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, Inc. 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. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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… Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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) Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 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 Allegro MicroSystems, Inc. 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. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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, Inc. 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, Inc. 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, Inc. 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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, Inc. 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 (<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 20 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 19 Ultra-Small Mobile Adjustable Xenon Photoflash Capacitor Charger with IGBT Driver A8739 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 20k TP_Gate 6 A8739 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 20 Ultra-Small Mobile Adjustable Xenon Photoflash Capacitor Charger with IGBT Driver A8739 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 ©2009-2010, 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 21