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A8740

Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8740

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 low-

to-high transition on the CHARGE pin is required to start the

charging. The ¯D¯¯¯O¯¯¯N¯¯E¯ open-drain indicator is pulled low when

CHARGE is high and target output voltage is reached.

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 A8740 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:

Timer Mode and Fast Charging Mode

The A8740 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 com-

pletely 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 time-

expanded view of a Timer mode interval is shown in figure 3.

VOUT

IIN

Where:

VOUT = K × N – Vd ,

K = 31.5 V typically,

Figure 2. Timer mode and Fast Charging mode: t = 1 s/div;

VOUT = 50 V/div; IIN = 150 mA/div., VIN = VBAT = 3.6 V;

COUT = 100 μF / 330 V; and ILIM = 1.0 A.

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 A8740 implements an adaptive on-time/off-time control. On-

time duration, ton , is approximately equal to

ton = ISWlim × LPRIMARY / VBAT .

Off-time duration, toff , depends on the operating conditions

during switch off-time. The A8740 applies two charging modes:

Fast Charging mode and Timer mode, according to the conditions

described in the next section.

ISW

C4

C2,C3

VSW

VBAT

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 = 750 mA / div., t = 5 μs / div.

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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