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Integrated High-Voltage LED Driver
with Analog and PWM Dimming Control
The overvoltage protection (OVP) comparator com-
pares the voltage at OV with a 1.238V (typ) internal ref-
erence. Use the following equation to calculate resistor
values:
REF
L_REG
R OV1 OV2 x ?
?
? V OV_LIM ? V OV ?
= R
? V OV ?
where V OV is the 1.238V OV threshold. Choose R OV1
and R OV2 to be reasonably high-value resistors to pre-
vent the discharge of filter capacitors. This prevents
degraded performance during dimming.
R DIM1
R DIM2
DIM
MAX16812
TGRM
AGND
R TGRM
C TGRM
Internal Oscillator Switching Frequency
The oscillator switching frequency is programmed by a
resistor connected from RT to AGND. To program the
oscillator frequency above 125kHz, choose the appro-
priate resistor RT from the curves shown in the
Oscillator Frequency vs. R T graph in the Typical
Operating Characteristics section.
PWM Dimming
PWM dimming can be achieved by driving DIM with an
analog voltage less than V REF . See Figure 7. An exter-
nal resistor on TGRM from L_REG in conjunction with
the ramp capacitor, C TGRM , from TGRM to AGND cre-
ates a sawtooth ramp that is compared with the DC
voltage on DIM. The output of the comparator is a pul-
sating dimming signal. The frequency f RAMP of the
sawtooth signal on TGRM is given by:
Figure 7. PWM Dimming from REF
PWM dimming can also be achieved by connecting
TGRM to a DC voltage less than V REF and applying the
PWM signal at DIM. The moment the internal dimming
signal goes low, gate drive DRV to the internal switching
MOSFET is turned off. The error amplifier goes to state B
(see the Internal Error Amplifier section and Figures 4
and 5). The peak current in the inductor prior to dis-
abling DRV is I LX . Gate drive DGT to the external dim-
ming MOSFET is held high. Then after a switchover
period, gate voltage V DGT on the external dimming
MOSFET is linearly controlled to reduce the LED current
f RAMP ?
3 . 67
C TGRM × R TGRM
to 0. The fall time of the LED current is controlled by an
internal timing circuit to 5μs for the MAX16812. During
this period, the gate (DRV) to the internal switching
Use the following formula to calculate the voltage V DIM ,
necessary for a given output duty cycle, D:
V DIM = D x 1.238V
where V DIM is the DC voltage applied to DIM in volts.
The DC voltage for DIM can also be created by con-
necting DIM to REF through a resistive voltage-divider.
Using the required dimming input voltage, V DIM , calcu-
late the resistor values for the divider string using the
following equation:
R DIM2 = [V DIM / (V REF - V DIM )] x R DIM1
where V REF is the voltage on REF.
MOSFET is enabled. After the fall time, the gate drive to
the external dimming MOSFET is turned off and the gate
drive to the internal switching MOSFET is still held high
after the switchover period. The peak current in the
inductor is controlled at I LX . Then after a time period of
20μs, the gate drive is disabled. The scope shots in
Figures 8–11 show the dimming waveforms.
______________________________________________________________________________________
13
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