MAX16821CATI+ Datasheet MAX16821CATI+, MAX16821BATI+, MAX16821AATI+ | P10-P12

MAX16821A/MAX16821B/MAX16821C

High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

10 ______________________________________________________________________________________

Synchronization

The MAX16821A/MAX16821B/MAX16821C synchronize

to an external clock connected to RT/SYNC. The appli-

cation of an external clock at RT/SYNC disables the

internal oscillator. Once the MAX16821A/MAX16821B/

MAX16821C are synchronized to an external clock, the

external clock cannot be removed if reliable operation

is to be maintained.

Control Loop

The MAX16821A/MAX16821B/MAX16821C use an

average current-mode control scheme to regulate the

output current (Figure 2). The main control loop con-

sists 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 combination 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 (R

), a current-sense amplifier

(CSA), a current-error amplifier (CEA), an oscillator pro-

viding the carrier ramp, and a PWM comparator

(CPWM) (Figure 2). The MAX16821A/MAX16821B/

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 (V

= 7V to 28V). The current-sense

amplifier senses the voltage across R

. The maximum

common-mode voltage is 3.2V when V

= 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 aver-

age current limit. The 60mV threshold for triggering the

peak-current limit is twice the full-scale average cur-

rent-limit voltage threshold. The peak-current compara-

tor has only a 260ns delay.

DIFF

SENSE+

SENSE-

EAN EAOUT CSN CSP CLP

LED

STRING

OUT

DIFF

REF

VEA

CEA

CPWM

DRIVER

MODE = SGND

Figure 2. MAX16821A/MAX16821B/MAX16821C Control Loop

MAX16821A/MAX16821B/MAX16821C

High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

______________________________________________________________________________________ 11

Current-Error Amplifier

The MAX16821A/MAX16821B/MAX16821C include a

transconductance current-error amplifier with a typical

of 550µS and 320µA output sink and source capa-

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

P-P

ramp signal. At the start of each clock cycle, an

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

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

MODE is connected to V

. The comparator sets the

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

CSP

CLP

CSN

MODE = GND

= 34.5

60mV

= 550µS

PEAK-CURRENT

COMPARATOR

BST

PGND

Figure 3. MAX16821A/MAX16821B/MAX16821C Phase Circuit

MAX16821A/MAX16821B/MAX16821C

High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

12 ______________________________________________________________________________________

Voltage-Error Amplifier (VEA)

The VEA sets the gain of the voltage control loop, and

determines the error between the differential amplifier

output and the internal reference voltage. The VEA out-

put clamps to 0.93V relative to the internal common-

mode voltage, V

(+0.6V), limiting the average maxi-

mum current. The maximum average current-limit

threshold is equal to the maximum clamp voltage of the

VEA divided by the gain (34.5) of the current-sense

amplifier. This results in accurate settings for the aver-

age maximum current.

MOSFET Gate Drivers

The high-side (DH) and low-side (DL) drivers drive the

gates of external n-channel MOSFETs. The drivers’ 4A

peak sink- and source-current capability provides

ample drive for the fast rise and fall times of the switch-

ing MOSFETs. Faster rise and fall times result in

reduced cross-conduction losses. Size the high-side

and low-side MOSFETs to handle the peak and RMS

currents during overload conditions. The driver block

also includes a logic circuit that provides an adaptive

nonoverlap time to prevent shoot-through currents dur-

ing transition. The typical nonoverlap time is 35ns

between the high-side and low-side MOSFETs.

BST

The MAX16821A/MAX16821B/MAX16821C provide

power to the low-side and high-side MOSFET drivers

through V

. A bootstrap capacitor from BST to LX pro-

vides the additional boost voltage necessary for the

high-side driver. V

supplies power internally to the

low-side driver. Connect a 0.47µF low-ESR ceramic

capacitor between BST and LX and a Schottky diode

from BST to V

Protection

The MAX16821A/MAX16821B/MAX16821C include out-

put overvoltage protection (OVP). During fault condi-

tions when the load goes to high impedance (output

opens), the controller attempts to maintain LED current.

The OVP disables the MAX16821A/MAX16821B/

MAX16821C whenever the output voltage exceeds the

OVP threshold, protecting the external circuits from

undesirable voltages.

Current Limit

The error amplifier (VEA) output is clamped between

-0.050V and +0.93V with respect to common-mode

voltage (V

). Average current-mode control limits the

average current sourced by the converter during a fault

condition. When a fault condition occurs, the VEA out-

put clamps to +0.93V with respect to the common-

mode voltage (0.6V) to limit the maximum current

sourced by the converter to I

LIMIT

= 0.0275 / R

Overvoltage Protection

The OVP comparator compares the OVI input to the

overvoltage threshold. The overvoltage threshold is typ-

ically 1.127 times the internal 0.6V reference voltage

plus V

(0.6V). A detected overvoltage event trips the

comparator output turning off both high-side and low-

side MOSFETs. Add an RC delay to reduce the sensi-

tivity of the overvoltage circuit and avoid unnecessary

tripping of the converter (Figure 4). After the OVI volt-

age falls below 1.076V (typ.), high-side and low-side

drivers turn on only after a 2048 clock-cycle POR and a

1024 clock-cycle soft-start have elapsed. Disable the

overvoltage function by connecting OVI to SGND.

MAX16821A

MAX16821B

MAX16821C

OVI

DIFF

EAN

EAOUT

OVI

OUT

Figure 4. Overvoltage Protection Input Delay

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MAX16821CATI+

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Manufacturer:

Maxim Integrated

Description:

LED Lighting Drivers Synchronous HB w/Rapid Current Puls

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MAX16821CATI+

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