ALT80800KLPATR datasheet(8/23 Pages) ALLEGRO

8 / 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

Switching Frequency

The ALT80800 operates in fixed on-time mode during switching.

The on-time (and hence switching frequency) is programmed

using an external resistor connected between the TON pin and

ground, as given by the following equation:

tON = k × (RON + RINT) × ( VOUT / VIN )

fSW = 1 / [ k × (RON + RINT)]

where k = 0.0127, with fSW in MHz, tON in µs, and RON and RINT

(internal resistance, 3 kΩ) in kΩ.

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

100

200

300

400

500

600

700

800

900

1000

RON (kΩ)

2.4

Figure 6: Switching Frequency vs. TON resistance

To minimize the peaks of switching frequency harmonics in EMC

measurement, a dithering feature is implemented. The dithering

range is internally set at ±5%. The actual switching frequency is

swept linearly between 0.95 × fSW and 1.05 × fSW, where fSW is

the programmed switching frequency. The rate of modulation for

fSW is fixed internally at 12.5 kHz.

ENABLE AND DIMMING

The ALT80800 is activated when a logic high signal is applied to

the EN (enable) pin and VIN = VVCCIN is above UVLO threshold

4.3 V. The buck converter ramps up the LED current to a target

level set by RSENSE when PWM pin = High.

The EN pin is high-voltage tolerant and can be directly connected

to a power supply. However, if VEN is higher than the VIN voltage

at any time, a series resistor (10 kΩ) is required to limit the current

flowing into the EN pin. This resistor is helpful in preventing EN

from damage in case of reverse-battery connection. This series

resistor is not necessary if EN is driven from a logic input.

The PWM pin is a logic input pin and is internally pulled up to

VCC through a resistor.

EN pin and PWM pin function as illustrated below:

EN pin

PWM pin

VCC

LED

High

Low

OFF

High

High/Open

Low

Shutdown

When the EN pin is forced from high to low, the LED current is

turned off, but the IC remains in standby mode for up to at least

10 ms. If EN goes high again within this period, the LED current

is turned on immediately if PWM pin is high. If EN pin is low for

more than tOFFDelay, the IC enters shutdown mode to reduce power

consumption. The next high signal on EN will initialize a full

startup sequence, which includes a startup delay of approximately

150 μs. This startup delay is not present during PWM operation.

Active dimming of the LED is achieved with 2 options: by send-

ing a PWM (pulse-width modulation) signal to the EN pin (while

PWM = High), or by sending a dimming PWM signal to the

PWM pin (while EN is enabled) as illustrated in the table above.

The resulting LED brightness is proportional to the duty cycle of

the applied PWM signal. A practical range for PWM dimming

frequency is between 100 Hz (period = 10 ms) and 2 kHz.

If the PWM dimming signal at PWM pin is low when the EN pin

is high, the LED will be off immediately and IC is alive waiting

for next PWM pulse. The internal LDO is still on and can provide

bias to the internal and external circuits.

In PWM dimming operation and when VIN is above 40 V, a

10 kΩ resistor is needed to be in parallel with a 0.047 µF output

capacitor across the LED string to facilitate BOOT charging dur-

ing PWM dimming OFF period.

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