ALT80800KLPATR datasheet(7/23 Pages) ALLEGRO

7 / 23 page

Automotive-Grade, Constant-Current 2.0 A

PWM Dimmable Synchronous Buck LED Driver

ALT80800

Allegro MicroSystems, LLC

955 Perimeter Road

Manchester, NH 03103-3353 U.S.A.

www.allegromicro.com

The ALT80800 is a synchronous buck regulator designed for

driving a high-current LED string. It uses average current mode

control to maintain constant LED current and consistent bright-

ness. The LED current level is easily programmable by selection

of an external sense resistor, with a value determined as follows:

iLED = VCSREG / RSENSE

where VCSREG = VCSH – VCSL = 0.2 V typical.

If necessary, a resistor can be inserted in series with the CSL pin

to fine-tune the LED current, as shown below:

CSH

CSL Radj

RSENSE

VCSREG

VSENSE

–

iLED

iCSH

iCSL

–

iCSL × Radj

VCSREG = iLED × RSENSE + iCSL × Radj

Therefore

iLED = (VCSREG – iCSL × Radj) / RSENSE

Figure 3: How To Fine-Tune LED Current Using Radj

For example, with a desired LED current of 1.4 A, the required

RSENSE = 0.2 V / 1.4 A = 0.143 Ω. But the closest power resistor

available is 0.13 Ω. Therefore, the difference is

Radj × iCSL = 0.2 V – 1.4 A × 0.13 Ω = 0.018 V

where iCSL = 75 µA typical

Radj = 0.018 V / 75 µA = 240 Ω

The LED current is further modulated by the ADIM (Analog

Dimming) pin voltage. This feature can be used for LED bright-

ness calibration, or for thermal foldback protection. See Analog

Dimming section for details.

Synchronous Regulation

The ALT80800 integrates an N-channel DMOS as the low-side

switch to implement synchronous regulation for LED drivers, as

shown in Figure 4.

CBOOT

BOOT

GND

VIN

Floating

Gate Driver

Integrated

Switch

i_L

Rsc

CLED

VOUT

Boot

Charger

ALT80800

Figure 4: Synchronous Buck LED Driver

The Synchronous configuration can effectively pull down SW

to ground by forcing the low-side synchronous switch on even

with small inductor current, as shown in Figure 5. Therefore, the

BOOT capacitor can be charged normally every switch cycle to

ensure the normal operation of buck LED drivers.

Figure 5: Normal SW waveform with SR configuration

when VIN ≈ VOUT: VIN = 5.4 V, VOUT = 5.14 V

(2 white LEDs)

FUNCTIONAL DESCRIPTION

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