A8438 Ver la hoja de datos (PDF) - Allegro MicroSystems

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Fabricante

A8438

Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8438

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).

When the A8438 charges a fully-discharged photoflash capacitor,

ISecondary decreases very slowly due to the low initial VOUT. The

A8438 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 .

Input Current Limiting

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

configuring the ILIM pin:

ISWLIM Setting

(A)

1.6

1.8

2.0

ILIM Pin

Connection

External ground

Float

Pull up to IC supply voltage

with a 1 to 10 k resistor

Lower input current offers the advantage of a longer battery life-

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

limit. ISWLIM may be adjusted during charging.

Allegro MicroSystems, LLC

12

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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