ALLEGRO A8732

A8732
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
Features and Benefits
Description
▪ Ultra small 2 × 2 DFN/MLP-8 package
▪ Low quiescent current draw (0.5 μA max. in shutdown mode)
▪ 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
▪ 1V logic (VHI(min)) compatibility
▪ Integrated IGBT driver with internal gate resistors
▪ Optimized for mobile phone, 1-cell Li+ battery applications
▪ Zero-voltage switching for lower loss
▪ >75% efficiency
▪ Charge complete indication
▪ Integrated 50 V DMOS switch with self-clamping
protection
The Allegro® A8732 is a Xenon photoflash charger IC designed
to meet the needs of ultra low power, small form factor cameras,
particularly camera phones. 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. To extend
battery life, the A8732 features very low supply current draw
(0.5 μA max in shutdown 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.
Package: 8-pin DFN/MLP (suffix EE)
The A8732 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.
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.
2 mm × 2 mm, 0.60 mm height
Not to scale
Typical Applications
Battery Input
2.3 to 5.5 V
+
C1
Battery Input
1.5 to 5.5 V
COUT
100F
315 V
VBAT
+
C1
VOUT Detect
VOUT Detect
SW
VIN_VDRV
SW
VIN_VDRV
Control
Block
C2
COUT
100F
315 V
VBAT
Control
Block
C2
ISW sense
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
8732-DS
100 kΩ
DONE
GATE
GND
(B)
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
Selection Guide
Part Number
A8732EEETR-T
Packing
Package
3000 pieces per reel
8-contact DFN/MLP with exposed thermal pad
Absolute Maximum Ratings
Characteristic
SW Pin
VIN_DRV, VBAT Pins
Symbol
Notes
VSW
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.)
ISW
DC current, pulse width = 1 ms
Care should be taken to limit the current when
–0.6 V is applied to these pins.
Remaining Pins
Operating Ambient Temperature
Maximum Junction
Storage Temperature
Units
–0.3 to 50
V
3
A
–0.3 to 6.0
V
–0.6 to VIN + 0.3 V
V
VIN
¯¯N¯¯E¯ Pins
CHARGE, TRIG, ¯D¯¯O
Rating
–0.3 to VIN + 0.3 V
V
–40 to 85
ºC
TJ(max)
150
ºC
Tstg
–55 to 150
ºC
TA
Range E
THERMAL CHARACTERISTICS may require derating at maximum conditions
Characteristic
Symbol
Package Thermal Resistance
RθJA
Test Conditions*
4-layer PCB, based on JEDEC standard
Value Units
49
ºC/W
*Additional thermal information available on Allegro Web site.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
Functional Block Diagram
SW
VBAT
VSW – VBAT
ILIM
Reference
DCM
Detector
Control Logic
DMOS
18 μs
VDSref
VIN_DRV
toff(max)
HmL
Triggered Timer
OCP
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.
GATE 3
PAD
GND 4
(Top View)
IGBT trigger input.
5
SW
6
VBAT
Drain connection of internal DMOS switch. Connect to transformer primary winding.
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.
Battery voltage.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
3
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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
Test Conditions
indicates specifications
Min.
Typ.
Max.
Unit
VBAT
(note 1)
1.5
–
5.5
V
VIN
(note 1)
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
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 Limit
Switch Current Limit (ILIM Programming Input
on CHARGE Pin)
ISWLIM
100% setting (note 2)
1.35
1.5
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
(note 3)
–
86
–
%
ISWLIM5
Four pulses applied to CHARGE pin
–
81
–
%
ISWLIM6
Five pulses applied to CHARGE pin (note 3)
–
76
–
%
ISWLIM7
Six pulses applied to CHARGE pin (note 3)
–
71
–
%
ISWLIM8
Seven pulses applied to CHARGE pin
(note 3)
–
67
–
%
ISWLIM9
Eight pulses applied to CHARGE pin
–
62
–
%
ISWLIM10
Nine pulses applied to CHARGE pin (note 3)
–
57
–
%
ISWLIM11
Ten pulses applied to CHARGE pin (note 3)
–
52
–
%
ISWLIM12
Eleven pulses applied to CHARGE pin
(note 3)
–
48
–
%
ISWLIM13
Twelve pulses applied to CHARGE pin
(note 3)
–
43
–
%
ISWLIM14
Thirteen pulses applied to CHARGE pin
(note 3)
–
38
–
%
ISWLIM15
Fourteen pulses applied to CHARGE pin
(note 3)
–
33
–
%
ISWLIM16
Fifteen pulses applied to CHARGE pin
(note 3)
–
29
–
%
Continued on the next page…
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
A8732
Ultra Small Mobile Phone 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
Switch On-Resistance
VIN_DRV = 3.6 V, ID = 600 mA, TA = 25°C
–
0.4
–
Ω
VSW = 5.5, over full temperature range
(note 1)
–
–
2
μA
–
100
–
kΩ
High, over input supply range (note 1)
1.0
–
–
V
Low, over input supply range (note 1)
–
–
0.4
V
Initial Pulse (note 3)
15
–
–
μs
tILIM(H)
Subsequent Pulses (note 3)
0.2
–
–
μs
tILIM(L)
(note 3)
0.2
–
–
μs
Total ILIM Setup Time at CHARGE Pin
tILIM(SU)
(note 3)
–
200
–
μs
Switch-Off Timeout
toff(max)
–
18
–
μs
Switch-On Timeout
ton(max)
–
18
–
μs
Switch Leakage Current
CHARGE Pull-down Resistance
CHARGE Input Voltage
RSWDS(on)
ISWLK
RCHGPD
VCHARGE
Charge Pin Programming
ILIM Programming High at CHARGE Pin
ILIM Programming Low at CHARGE Pin
Output Comparator Trip Voltage
tILIM(H)init
Measured as VSW – VBAT (note 4)
31
31.5
32
V
Output Comparator Voltage Overdrive
VOUTOV
Pulse width = 200 ns (90% to 90%)
–
200
400
mV
¯D¯¯O
¯¯N¯¯E¯ Leakage Current
IDONELK
(note 1)
1
μA
¯D¯¯O
¯¯N¯¯E¯ Output Low Voltage
VDONEL
dV/dt Threshold for ZVS Comparator
VOUTTRIP
dV/dt
¯¯N¯¯E¯ pin (note 1)
32 μA into ¯D¯¯O
–
–
100
mV
Measured at SW pin
–
20
–
V/μs
IGBT Driver
VTRIG(H)
Input = logic high, over input supply range
(note 1)
1.0
–
–
V
VTRIG(L)
Input = logic low, over input supply range
(note 1)
–
–
0.4
V
–
100
–
kΩ
TRIG Input Voltage
TRIG Pull-Down Resistor
RTRIGPD
GATE Resistance to VIN_DRV
RSrcDS(on)
VGATE = 1.8 V
–
21
–
Ω
GATE Resistance to GND
RSnkDS(on)
VGATE = 1.8 V
–
27
–
Ω
–
25
–
ns
Propagation Delay (Rising)
tDr
Measurement taken at GATE pin,
CL= 6500 pF (notes 3, 5)
Propagation Delay (Falling)
tDf
(notes 3, 5)
–
60
–
ns
Output Rise Time
tr
(notes 3, 5)
–
290
–
ns
Output Fall Time
tf
(notes 3, 5)
–
380
–
ns
–
20
–
kΩ
GATE Pull-Down Resistor
RGTPD
1 Specifications
throughout the range TA = –40°C to 85°C guaranteed by design and characterization.
2Current limit guaranteed by design and correlation to static test.
3Guaranteed by design and characterization.
4Specifications throughout the range T = –20°C to 85°C guaranteed by design and characterization.
A
5See IGBT Drive Timing Definition diagram for further information.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
IGBT Drive Timing Definition
50%
TRIG
50%
tDr
tr
tDf
90%
GATE
tf
90%
10%
10%
Operation Timing Diagram
VBAT
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.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
7
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
Characteristic Performance
Charge Time versus Battery Voltage at Various ILIM
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
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
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
8
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
Final Output Voltage versus Battery Voltage at Various ILIM
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
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)
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
9
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
Average Input Current versus Battery Voltage at Various ILIM
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.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
10
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
11
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
12
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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 A8732 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 A8732 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 A8732 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 A8732 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.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
13
A8732
Ultra Small Mobile Phone 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, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
14
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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 A8732 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, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
15
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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 8.
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 8. Transformer Selection Relationships
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
16
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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 9.
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 10,
11, and 12. 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 10. 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 11. 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
A8732
C3
IBAT
C1
Figure 9. Typical input section with input inductance (inductance, LIN, may
be an input filter inductor or inductance due to long wires in test setup)
Figure 12. 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, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
17
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
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 (<1.2 V) add 1 nF capacitors across the
CHARGE and TRIGGER pins to GND to avoid malfunction due
to noise.
Connect the EE package PAD to the ground pad for better thermal performance. Use ground planes on the top and bottom layers
below the IC and connect them through multiple thermal vias.
Refer to the figures on page 18 for recommended layout.
Recommended Components
Component
Rating
C1, Input Capacitor
10 μF, ±10%, 6.3 V, X5R ceramic
capacitor (0805)
C2
0.1uF, 6.3V X5R ceramic capacitor
COUT, Photoflash
Capacitor
Part Number
Source
JMK212BJ106K
Taiyo Yuden
100 μF / 330 V
EPH-31ELL101B131S
Chemi-Con
D1, Output Diode
2 x 250 V, 225 mA, 5 pF
BAV23S
Philips Semiconductor,
Fairchild Semiconductor
T1, Transformer
LPRIMARY = 12.8 μH, N= 10.25,
6.5 × 8 × 4 mm
T-16-024A
Tokyo Coil Electric
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
18
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
Recommended layout:
Vout
3
D1
VBAT
1
2
BAV23S
X2
C2
C4
Schematic
10uF
0.1uF
TCE_T-16-024A
1
DONE
CHARGE
Cout1
100uF
4
2
R10
2k
TP_Gate
6
A8732
VBAT
5
SW
7
1
3
VIN_DRV
GATE
DONE
TRIG
2
4
8
CHARGE
C5
1nF
3
C6
1nF
GND
Rg
12
U1
TRIGGER
Top side
Bottom side
Top components
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
19
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
A8732
Package EE 8-Contact DFN/MLP with Exposed Thermal Pad
0.30
0.50
8
2.00 ±0.15
8
0.83
2.00 ±0.15
0.90
2.13
A
1
9X
2
D
SEATING
PLANE
0.08 C
+0.05
0.55 –0.04
0.25 ±0.05
0.50 BSC
1
C
1.60
PCB Layout Reference View
All dimensions nominal, not for tooling use
(reference JEDEC MO-229UCCD)
Dimensions in millimeters
Exact case and lead configuration at supplier discretion within limits shown
2
0.325 ±0.050
0.90
B
8
1
C
1.60
A Terminal #1 mark area
B Exposed thermal pad (reference only, terminal #1
identifier appearance at supplier discretion)
C Reference land pattern layout (reference IPC7351
SON50P200X200X100-9M);
All pads a minimum of 0.20 mm from all adjacent pads; adjust as
necessary to meet application process requirements and PCB layout
tolerances; when mounting on a multilayer PCB, thermal vias at the
exposed thermal pad land can improve thermal dissipation (reference
EIA/JEDEC Standard JESD51-5)
D Coplanarity includes exposed thermal pad and terminals
Copyright ©2008-2010, Allegro MicroSystems, Inc.
The products described here are manufactured under one or more U.S. patents or U.S. patents pending.
Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the
information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use;
nor for any infringement of patents or other rights of third parties which may result from its use.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
20