A8436EEJTR-T Просмотр технического описания (PDF) - Allegro MicroSystems

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A8436EEJTR-T

Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8436

Photoflash Capacitor Charger with IGBT Driver

To understand the Timer mode, it is noted that the secondary

winding charge current, ISecondary, decreases linearly at a rate of:

dISecondary

dt

=

VOUT

LPrimary N 2

(1)

where:

ISecondary is the secondary side current,

LPrimary is the primary side inductance, and

N is the transformer turns ratio (NSecondary / NPrimary).

As shown in the three panels of figure 4, when the A8436 charges

a fully-discharged photoflash capacitor, ISecondary decreases very

slowly due to the low initial VOUT. The A8436 internal timer

(Timer mode) sets a maximum timeframe of 18 μs for the off-

time as long as the SW node voltage is greater than 1.2 V. When

the off-time passes 18 μs, the internal MOSFET switch is turned

on, initiating the next charging cycle .

Lower input current offers the advantage of a longer battery

lifetime. For faster charging time, however, use the highest

current limit.

ISWLIM may be adjusted during charging. Figure 5 shows a

waveform of ISWLIM being adjusted during charging. ISWLIM is

lowered from 1.4 to 1.2 A, and charging slows slightly at the

lower current level.

ISWLIM = 1.4 A

ISWLIM = 1.2 A

IIN(Avg)

Input Current Limiting

The peak input current, ISWLIM , can be set to three levels by

configuring the ILIM pin:

VOUT

ISWLIM Setting

(A)

1.0

1.2

1.4

ILIM Pin

Connection

External ground

Float

Pull up to IC supply voltage

with a 1 to 10 kΩ resistor

Figure 5. Reducing ISWLIM during charging

Allegro MicroSystems, Inc.

11

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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