MAX16821 datasheet(10/24 Pages) MAXIM

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Once the MAX16821A–MAX16821C are synchronized to

an external clock, the external clock cannot be removed if

reliable operation is to be maintained.

Control Loop

The MAX16821A–MAX16821C use an average current-

mode control scheme to regulate the output current

(Figure 2). The main control loop consists of an inner

current regulation loop for controlling the inductor current

and an outer current regulation loop for regulating the

LED current. The inner current regulation loop absorbs

the double pole of the inductor and output capacitor com-

bination reducing the order of the outer current regulation

loop to that of a single-pole system. The inner current

regulation loop consists of a current-sense resistor (RS),

a current-sense amplifier (CSA), a current-error amplifier

(CEA), an oscillator providing the carrier ramp, and a

PWM comparator (CPWM) (Figure 2). The MAX16821A–

MAX16821C outer LED-current control loop consists of

a differential amplifier (DIFF), a reference voltage, and a

voltage-error amplifier (VEA).

Inductor Current-Sense Amplifier

The differential current-sense amplifier (CSA) provides

a 34.5V/V DC gain. The typical input offset voltage of

the current-sense amplifier is 0.1mV with a 0 to 5.5V

common-mode voltage range (VIN = 7V to 28V). The cur-

rent-sense amplifier senses the voltage across RS. The

maximum common-mode voltage is 3.2V when VIN = 5V.

Inductor Peak-Current Comparator

The peak-current comparator provides a path for fast

cycle-by-cycle current limit during extreme fault condi-

tions, such as an inductor malfunction (Figure 3). Note the

average current-limit threshold of 27.5mV still limits the

output current during short-circuit conditions. To prevent

inductor saturation, select an inductor with a saturation

current specification greater than the average current limit.

The 60mV threshold for triggering the peak-current limit is

twice the full-scale average current-limit voltage threshold.

The peak-current comparator has only a 260ns delay.

Current-Error Amplifier

The MAX16821A–MAX16821C include a transconduc-

tance current-error amplifier with a typical gm of 550µS

and 320µA output sink and source capability. The current-

Figure 2. MAX16821A

–MAX16821C Control Loop

RIN

DIFF

SENSE+

SENSE-

EAN

EAOUT

CSN

CSP

CLP

CCZ

CCP

RCF

RLS

LED

STRING

COUT

DIFF

VREF

VEA

CEA

CPWM

DRIVER

MODE = SGND

VIN

MAX16821A/MAX16821B/

MAX16821C

High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

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