A8521 datasheet(21/35 Pages) ALLEGRO

21 / 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

Figure 25. Normal operation of the switch node (SW); inductor current

(IL) and output voltage (VOUT) for 9 series LEDs in each of four strings

configuration; shows SW node (ch1, 20 V/div.), inductor current IL (ch2,

1 A/div.), VOUT (ch3, 10 V/div.), and PWM/EN (ch4, 5 V/div.),

time = 2 μs/div.

Figure 26. Cycle-by-cycle current limiting; inductor current (yellow trace, IL),

note reduction in output voltage as compared to normal operation with the

same configuration (figure 26); shows SW node (ch1, 20 V/div.), inductor

current IL (ch2, 1 A/div.), VOUT (ch3, 10 V/div.), and PWM/EN (ch4, 5 V/div.),

time = 2 μs/div.

Figure 27. Secondary boost switch current limit; when this limit is hit, the

A8521 immediately shuts down; shows PWM (ch1, 5 V/div.), VOUT (ch2,

5 V/div.), SW node (ch3, 50 V/div.), and inductor current IL (ch4, 2 A/div.),

time = 100 ns/div.

VOUT

PWM/EN

SW node

VOUT

PWM/EN

SW node

PWM/EN

SW node

FAULT

Boost switch overcurrent protection

The boost switch is protected with cycle-by-cycle current limiting

set at a minimum of 3.0 A. There is also a secondary current limit

that is sensed on the boost switch. When detected this current

limit immediately shuts down the A8521. The level of this cur-

rent limit is set above the cycle-by-cycle current limit to protect

the switch from destructive currents when the boost inductor is

shorted. Various boost switch overcurrent conditions are shown in

figures 25 through 27.

A8521 Datasheet(PDF) 21 Page - Allegro MicroSystems