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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:
I SW x R SRC ≥ V COMP - V OFFSET - V SCOMP
where I SW is the current through the internal n-channel
switching MOSFET, R SRC is the switch current-sense
resistor, V COMP is the output voltage of the internal
amplifier, V OFFSET is the internal DC offset, which is a
V BE drop, and V SCOMP 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 (R SRC ), connected between
the source of the internal MOSFET and ground, sets the
current limit. The SRC input has a voltage trip level
(V SRC ) of 600mV for the cycle-by-cycle current limit. Use
the following equation to calculate the value of R SRC :
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.
R SRC =
V SRC
I LXLIM
Connect OV to HV through a resistive voltage-divider to
ground to set the overvoltage threshold at the output.
Setting the Overvoltage Threshold
where I LXLIM is the peak current that flows through the
switching MOSFET at full load and low line. When the
V LED+
HV
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).
V LED+
R OV1
R OV2
OV
MAX16812
AGND
R OV1
R OV2
OV
MAX16812
AGND
Figure 6. OVP Setting
12
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