A8732EEETR-T

A8732 Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver Features and Benefits ▪ 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 Description 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. 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 × 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 × 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 100F 315 V COUT Battery Input 1.5 to 5.5 V VBAT + C1 100F 315 V COUT VOUT Detect SW VIN_VDRV Control Block ISW sense VPULLUP C2 ISW sense VPULLUP CHARGE DONE DONE VIN_VDRV 100 kΩ CHARGE DONE DONE VIN_VDRV 100 kΩ 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 μJ 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 ºC ºC ºC THERMAL CHARACTERISTICS may require derating at maximum conditions Characteristic Package Thermal Resistance Symbol 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 A8732 Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver Functional Block Diagram VBAT SW VSW – VBAT DCM Detector toff(max) 18 μs S Q Q Control Logic DMOS ILIM Reference VDSref VIN_DRV OCP ton(max) 18 μs 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 μF 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 3 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 μA μA mA μA μA μA A % % % % % % % % % % % % % % % % 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 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 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 specifications guaranteed from −40°C to 85°C 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 Ω μA kΩ V V μs μs μs μs μs μs V mV μA mV V/μs RSWDS(on) ISWLK RCHGPD VCHARGE VIN_DRV = 3.6 V, ID = 600 mA, TA = 25°C 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 μA 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 kΩ Ω Ω ns ns ns ns kΩ RTRIGPD RSrcDS(on) RSnkDS(on) tDr tDf tr tf RGTPD 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 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 6 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 7 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 μH, N =10.25, VIN =3.6 V, COUT = 100 μF / 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 μH, 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 8 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 μH, 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 × 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) 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 9 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 μH, 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 10 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 μF / 330 V UCC, transformer = T-16-024A (LPRIMARY =12.8 μH, 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 11 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 μF / 330 V UCC, transformer = T-16-024A (LPRIMARY =12.8 μH, 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 12 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 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 , 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 × 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 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. 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. 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 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 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 μs / 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 μs / div. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 14 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 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 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 × 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 . 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. 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 16 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 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. The resonant period is given by: Tres = 2 (L × 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 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 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: IDPeak = IPRIMARY_Peak / N . 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 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 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 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 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 (