A8437ECGLT datasheet(9/22 Pages) ALLEGRO

9 / 22 page

Mobile Phone Xenon Photoflash Capacitor Charger

with IGBT Driver

A8437

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Charge Time versus Battery Voltage

Transformer Lp= 8 μH, N = 10.2; COUT= 20 μF / 330 V UCC; TA=25°

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

VBAT (V)

≈ 0.65

≈ 0.8

≈ 0.9

33.2

≈ 1.0

26.7

≈ 1.2

RSET

(kΩ)

(A)

COUT= 20 μF. For larger or smaller capacitances, charging time

scales proportionally.

Efficiency versus Battery Voltage

2.02.5

3.03.5

4.04.5

5.0

5.5

6.0

VBAT (V)

Transformer Lp= 8 μH, N = 10.2; COUT= 20 μF / 330 V UCC; TA=25°

≈ 0.65

≈ 0.8

≈ 0.9

33.2

≈ 1.0

26.7

≈ 1.2

RSET

(kΩ)

(A)

Special low-profile transformer with relatively low inductance

(Lp= 8 μH) and high winding resistance (Rp = 0.37 Ω). Higher efficien-

cy can be achieved by using transformers with higher Lp, which reduces

switching frequency and therefore switching loses, and lower resistance,

which reduces conduction losses.

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

VBAT (V)

Average Input Current versus Battery Voltage

XFM Lp= 8 μH, N = 10.2, COUT= 20 μF 330 V UCC, TA=25°

≈ 0.65

RSET

(kΩ)

(A)

≈ 0.8

≈ 0.9

33.2

≈ 1.0

26.7

≈ 1.2

An increase in ISWlim with respect to VBAT actually keeps the average input current

roughly constant throughout the battery voltage range. Normally, if ISWlim is kept

constant, the average current will drop as VBAT goes higher.

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