A8521 datasheet(29/35 Pages) ALLEGRO

29 / 35 page

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

A8521

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Step 9

Choosing the input disconnect switch components. Set

the input disconnect 1X current limit to 3 A by choosing a sense

resistor. The calculated maximum value of the sense resistor is:

RSC(max) = VSENSEtrip / 3.0 (A)

(28)

= 0.104 (V) / 3.0 (A) = 0.035 Ω

The RSC chosen is 0.033 Ω, a standard.

The trip point voltage must be:

VADJ = 3.0 (A) × 0.033 (Ω) = 0.099 (V)

RADJ = (VSENSEtrip – VADJ ) / IADJ (typ)

(29)

RADJ = (0.104 (V) – 0.099 (V)) / 20.3 (μA) = 246.31 Ω

A value of 249 Ω was chosen for this design.

GATE

CVDD

OVP

VOUT

ROVP

COUT

RSC

RADJ

VSENSE

VIN

VDD

PWM/EN

APWM

ISET

FSET/SYNC

AGND

PGND

COMP

FAULT

PAD

A8521

150 Ω

10 μH

2 A / 60 V

137 kΩ

0.033 Ω

249 Ω

100 kΩ

RISET

11 kΩ

RFSET

10 kΩ

4.7 μF

CIN

4.7 μF

22 nF

20 Ω

0.1 μF

0.47 μF

120 pF

VIN

10 to 14 V

LED4

LED1

10 LEDs each string

LED2

LED3

Figure 33. The schematic diagram showing calculated values from the design example above.

A8521 Datasheet(PDF) 29 Page - Allegro MicroSystems

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