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A8439EEJTR-T

Photoflash Capacitor Charger with IGBT Driver and Refresh

Allegro MicroSystems 

A8439

Photoflash Capacitor Charger with IGBT Driver and Refresh

Functional Description

Overview

Auto-Refresh

The A8439 is a photoflash capacitor charger control IC with

adjustable input current limiting and automatic refresh. It also

integrates an IGBT driver for strobe operation of the flash tube,

dramatically saving board space in comparison to discrete solu-

tions for strobe flash operation. The control logic is shown in the

functional block diagram.

The charging operation of the A8439 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. The primary peak current is set by input

clock signals from the CHARGE pin. 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 , 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

recharging cycle starts again, either after the transformer flux

is reset, or after a predetermined time period, tOFF(Max) (18 μs),

whichever occurs first.

The A8439 features auto-refresh when the feedback resistor

network is connected at the output. Auto-refresh initiates when

the output voltage drops to ≈90 % of the set stop voltage of the

resistor network. The operation is shown in figure 3.

Input Current Limiting

The peak current limit can be adjusted to eight different levels,

from 270 mA to 1.4 A, by clocking the CHARGE pin. An internal

digital circuit decodes the input clock signals to a counter, which

sets the charging time. This flexible scheme allows the user

to operate the flash circuit according to different battery input

voltages. The battery life can be effectively extended by setting a

lower current limit at low battery voltages.

Figure 4 shows the ILIM clock timing scheme protocol. The

total ILIM setup time, tILIM(SU) , denotes the time needed for the

The output voltage, VOUT, is sensed by a resistor string, R1, R2 ,

and R3 (see application circuit diagrams), connected between

the positive terminal of the output capacitor 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 volt-

age 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 A8439 automatically starts a new charging cycle

when the internal voltage sensing circuit detects a 10 % drop in

the output voltage. Toggling the CHARGE pin can also start a

refresh operation.

Figure 3. Auto-refresh waveform of A8439.

Allegro MicroSystems, Inc.

9

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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