High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

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11

Current-Error Amplifier

The MAX16821A/MAX16821B/MAX16821C include a

transconductance current-error amplifier with a typical

bility. The current-error amplifier output (CLP) is con-

nected to the inverting input of the PWM comparator.

CLP is also externally accessible to provide frequency

compensation for the inner current regulation loop

(Figure 2). Compensate CEA so the inductor current

negative slope, which becomes the positive slope to

the inverting input of the PWM comparator, is less than

the slope of the internally generated voltage ramp (see

the

Compensation section). In applications without syn-

chronous rectification, the LED driver can be turned off

and on instantaneously by shorting or opening the CLP

to ground.

PWM Comparator and R-S Flip-Flop

An internal PWM comparator sets the duty cycle by

comparing the output of the current-error amplifier to a

R-S flip-flop resets and the high-side driver (DH) turns

on if MODE is connected to SGND, and DL turns on if

flip-flop as soon as the ramp signal exceeds the CLP

voltage, thus terminating the ON cycle. See Figure 3.

Differential Amplifier

The differential amplifier (DIFF) allows LED current sens-

ing (Figure 2). It provides true-differential LED current

sensing, and amplifies the sense voltage by a factor of 1

(MAX16821A), 6 (MAX16821B), and 20 (MAX16821C),

while rejecting common-mode voltage errors. The VEA

provides the difference between the differential amplifier

output (DIFF) and the desired LED current-sense volt-

age. The differential amplifier has a bandwidth of 1.7MHz

(MAX16821A), 1.6MHz (MAX16821B), and 550kHz

(MAX16821C). The difference between SENSE+ and

SENSE- is regulated to +0.6V (MAX16821A), +0.1V

(MAX16821B), or +0.03V (MAX16821C).

PWM

COMPARATOR

CLK

SHDN

RAMP

IN

CSP

CLP

CSN

MODE = GND

60mV

SQ

RQ

PEAK-CURRENT

COMPARATOR

BST

DH

LX

DL

PGND

Figure 3. MAX16821A/MAX16821B/MAX16821C Phase Circuit