High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

8

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Detailed Description

The MAX16821A/MAX16821B/MAX16821C are high-per-

formance average current-mode PWM controllers for

high-power and high-brightness LEDs (HBLEDs). The

average current-mode control technique offers inherently

stable operation, reduces component derating and size

by accurately controlling the inductor current. The

devices achieve high efficiency at high currents (up to

30A) with a minimum number of external components. A

logic input (MODE) allows the LED driver to switch

between buck and boost modes of operation.

The MAX16821A/MAX16821B/MAX16821C feature a

CLKOUT output 180° out-of-phase with respect to either

the high-side or low-side driver, depending on MODE’s

logic level. CLKOUT provides the drive for a second

out-of-phase LED driver for applications requiring

reduced input capacitor ripple current while operating

another LED driver.

The MAX16821A/MAX16821B/MAX16821C consist of

an inner average current regulation loop controlled by

an outer loop. The combined action of the inner current

loop and outer voltage loop corrects the LED current

errors by adjusting the inductor current resulting in a

tightly regulated LED current. The differential amplifier

(SENSE+ and SENSE- inputs) senses the LED current

using a resistor in series with the LEDs and produces

an amplified version of the sense voltage at DIFF. The

resulting amplified sensed voltage is compared against

an internal 0.6V reference at the error amplifier input.

Input Voltage

The MAX16821A/MAX16821B/MAX16821C operate

with a 4.75V to 5.5V input supply range when the inter-

28V input supply range when the internal LDO is

enabled. For a 7V to 28V input voltage range, the inter-

nal LDO provides a regulated 5V output with 60mA of

and 0.1µF low-ESR ceramic capacitors.

provides supply voltage for the low-side and the high-

R-C filter to isolate the analog circuits from the MOSFET

drivers. The internal LDO powers up the MAX16821A/

MAX16821B/MAX16821C. For applications utilizing a

5V input voltage, disable the internal LDO by connect-

in the 4.75V to 5.5V range of for proper operation of the

MAX16821A/MAX16821B/MAX16821C.

Undervoltage Lockout (UVLO)

The MAX16821A/MAX16821B/MAX16821C include

UVLO and a 2048 clock-cycle power-on-reset circuit.

The UVLO rising threshold is set to 4.3V with 200mV

hysteresis. Hysteresis at UVLO eliminates chattering

during startup. Most of the internal circuitry, including

the oscillator, turns on when the input voltage reaches

4V. The MAX16821A/MAX16821B/MAX16821C draw up

to 3.5mA of quiescent current before the input voltage

reaches the UVLO threshold.

Soft-Start

The MAX16821A/MAX16821B/MAX16821C include an

internal soft-start for a glitch-free rise of the output volt-

age. After 2048 power-on-reset clock cycles, a 0.6V

reference voltage connected to the positive input of the

internal error amplifier ramps up to its final value after

1024 clock cycles. Soft-start reduces inrush current

and stress on system components. During soft-start,

the LED current will ramp monotonically towards its

final value.

Internal Oscillator

The internal oscillator generates a clock with the fre-

the

Typical Operating Circuit). The oscillator frequency

is adjustable from 125kHz to 1.5MHz range using a sin-

gle resistor connected from RT/SYNC to SGND. The

frequency accuracy avoids the overdesign, size, and

cost of passive filter components like inductors and

capacitors. Use the following equation to calculate the

oscillator frequency:

the PWM comparator and a 180° out-of-phase clock

signal at CLKOUT to drive a second out-of-phase LED

current regulator.

f

x

R

Hz

SW

T

.

()

=

640

10

10

f

x

R

Hz

SW

T

.

()

=

625

10

10