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A8732 Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Features and Benefits
Ultra small 2 x 2 DFN/MLP-8 package Low quiescent current draw (0.5 A 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
Description
The Allegro(R) 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 A 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. 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 A8732 is available in an 8-contact 2 mm x 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.
Package: 8-pin DFN/MLP (suffix EE)
2 mm x 2 mm, 0.60 mm height Not to scale
Typical Applications
Battery Input 2.3 to 5.5 V VBAT VOUT Detect SW VIN_VDRV C2
Control Block
+ C1 COUT
100F 315 V
Battery Input 1.5 to 5.5 V VBAT
+ C1 COUT
100F 315 V
VOUT Detect SW VIN_VDRV
Control Block
ISW sense VPULLUP
C2
ISW sense VPULLUP
CHARGE
DONE DONE VIN_VDRV
100 k
CHARGE
DONE DONE VIN_VDRV
100 k
TRIG
IGBT Driver IGBT Gate GATE
TRIG
IGBT Driver IGBT Gate GATE
GND
GND
(A) Figure 1. Typical applications: (A) with single battery supply and (B) with separate bias supply 8732-DS
(B)
A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Selection Guide
Part Number
A8732EEETR-T
Packing
3000 pieces per reel
Package
8-contact DFN/MLP with exposed thermal pad
Absolute Maximum Ratings
Characteristic Symbol 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 J at frequency 400 kHz.) DC current, pulse width = 1 ms Care should be taken to limit the current when -0.6 V is applied to these pins. TA TJ(max) Tstg Range E Rating Units
SW Pin
VSW
-0.3 to 50
V
ISW VIN_DRV, VBAT Pins CHARGE, TRIG, Pins DONE Remaining Pins Operating Ambient Temperature Maximum Junction Storage Temperature VIN
3 -0.3 to 6.0 -0.6 to VIN + 0.3 V -0.3 to VIN + 0.3 V -40 to 85 150 -55 to 150
A V V V C C C
THERMAL CHARACTERISTICS may require derating at maximum conditions
Characteristic Package Thermal Resistance Symbol RJA Test Conditions* 4-layer PCB, based on JEDEC standard Value Units 49 C/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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Functional Block Diagram
VBAT
SW VSW - VBAT DCM Detector toff(max)
18 s S Q Q
Control Logic
DMOS
ILIM Reference VDSref VIN_DRV
OCP ton(max)
18 s
H L Triggered Timer
R
Enable
Decoder
S R
Q
DONE
Q
CHARGE
VIN_DRV IGBT Driver
TRIG
GATE
GND
Pin-out Diagram
Terminal List
Number 1 Name DONE TRIG GATE GND SW VBAT VIN_DRV CHARGE PAD Function Open collector output, pulls low when output reaches target value and CHARGE is high. Goes high during charging or whenever CHARGE is low. IGBT trigger input. IGBT gate drive output. Ground connection. Drain connection of internal DMOS switch. Connect to transformer primary winding. Battery voltage. Input voltage. Connect to 3 to 5.5 V bias supply. Decouple VIN voltage with 0.1 F ceramic capacitor placed close to this pin. Charge enable and current limit serial programming pin. Set this pin low to shut down the chip. Exposed pad for enhanced thermal dissipation. Connect to ground plane.
DONE 1 TRIG 2 GATE 3 GND 4 PAD
8 7 6 5
CHARGE VIN_DRV VBAT SW
2 3 4 5 6 7 8 -
(Top View)
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
indicates specifications
Min. 1.5 2.3 - - Shutdown (CHARGE = 0 V, TRIG = 0 V) - - - - - - 1.35 - - - - - - - - - - - - - - - - Charging complete Charging (CHARGE = VIN, TRIG = 0 V) Shutdown (CHARGE = 0 V, TRIG = 0 V) Typ. - - 2.05 150 0.02 50 2 0.01 - - 1.5 100 95 90 86 81 76 71 67 62 57 52 48 43 38 33 29 Max. 5.5 5.5 2.2 - 0.5 100 - 1 5 50 1.65 - - - - - - - - - - - - - - - - Unit V V V mV A A mA A A A A % % % % % % % % % % % % % % % %
ELECTRICAL CHARACTERISTICS Typical values are valid at VIN = VBAT = 3.6 V; TA = 25C, except guaranteed from -40C to 85C ambient, unless otherwise noted
Characteristics VBAT Voltage Range VIN_DRV Voltage Range UVLO Enable Threshold UVLO Hysteresis VIN Supply Current Symbol VBAT VIN VINUV VINUV(hys) IIN (note 1) (note 1) VIN rising Test Conditions
VBAT Pin Supply Current IBAT
Charging done (CHARGE = VIN, = 0 V) DONE Charging (CHARGE = VIN, TRIG = 0 V)
Current Limit Primary-Side Current Limit ISWLIM ISWLIM1 ISWLIM2 ISWLIM3 ISWLIM4 ISWLIM5 ISWLIM6 ISWLIM7 ISWLIM8 Switch Current Limit (ILIM Programming Input on CHARGE Pin) ISWLIM9 ISWLIM10 ISWLIM11 ISWLIM12 ISWLIM13 ISWLIM14 ISWLIM15 ISWLIM16 100% setting (note 2) Default setting One pulse applied to CHARGE pin Two pulses applied to CHARGE pin Three pulses applied to CHARGE pin (note 3) Four pulses applied to CHARGE pin Five pulses applied to CHARGE pin (note 3) Six pulses applied to CHARGE pin (note 3) Seven pulses applied to CHARGE pin (note 3) Eight pulses applied to CHARGE pin Nine pulses applied to CHARGE pin (note 3) Ten pulses applied to CHARGE pin (note 3) Eleven pulses applied to CHARGE pin (note 3) Twelve pulses applied to CHARGE pin (note 3) Thirteen pulses applied to CHARGE pin (note 3) Fourteen pulses applied to CHARGE pin (note 3) Fifteen pulses applied to CHARGE pin (note 3)
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
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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 = 25C, except
specifications guaranteed from -40C to 85C ambient, unless otherwise noted
Switch On-Resistance Switch Leakage Current CHARGE Pull-down Resistance CHARGE Input Voltage Charge Pin Programming ILIM Programming High at CHARGE Pin ILIM Programming Low at CHARGE Pin Total ILIM Setup Time at CHARGE Pin Switch-Off Timeout Switch-On Timeout Output Comparator Trip Voltage Output Comparator Voltage Overdrive Leakage Current DONE Output Low Voltage DONE dV/dt Threshold for ZVS Comparator IGBT Driver VTRIG(H) TRIG Input Voltage VTRIG(L) TRIG Pull-Down Resistor GATE Resistance to VIN_DRV GATE Resistance to GND Propagation Delay (Rising) Propagation Delay (Falling) Output Rise Time Output Fall Time GATE Pull-Down Resistor
1 Specifications
indicates
- 2 - - 0.4 - - - - - - 32 400 1 A k V V s s s s s s V mV A mV V/s
RSWDS(on) ISWLK RCHGPD VCHARGE
VIN_DRV = 3.6 V, ID = 600 mA, TA = 25C VSW = 5.5, over full temperature range (note 1) High, over input supply range (note 1) Low, over input supply range (note 1) Initial Pulse (note 3) Subsequent Pulses (note 3) (note 3) (note 3)
- - - 1.0 - 15 0.2 0.2 - - -
0.4 - 100 - - - - - 200 18 18 31.5 200
tILIM(H)init tILIM(H) tILIM(L) tILIM(SU) toff(max) ton(max) VOUTTRIP VOUTOV IDONELK VDONEL dV/dt
Measured as VSW - VBAT (note 4) Pulse width = 200 ns (90% to 90%) (note 1) 32 A into pin (note 1) DONE Measured at SW pin Input = logic high, over input supply range (note 1) Input = logic low, over input supply range (note 1) VGATE = 1.8 V VGATE = 1.8 V Measurement taken at GATE pin, CL= 6500 pF (notes 3, 5) (notes 3, 5) (notes 3, 5) (notes 3, 5)
31 -
- -
- 20
100 -
1.0 - - - - - - - - -
- - 100 21 27 25 60 290 380 20
- 0.4 - - - - - - - -
V V k ns ns ns ns k
RTRIGPD RSrcDS(on) RSnkDS(on) tDr tDf tr tf RGTPD
throughout the range TA = -40C to 85C 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 = -20C to 85C 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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
IGBT Drive Timing Definition
TRIG
50% 50%
tDr
tr
tDf
tf
90%
90% 10%
GATE
10%
Operation Timing Diagram
VBAT VIN CHARGE SW
Target VOUT UVLO
VOUT DONE
T1 T2 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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Characteristic Performance
IGBT Drive Performance
IGBT drive waveforms are measured at pin, with capacitive load of 6800 pF
Rising Signal
VIN
tr
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
Falling Signal
tf
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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Characteristic Performance
Charge Time versus Battery Voltage at Various ILIM
Transformer LPRIMARY = 12.8 H, N =10.25, VIN =3.6 V, COUT = 100 F / 330 V UCC, at room temperature
20 18 16 14
ILIM13 (0.65A)
ILIM11 (0.79A)
Time (Sec)
12 10 8 6 4
ILIM9 (0.93A)
ILIM7 (1.07A)
ILIM5 (1.22A)
ILIM3 (1.36A)
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 H, N =10.25, VIN =3.6 V, at room temperature
86% 84% 82% 80% 78% 76% 74% 72% 70% 68% 66% 64% 62% 60% 58% 56% 54% 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
ILIM5 (1.22A) ILIM3 (1.36A) ILIM1 (1.50A) ILIM9 (0.93A) ILIM7 (1.07A) ILIM13 (0.65A) ILIM11 (0.79A)
Efficiency (%)
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Final Output Voltage versus Battery Voltage at Various ILIM
Transformer LPRIMARY = 12.8 H, N =10.25, VIN =3.6 V, at room temperature
328 327
ILIM15 (0.51A)
326 325
ILIM13 (0.65A) ILIM11 (0.79A) ILIM9 (0.93A) ILIM7 (1.07A)
VOUT (V)
324 323 322 321 320 1.5 2.0 2.5 3.0 3.5 4.0 Battery Voltage (V) 4.5 5.0 5.5
ILIM5 (1.22A) ILIM3 (1.36A) ILIM1 (1.50A)
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 x LPRIMARY x N) / VOUT .
Final Output Voltage versus Secondary Side Conduction Time at Various Battery Voltages
Transformer LPRIMARY = 12.8 H, N =10.25, VIN =3.6 V, at room temperature
328 327 326
VBAT (V)
325
VOUT (V)
5.0 4.2
324
3.6
323 322 321 320 150 200 250 300 350 400 450 500 550 600 650
toff (ns)
2.2
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Average Input Current versus Battery Voltage at Various ILIM
Transformer LPRIMARY = 12.8 H, N =10.25, VIN =3.6 V, at room temperature
0.7
0.6
ILIM1 (1.50A)
0.5
ILIM3 (1.36A) ILIM5 (1.22A)
Current (A)
0.4
ILIM7 (1.07A) ILIM9 (0.93A) ILIM11 (0.79A)
0.3
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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Charging Waveforms
Output Capacitor Charging at Various Peak Current Limits
Test conditions: VIN = 3.0 V, VBAT = 3.7 V, COUT = 100 F / 330 V UCC, transformer = T-16-024A (LPRIMARY =12.8 H, N = 10.25), at room temperature Oscilloscope settings: Ch1 = (5 V / div), Ch2 = Output Voltage (50 V / div), Ch3 = Input Current (100 mA / div), Time scale = 1 sec / div DONE
VOUT
ILIM1 (1.5 A)
IIN C2,C3 VDONE C1 VOUT
ILIM5 (1.22 A)
IIN C2,C3 C1 VDONE VOUT
ILIM9 (0.93 A)
IIN C2,C3 C1 VDONE VOUT
ILIM11 (0.79 A)
IIN
C2,C3 C1
VDONE t Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Output Capacitor Charging at Various Battery Voltages
Test conditions: VIN = 3.0 V, ILIM3 (1.36 A), COUT = 100 F / 330 V UCC, transformer = T-16-024A (LPRIMARY =12.8 H, N = 10.25), at room temperature Oscilloscope settings: Ch1 = (5 V / div), Ch2 = Battery Voltage (1 V / div), Ch3 = Output Voltage (50 V / div), Ch4 = Input Current (100 mA V / div), DONE 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 C1 VOUT VDONE
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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
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. The open-drain indicator is pulled low when CHARGE DONE 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. 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 s), 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 x N - Vd , 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 ton = ISWlim x 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 C4
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 s, 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.
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 F / 330 V, ILIM = 1.0 A, and t = 1 s/div.
ISW
VSW VBAT C2,C3
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 s / div.
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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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
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.
C4
ISW
C4
ISW
VSW VBAT C2,C3 VOUT C2,C3
VSW VBAT
VOUT C1 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 s / div.
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 s / div.
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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A8732
Ultra Small Mobile Phone 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 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 s. 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 s long. Subsequent pulses (up to 15 more) can be as short as 0.2 s. The last lowto-high edge must arrive within 200 s 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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Applications Information
Transformer Design 1. The transformer turns ratio, N, determines the output voltage: N = NS / NP VOUT = 31.5 x 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 ) x ln (1 - ISWlim x 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 x R, then ton can be approximated by: ton = ISWlim x LPRIMARY /VIN . 3. The secondary inductance, LSECONDARY, determines the offtime of the switch. Given: LSECONDARY / LPRIMARY = N x N , then toff = (ISWlim / N) x LSECONDARY /VOUT = (ISWlim x LPRIMARY x N) /VOUT . 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 H 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.
ton VSW
toff ISW
Vr
tf VIN VSW tneg
VIN ISW
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
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A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
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 s off-time. The resonant period caused by input filter inductor and capacitor should be at least 2 times greater or smaller than the 18 s Timer period, to reduce input ripple current during this period. The typical input LC filter is shown in figure 9. The resonant period is given by: Tres = 2 (L x CIN)1/2 .
VOUT
VBAT
C2 C3
IBAT
C1
The effects of input filter components are shown in figures 10, 11, and 12. It is recommended to use at least 10 F / 6.3 V to decouple the battery input, VBAT , at the primary of the transformer. Decouple the VIN pin using 0.1 F / 6.3 V bypass capacitor. Output Diode Selection 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 x VBAT . The peak current of the rectifying diode, IDPeak, is calculated as: IDPeak = IPRIMARY_Peak / N .
Figure 10. Input current waveforms with Li+ battery connected by 5-in. wire and decoupled by 4.7 F capacitor, COUT = 100 F, 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 C3
IBAT
C1
Figure 11. Input current waveforms with Li+ battery connected through 4.7 H inductor and 4.7 F capacitor, COUT = 100 F, VIN = VBAT = 3.6 V, Ch1: VOUT = 50 V/div, Ch2: VBAT = 2 V/div, Ch3: IBAT = 200 mA/div, t = 1 s/div
VOUT
LIN +
C2
VBAT IBAT
VBAT
CIN
A8732
C3
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 H inductor and 10 F capacitor, COUT = 100 F, 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
A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
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 and SW pin. It is important that the signal trace and other DONE 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 C1, Input Capacitor C2 COUT, Photoflash Capacitor D1, Output Diode T1, Transformer Rating 10 F, 10%, 6.3 V, X5R ceramic capacitor (0805) 0.1uF, 6.3V X5R ceramic capacitor 100 F / 330 V 2 x 250 V, 225 mA, 5 pF LPRIMARY = 12.8 H, N= 10.25, 6.5 x 8 x 4 mm EPH-31ELL101B131S BAV23S T-16-024A Chemi-Con Philips Semiconductor, Fairchild Semiconductor Tokyo Coil Electric Part Number JMK212BJ106K Source Taiyo Yuden
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
18
A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Recommended layout:
VBAT
3
D1
1
Vout
2
BAV23S C2 C4
2
X2
4
10uF TCE_T-16-024A
1 3
Cout1 100uF
Schematic
DONE CHARGE
0.1uF R10 2k
TP_Gate
6 VBAT 7 VIN_DRV 1 8 CHARGE GND DONE GATE TRIG 2 4
A8732
5 SW 3
C5 1nF
C6 1nF
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
A8732
Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver
Package EE 8-Contact DFN/MLP with Exposed Thermal Pad
0.30 2.00 0.15 8 0.83 8 0.50
2.00 0.15 A 1 2 1 1.60
0.90
2.13
9X
D 0.08 C 0.25 0.05 0.50 BSC
SEATING PLANE +0.05 0.55 -0.04
C C
PCB Layout Reference View
1
2
0.325 0.050 0.90 B
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 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
8
1.60
Copyright (c)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

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