A8437 Просмотр технического описания (PDF) - Allegro MicroSystems

Номер в каталоге

Компоненты Описание

Список матч

A8437

Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8437

Mobile Phone Xenon Photoflash Capacitor Charger

with IGBT Driver

by selecting the value of an external resistor RSET,

connected from the ISET pin to GND, which deter-

mines the ISET bias current, and therefore the switch-

ing current limit, ISWlim.

To the first order approximation, ISWlim is related to

ISET and RSET according to the following equations:

ISWlim = ISET × K = VSET / RSET × K , (3)

where K = 28000 when battery voltage is 3.6 V.

In real applications, the actual switching current

limit is affected by input battery voltage, and also the

transformer primary inductance, Lp. If necessary, the

following expressions can be used to determine ISWlim

more accurately:

ISET = VSET / (RSET + RSET(INT) – K × RGND(INT) ), (4)

where:

RSET(INT) is the internal resistance of the ISET pin

(1 kΩ typical),

RGND(INT) is the internal resistance of the bonding

wire for the GND pin (27 mΩ typical), and

K = (K′ + VIN × K″), with K′ = 24350 and K″ ≈

1040 at TA = 25°C. Then,

ISWlim = ISET × K + VBAT / LP × tD ,

(5)

where tD is the delay in SW turn-off (0.1 μs typical).

The chart at the bottom of the page can be used to

determine the relationship between RSET and ISWlim at

various battery voltages.

Peak Current Limit versus ISET Resistance

VIN = VBAT, XFM Lp = 8 μH, TA=25°C

1.3

1.2

1.1

VIN = 5.5 V

1.0

VIN = 4.5 V

VIN = 3.6 V

0.9

VIN = 3.0 V

0.8

VIN = 2.3 V

0.7

0.6

0.5

0.4

25 30 35 40 45 50 55 60 65 70 75 80 85 90

RSET (kΩ)

Allegro MicroSystems, Inc.

10

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Share Link: