A8425 データシートの表示（PDF） - Allegro MicroSystems

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A8425

High Current Photoflash Capacitor Charger with IGBT Driver for Two Li+ Batteries

Allegro MicroSystems 

A8425

High Current Photoflash Capacitor Charger

with IGBT Driver for Two Li+ Batteries

Application Information

Circuit Description

The A8425 is a photoflash capacitor charger control

IC with a high current limit (up to 3.2 A) and low

RDS(on) (0.23 Ω maximum). The IC also integrates an

IGBT driver for strobe operation of the flash, dramati-

cally saving board space in comparison with discrete

solutions for strobe flash operation.

The IC is turned on by a low-to-high signal on the

CHARGE pin. When the charging cycle is initiated,

the primary current ramps up linearly at a rate deter-

mined by the battery voltage and the primary side

inductance. When the primary current reaches the set

limit, the internal MOSFET is turned off immediately

to allow the energy to be dumped into the photoflash

capacitor through the secondary winding. The sec-

ondary current drops linearly as the output capacitor

is charged. The charging cycle starts again when the

transformer flux is reset or after a predetermined time

period (18 μs maximum off-time) has passed, which-

ever occurs first.

Timer Mode and Fast Charging Mode

The A8425 achieves fast charging times and high effi-

ciency 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 1.

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 40 V (actual value depends on input

voltage and transformer inductance). 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 time

expanded view of a Timer Mode interval is shown in

figure 2.

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

than previously possible, thereby reducing the overall

VOUT

Timer Mode

VSW

Fast Charging Mode

VOUT

VBAT

ISW

ISW

Figure 1. Relationship of Timer mode and Fast Charging mode

t = 2 μs/div; VOUT =10 V/div; VBAT =2 V/div.; VSW =2 V/div;

ISW =200 mA/div. VIN = 3.6 V; VBAT =5.5 V; RSET = 66.5 kΩ;

Transformer LP= 7.5 μH, N = 10

Figure 2. Timer Mode

Allegro MicroSystems, Inc.

10

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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