RL1632R-R150-F datasheet(8/17 Pages) ALLEGRO

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Constant-Current 3-Ampere PWM Dimmable

Buck Regulator LED Driver

A6211

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

The A6211 is a buck regulator designed for driving a high-current

LED string. It utilizes average current mode control to maintain

constant LED current and consistent brightness. 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 = 0.2 V typical.

Switching Frequency

The A6211 operates in fixed on-time mode during switching. The

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

an external resistor connected between the VIN and TON pins, as

given by the following equation:

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

fSW = 1 / [ k × (RON + RINT )]

where k = 0.0139, with fSW in MHz, tON in μs, and RON and RINT

(internal resistance, 5 kΩ) in kΩ (see figure 6).

Enable and Dimming

The IC is activated when a logic high signal is applied to the EN

(enable) pin. The buck converter ramps up the LED current to a

target level set by RSENSE.

When the EN pin is forced from high to low, the buck converter

is turned off, but the IC remains in standby mode for up to 10 ms.

If EN goes high again within this period, the LED current is

turned on immediately. Active dimming of the LED is achieved

by sending a PWM (pulse-width modulation) signal to the EN

pin. The resulting LED brightness is proportional to the duty cycle

(TON/Period) of the PWM signal. A practical range for PWM

dimming frequency is between 100 Hz ( Period = 10 ms) and

2 kHz. At a 200 Hz PWM frequency, the dimming duty cycle can

be varied from 100% down to 1% or lower.

If EN is low for more than 17 ms, 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 130 μs. This startup delay is not present during

PWM operation.

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

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

Functional Description

Figure 6. Switching Frequency versus RTON Resistance

Figure 7. Simplified buck controller equations, and reference circuit and waveforms

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

80 100 120 140 160 180 200 220 240 260

RTON (kΩ)

• During SW on-time:

iRIPPLE = [(VIN – VOUT) / L] × tON = [(VIN – VOUT) / L] × T × D

iRIPPLE = [(VOUT – VD) / L] × tOFF = [(VOUT – VD) / L] × T × (1 – D)

VOUT = VIN × D – VD × (1 – D)

VOUT = (VIN – Iav × RDS(on)) × D – VD × (1 – D) – RL × Iav

where D = tON / T.

where RL is the resistance of the inductor.

• During SW off-time:

Therefore (simplified equation for Output Voltage):

More precisely:

If VD << VOUT, then VOUT ≈ VIN × D.

VSW

VIN

i(max)

iav

i(min)

iRIPPLE

tON

tOFF

–VD

CIN

VIN

A6211

VOUT

RSENSE

MOS

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