MAX16821A Datasheet, PDF(12/25 Page) Maxim Integrated Products – High-Power Synchronous HBLED Drivers with Rapid Current Pulsing

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MAX16821A Datasheet, PDF (12/25 Pages) Maxim Integrated Products – High-Power Synchronous HBLED Drivers with Rapid Current Pulsing

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High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

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, VCM (+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 driversâ 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 VDD. A bootstrap capacitor from BST to LX pro-

vides the additional boost voltage necessary for the

high-side driver. VDD supplies power internally to the

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

capacitor between BST and LX and a Schottky diode

from BST to VDD.

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 (VCM). 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 ILIMIT = 0.0275 / RS.

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 VCM (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.

OVI

MAX16821A

MAX16821B DIFF

MAX16821C

EAN

EAOUT

COVI

VOUT

RIN

Figure 4. Overvoltage Protection Input Delay

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