MAX16812 Maxim Integrated Products, MAX16812 Datasheet - Page 13

• Manufacturer Part Number: MAX16812
• Description: Integrated High-Voltage LED Driver
• Manufacturer: Maxim Integrated Products

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:
where V
and R
vent the discharge of filter capacitors. This prevents
degraded performance during dimming.
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
Operating Characteristics section.
PWM dimming can be achieved by driving DIM with an
analog voltage less than V
nal resistor on TGRM from L_REG in conjunction with
the ramp capacitor, C
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
sawtooth signal on TGRM is given by:
Use the following formula to calculate the voltage V
necessary for a given output duty cycle, D:
where V
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
late the resistor values for the divider string using the
following equation:
where V
Internal Oscillator Switching Frequency
OV2
R
OV1
DIM
REF
OV
R
DIM2
to be reasonably high-value resistors to pre-
is the 1.238V OV threshold. Choose R
is the DC voltage applied to DIM in volts.
is the voltage on REF.
=
f
RAMP
= [V
R
OV2
______________________________________________________________________________________
V
DIM
DIM
≅
with Analog and PWM Dimming Control
x
TGRM
C
⎛
⎜
⎝
/ (V
= D x 1.238V
V
TGRM
OV_LIM
REF
REF
, from TGRM to AGND cre-
T
3 67
. See Figure 7. An exter-
V
Integrated High-Voltage LED Driver
- V
.
×
OV
graph in the Typical
DIM
−
R
TGRM
V
PWM Dimming
)] x R
OV
⎞
⎟
⎠
DIM1
RAMP
DIM
, calcu-
of the
DIM
OV1
,
PWM dimming can also be achieved by connecting
TGRM to a DC voltage less than V
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
ming MOSFET is held high. Then after a switchover
period, gate voltage V
MOSFET is linearly controlled to reduce the LED current
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
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
20µs, the gate drive is disabled. The scope shots in
Figures 8–11 show the dimming waveforms.
Figure 7. PWM Dimming from REF
R
R
DIM1
DIM2
LX
. Gate drive DGT to the external dim-
REF
DIM
MAX16812
DGT
LX
AGND
. Then after a time period of
on the external dimming
L_REG
TGRM
www.DataSheet4U.com
REF
and applying the
R
C
TGRM
TGRM
13