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A8438

Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8438

Photoflash Capacitor Charger with IGBT Driver

Functional Description

Overview

The A8438 is a photoflash capacitor charger control IC with

adjustable input current limiting. It also integrates an IGBT

driver for strobe operation of the flash tube, dramatically saving

board space in comparison to discrete solutions for strobe flash

operation. The control logic is shown in the functional block

diagram.

The charging operation of the A8438 is started by a low-to-high

signal on the CHARGE pin, provided that VIN is above VUVLO

level. If CHARGE is already high before VIN reaches VUVLO ,

another low-to-high transition on the CHARGE pin is required

to start the charging. 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,

VBATT , and the primary side inductance, LPrimary. When IPrimary

reaches the current limit, ISWLIM , set by configuring the ILIM

pin, 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.

While the internal MOSFET switch is turned off, the output

voltage, VOUT, is sensed by a resistor string, R1 through R3, con-

nected between the anode of the output diode, D1, and ground.

This resistor string forms a voltage divider that feeds back to the

FB pin. The resistors must be sized to achieve a desired output

voltage level based on a typical value of 1.205 V at the FB pin.

As soon as VOUT reaches the desired value, the charging process

is terminated. The user may toggle the CHARGE pin to refresh

the photoflash capacitor.

Fast Charging and Timer Modes

The IC operates in the Fast Charging mode when the photoflash

capacitor, COUT, is only partially discharged. In Fast Charging

mode, the converter operates near the discontinuous bound-

ary, and a sensing circuit tracks the fly-back voltage at the SW

node. As soon as this voltage swings below 1.2 V, the internal

MOSFET switch is turned on again, starting the next charging

cycle.

The IC operates in the Timer mode when beginning to charge a

completely discharged photoflash capacitor, usually when the out-

put voltage, VOUT, is less than approximately 10 to 20 V. Timer

mode is a fixed 18 μs off-time control. One advantage of the

A8438 watchdog timer control scheme is that it limits the initial

current surge and thus acts as a “soft-start.” As shown in figure 3,

the timer mode only lasts a small fraction of a second (usually <

100 ms). It can be recognized by its lower initial input charging

current as a result of a lower duty cycle. As output voltage rises to

more than 10 to 20 V, the adaptive Fast Charging mode takes over

the control, raising the average input current level.

Timer

Mode

Fast Charging Mode

Switch On-Time and Off-Time Control

The A8438 implements an adaptive on-time/off-time control. (For

circuit details, please refer to the the Control Logic block in the

simplified Functional Block Diagram on page 2.) On-time dura-

tion, tON , is determined by input voltage, VIN, transformer primary

inductance, LPrimary, and the set current limit, ISWLIM . Off-time

duration, tOFF , depends on the operating conditions during switch

off-time. The A8438 applies its two charging modes, Fast Charg-

ing mode and Timer mode, according to those conditions.

IBATT(Avg)

VOUT

Figure 3. Sequencing of Timer mode and Fast Charging mode (time

axis scale is 1 s per division)

Allegro MicroSystems, LLC

11

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

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