MAX16812 Datasheet, PDF(12/21 Page) Maxim Integrated Products – Integrated High-Voltage LED Driver with Analog and PWM Dimming Control

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MAX16812 Datasheet, PDF (12/21 Pages) Maxim Integrated Products – Integrated High-Voltage LED Driver with Analog and PWM Dimming Control

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Integrated High-Voltage LED Driver

with Analog and PWM Dimming Control

Analog Dimming

The MAX16812 offers analog dimming of the LED cur-

rent by allowing the application of an external voltage

at REFI. The output current is proportional to the volt-

age at REFI. Use a potentiometer from REF or directly

apply an external voltage source at REFI.

PWM Comparator

The PWM comparator uses the instantaneous switch

current, the error-amplifier output, and the slope com-

pensation to determine when the gate drive DRV to the

internal n-channel switching MOSFET turns off. In nor-

mal operation, gate drive DRV to the n-channel MOS-

FET turns off when:

ISW x RSRC â¥ VCOMP - VOFFSET - VSCOMP

where ISW is the current through the internal n-channel

switching MOSFET, RSRC is the switch current-sense

resistor, VCOMP is the output voltage of the internal

amplifier, VOFFSET is the internal DC offset, which is a

VBE drop, and VSCOMP is the ramp function that starts

at zero and slews at the programmed slew rate (SR).

Internal Switching MOSFET Current Limit

The current-sense resistor (RSRC), connected between

the source of the internal MOSFET and ground, sets the

current limit. The SRC input has a voltage trip level

(VSRC) of 600mV for the cycle-by-cycle current limit. Use

the following equation to calculate the value of RSRC:

RSRC

VSRC

ILXLIM

where ILXLIM is the peak current that flows through the

switching MOSFET at full load and low line. When the

voltage produced by this current (through the current-

sense resistor) exceeds the current-limit (ILIM) com-

parator threshold, the MOSFET driver (DRV) quickly

terminates the current on-cycle. The 200ns leading-

edge blanking circuit suppresses the leading-edge

spike on the current-sense waveform from appearing at

the current-limit comparator. There is also a hiccup

comparator (HICCUP) that limits the peak current in the

internal switch set at twice the peak limit setting.

Internal n-Channel

Switching MOSFET Driver (DRV)

L_REG provides power for the DRV output. Connect a

resistor from DRV to gate GT of the internal switching

MOSFET to control the switching MOSFET rise and fall

times, if necessary.

External Dimming

MOSFET Gate Drive (DGT)

DGT is the gate drive to the external dimming MOSFET

referenced to LV. H_REG provides the power to the

gate drive.

Overvoltage Protection

The overvoltage protection (OVP) comparator com-

pares the voltage at OV with a 1.238V (typ) internal ref-

erence. When the voltage at OV exceeds the internal

reference, the OVP comparator terminates PWM switch-

ing and no further energy is transferred to the load.

Connect OV to HV through a resistive voltage-divider to

ground to set the overvoltage threshold at the output.

Setting the Overvoltage Threshold

Connect OV to HV or to the high-side of the LEDs

through a resistive voltage-divider to set the overvolt-

age threshold at the output (Figure 6).

VLED+

ROV1

ROV2

MAX16812

AGND

VLED+

ROV1

MAX16812

ROV2

AGND

Figure 6. OVP Setting

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