room temperature
¯¯N¯¯E
¯ (5 V / div), Ch2 = Output Voltage (50 V / div), Ch3 = Input Current (100 mA / div), Time scale = 1 sec / div
Oscilloscope settings: Ch1 = ¯D¯¯O
VOUT
ILIM1 (1.5 A)
IIN
C2,C3
VDONE
C1
VOUT
ILIM5 (1.22 A)
IIN
C2,C3
VDONE
C1
VOUT
ILIM9 (0.93 A)
IIN
C2,C3
VDONE
C1
VOUT
ILIM11 (0.79 A)
IIN
VDONE
C2,C3
C1
t
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115 Northeast Cutoff
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1.508.853.5000; www.allegromicro.com
11
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Output Capacitor Charging at Various Battery Voltages
Test conditions: VIN = 3.0 V, ILIM3 (1.36 A), COUT = 100 μF / 330 V UCC, transformer = T-16-024A (LPRIMARY =12.8 μH, N = 10.25), at room temperature
¯¯N¯¯E¯ (5 V / div), Ch2 = Battery Voltage (1 V / div), Ch3 = Output Voltage (50 V / div), Ch4 = Input Current (100 mA V / div),
Oscilloscope settings: Ch1 = ¯D¯¯O
Time scale = 1 sec / div
VOUT
VBAT = 3.0 V
VBAT
IIN
C2,C3,C4
VDONE
C1
VOUT
VBAT
VBAT = 3.7 V
IIN
C2,C3,C4
VDONE
C1
VOUT
VBAT
VBAT = 4.2 V
IIN
C2,C3,C4
VDONE
C1
VOUT
VBAT
VBAT = 5.0 V
IIN
C2,C3,C4
C1
VDONE
t
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
12
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Functional Description
General Operation Overview
The charging operation is started by a low-to-high signal on the
CHARGE pin, provided that VIN is above the VUVLO level. It is
strongly recommended to keep the CHARGE pin at logic low
during power-up. After VIN exceeds the UVLO level, a lowto-high transition on the CHARGE pin is required to start the
charging.
¯ open-drain indicator is pulled low when CHARGE
The ¯D¯¯ ¯O¯¯¯N¯¯E
is high and target output voltage is reached. The primary peak
current is set to 1.5 A by default, but it can be programmed from
1.5 A down to approximately 0.44 A in 15 steps. See the ILIM
Programming section for details.
Timer Mode and Fast Charging Mode
The A8739 achieves fast charging times and high efficiency by
operating in discontinuous conduction mode (DCM) through
most of the charging process. The relationship of Timer mode and
Fast Charging mode is shown in figure 2.
The IC operates in Timer mode when beginning to charge a completely discharged photoflash capacitor, usually when the output
voltage, VOUT , is less than approximately 30 V (depending on
transformer used). Timer mode is a fixed period, 18 μs, off-time
control. One advantage of having Timer mode is that it limits the
initial battery current surge and thus acts as a “soft-start.” A timeexpanded view of a Timer mode interval is shown in figure 3.
When a charging cycle is initiated, the transformer primary side
current, IPRIMARY , ramps-up linearly at a rate determined by the
combined effect of the battery voltage, VBAT , and the primary
side inductance, LPRIMARY . When IPRIMARY reaches the current
limit, ISWLIM , the internal MOSFET is turned off immediately,
allowing the energy to be pushed into the photoflash capacitor,
COUT , from the secondary winding. The secondary side current
drops linearly as COUT charges. The switching cycle starts again,
either after the transformer flux is reset, or after a predetermined
time period, tOFF(max) (18 μs), whichever occurs first.
The A8739 senses output voltage indirectly on primary side. This
eliminates the need for high voltage feedback resistors required
for secondary sensing. Flyback converter stops switching when
output voltage reaches:
VOUT = K × N – Vd ,
VOUT
IIN
Figure 2. Timer mode and Fast Charging mode: VOUT = 50 V/div,
IIN = 100 mA/div., VIN = VBAT = 3.6 V, COUT = 100 μF / 330 V, ILIM = 1.0 A,
and t = 1 s/div.
Where:
K = 31.5 V typically,
Vd is the forward drop of the output diode (approximately 2 V),
and
N is transformer turns ratio.
Switch On-Time and Off-Time Control
The A8739 implements an adaptive on-time/off-time control. Ontime duration, ton , is approximately equal to
ISW
C4
VSW
VBAT
C2,C3
ton = ISWlim × LPRIMARY / VBAT .
Off-time duration, toff , depends on the operating conditions
during switch off-time. The A8739 applies two charging modes:
Fast Charging mode and Timer mode, according to the conditions
described in the next section.
VOUT
C1
Figure 3. Expanded view of Timer mode: VOUT ≤ 10 V, VBAT = 5.5 V,
Ch1: VOUT = 20 V / div., Ch2: VBAT = 5 V / div., Ch3: VSW = 5 V / div.,
Ch4: ISW = 500 mA / div., t = 5 μs / div.
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
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13
A8739
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
As soon as a sufficient voltage has built up at the output capacitor, the IC enters Fast-Charging mode. In this mode, the next
switching cycle starts after the secondary side current has stopped
flowing, and the switch voltage has dropped to a minimum value.
A proprietary circuit is used to allow minimum-voltage switching, even if the SW pin voltage does not drop to 0 V. This enables
C4
ISW
Fast-Charging mode to start earlier, thereby reducing the overall
charging time. Minimum-voltage switching is shown in figure 4.
During Fast-Charging mode, when VOUT is high enough (over
50 V), true zero-voltage switching (ZVS) is achieved. This
further improves efficiency as well as reduces switching noise. A
ZVS interval is shown in figure 5.
ISW
C4
VSW
VSW
VBAT
VBAT
C2,C3
C2,C3
VOUT
VOUT
C1
Figure 4. Minimum-voltage switching: VOUT ≥ 35 V, VBAT = 5.5 V,
Ch1: VOUT = 20 V / div., Ch2: VBAT = 5 V / div., Ch3: VSW = 5 V / div.,
Ch4: ISW = 500 mA / div., t = 1 μs / div.
C1
Figure 5. True zero-voltage switching (ZVS): VOUT = 75 V, VBAT = 5.5 V,
Ch1: VOUT = 20 V / div., Ch2: VBAT = 5 V / div., Ch3: VSW = 5 V / div.,
Ch4: ISW = 500 mA / div., t = 0.5 μs / div.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
14
A8739
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
ILIM Programming
The peak current limit can be programmed to sixteen different levels, from 100% to 29%, with programming through the
CHARGE pin. An internal digital circuit decodes the input
clock signals, which sets the switch current limit. This flexible
scheme allows the user to operate the A8739 at required current
limits. The battery life can be effectively extended by setting a
lower current limit at low battery voltages. Figure 6 shows the
ILIM clock timing scheme protocol. The total ILIM setup time,
tILIM(SU) , denotes the time needed for the decoder circuit to
receive ILIM inputs and set ISWLIM , and has a typical duration of
200 μs.
Figure 7 shows the timing definition of the primary current
limiting circuit. At the end of the setup period, tILIM(SU) , primary
current starts to ramp up to the set ISWLIM . The ISWLIM setting
remains in effect as long as the CHARGE pin is high. To reset the
ILIM decoder, pull the CHARGE pin low before clocking-in the
new setting.
After the first start-up or an ILIM decoder reset, each new current
limit can be set by sending a burst of pulses to the CHARGE
pin. The first rising edge starts the ILIM decoder, and up to 16
rising edges will be counted to set the ISWLIM level. The first
pulse width, tILIM1(H), must be at least 15 μs long. Subsequent
pulses (up to 15 more) can be as short as 0.2 μs. The last lowto-high edge must arrive within 200 μs from the first edge. The
CHARGE pin will stay high afterwards.
(0 to 15)
Figure 6. ILIM programming timing definition
Figure 7. Current limit timing example (ISWLIM4 selected)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
15
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Output Voltage Adjustment
The target output voltage of the A8739 is primarily determined
by the transformer turns ratio, N:
VOUT = 31.5 (V) × N
Typically N = 10, so the output voltage is approximately 315 V.
This output voltage can be further programmed in 5 V decrements down to 295 V, by using an external resistor at the VBAT
pin. This enables fine-tuning to achieve better accuracy, and provides greater flexibility in the selection of transformers. Refer to
figure 8 for example: An external resistor RBAT = 2.32 kΩ is used
to set the target output voltage at two steps lower, or 305 V when
the transformer turns ratio = 10. Capacitor C3 is needed in case
excessive switching noises is coupled into the VBAT pin, which
may affect accuracy of target voltage.
The relationship of output voltage and VBAT resistance is shown
in figure 9. To ensure proper voltage selection, use only
1% resistors. If C3 is used, make sure the RC filter time constant
(RBAT × C3) is less than 1 μs approximately.
Output Voltage versus VBAT Resistance
Transformer turns ratio N = 10
320
1:N
(N = 9 -11)
RBAT
2.32 kΩ
C3
100 pF
C1
COUT
100F
330 V
VBAT
Bias Input
2.3 to 5.5 V
A8739
VIN_VDRV
C2
VSW
Control
Block
SW
310
305
300
ISW sense
VPULLUP
DONE
295
100 kΩ
CHARGE
DONE
290
0
VIN_VDRV
TRIG
Output Voltage (V)
315
VP = Power Supply
2.0 to 5.5 V
1
2
3
4
5
6
Resistance (kΩ)
7
8
9
10
IGBT Driver
IGBT Gate
GATE
GND
Figure 8. Adjustment of VOUT using a series resistor at the VBAT pin
(VOUT = 30.5 × N).
Resistance,
RBAT
(kΩ)
Output Voltage,
VOUT
(V)
Recommended 1%
Resistor Values
(kΩ)
Recommended
C3 capacitance
(pF)
Optional
0 to 0.10
31.5 × N
0.
0.65 to 1.03
31.0 × N
0.845
100
2.15 to 2.49
30.5 × N
2.32
100
4.58 to 5.08
30.0 × N
4.87
100
8.68 to 9.76
29.5 × N
9.09
47
Figure 9. Relationship of VOUT and a series resistor at the VBAT pin
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
16
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Applications Information
Transformer Design
1. The transformer turns ratio, N, determines the output voltage:
N = NS / NP
VOUT = 31.5 × N – Vd ,
where 31.5 is the typical value of VOUTTRIP , and Vd is the forward drop of the output diode.
2. The primary inductance, LPRIMARY , determines the on-time of
the switch:
ton = (–LPRIMARY / R ) × ln (1 – ISWlim × R /VIN) ,
where R is the total resistance in the primary current path (including RSWDS(on) and the DC resistance of the transformer).
If VIN is much larger than ISWlim × R, then ton can be approximated by:
ton = ISWlim × LPRIMARY /VIN .
3. The secondary inductance, LSECONDARY, determines the offtime of the switch. Given:
LSECONDARY / LPRIMARY = N × N , then
toff = (ISWlim / N) × LSECONDARY /VOUT
= (ISWlim × LPRIMARY × N) /VOUT .
ton
The minimum pulse width for toff determines what is the minimum LPRIMARY required for the transformer. For example, if
ILIM8 = 1.0 A, N = 10, and VOUT = 315 V, then LPRIMARY must
be at least 6.3 μH in order to keep toff at 200 ns or longer. These
relationships are illustrated in figure 10.
In general, choosing a transformer with a larger LPRIMARY results
in higher efficiency (because a larger LPRIMARY corresponds to
a lower switch frequency and hence lower switching loss). But
transformers with a larger LPRIMARY also require more windings
and larger magnetic cores. Therefore, a trade-off must be made
between transformer size and efficiency.
Leakage Inductance and Secondary Capacitance
The transformer design should minimize the leakage inductance to ensure the turn-off voltage spike at the SW node does
not exceed the absolute maximum specification on the SW pin
(refer to the Absolute Maximum Ratings table). An achievable
minimum leakage inductance for this application, however, is
usually compromised by an increase in parasitic capacitance.
Furthermore, the transformer secondary capacitance should be
minimized. Any secondary capacitance is multiplied by N2 when
reflected to the primary, leading to high initial current swings
when the switch turns on, and to reduced efficiency.
toff
VSW
ISW
Vr
tf
VIN
VIN
VSW
ISW
tneg
Figure 10. Transformer Selection Relationships
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17
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Effects of Input Filters
Input Capacitor Selection
Ceramic capacitors with X5R or X7R dielectrics are recommended for the input capacitor, CIN. During initial Timer mode
the device operates with 18 μs off-time. The resonant period
caused by input filter inductor and capacitor should be at least
2 times greater or smaller than the 18 μs Timer period, to reduce
input ripple current during this period. The typical input LC filter
is shown in figure 11.
VBAT
C2
C3
IBAT
C1
The resonant period is given by:
Tres = 2
VOUT
(L × CIN)1/2 .
The effects of input filter components are shown in figures 12,
13, and 14. It is recommended to use at least 10 μF / 6.3 V to
decouple the battery input, VBAT , at the primary of the transformer. Decouple the VIN pin using 0.1 μF / 6.3 V bypass
capacitor.
Output Diode Selection
Figure 12. Input current waveforms with Li+ battery connected by
5-in. wire and decoupled by 4.7 μF capacitor, COUT = 100 μF,
VIN = VBAT = 3.6 V, Ch1: VOUT = 50 V/div, Ch2: VBAT = 2 V/div,
Ch3: IBAT = 500 mA/div, t = 1 s/div
VOUT
VBAT
C2
Choose rectifying diodes, D1, to have small parasitic capacitance
(short reverse recovery time) while satisfying the reverse voltage
and forward current requirements. The peak reverse voltage of
the diodes, VDPeak , occurs when the internal MOSFET switch is
closed. It can be calculated as:
VDPeak = VOUT + N × VBAT .
The peak current of the rectifying diode, IDPeak, is calculated as:
C3
IBAT
C1
Figure 13. Input current waveforms with Li+ battery connected through
4.7 μH inductor and 4.7 μF capacitor, COUT = 100 μF, VIN = VBAT = 3.6 V,
Ch1: VOUT = 50 V/div, Ch2: VBAT = 2 V/div, Ch3: IBAT = 200 mA/div,
t = 1 s/div
IDPeak = IPRIMARY_Peak / N .
VOUT
LIN
+
VBAT
C2
VBAT
CIN
A8739
C3
IBAT
C1
Figure 11. Typical input section with input inductance (inductance, LIN, may
be an input filter inductor or inductance due to long wires in test setup)
Figure 14. Input current waveforms with Li+ battery connected through
4.7 μH inductor and 10 μF capacitor, COUT = 100 μF, VIN = VBAT = 3.6 V,
Ch1: VOUT = 50 V/div, Ch2: VBAT = 2 V/div, Ch3: IBAT = 200 mA/div,
t = 1 s/div
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
18
Ultra-Small Mobile Adjustable Xenon Photoflash
Capacitor Charger with IGBT Driver
A8739
Layout Guidelines
Key to a good layout for the photoflash capacitor charger circuit
is to keep the parasitics minimized on the power switch loop
(transformer primary side) and the rectifier loop (secondary side).
Use short, thick traces for connections to the transformer primary
¯ signal trace and other
and SW pin. It is important that the ¯D¯¯ ¯O¯¯¯N¯¯E
signal traces be routed away from the transformer and other
switching traces, in order to minimize noise pickup. In addition,
high voltage isolation rules must be followed carefully to avoid
breakdown failure of the circuit board.
Avoid placing any ground plane area underneath the transformer
secondary and diode, to minimize parasitic capacitance.
For low threshold logic (
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