MAX16812 Datasheet, PDF (14/21 Pages) Maxim Integrated Products – Integrated High-Voltage LED Driver with Analog and PWM Dimming Control

Integrated High-Voltage LED Driver
with Analog and PWM Dimming Control
MAX16812 fig08
MAX16812 fig10
V_OUT
10V/div
I_LED
100mA/div
0A, 0V
2V/div
V_DRV
0V
10µs/div
Figure 8. LED Current, Output Voltage, and DRV Waveforms
when DIM Signal Goes Low
MAX16812 fig09
V_DIM
5V/div
I_LED
100mA/div
0A, 0V
V_DRV
2V/div
0V
10µs/div
Figure 9. LED Current, DIM Signal, and DRV Waveforms when
DIM Signal Goes Low
V_OUT
10V/div
100mA/div
I_LED
0A, 0V
2V/div
V_DRV
0V
10µs/div
Figure 10. LED Current, Output Voltage, and DRV Waveforms
when DIM Signal Goes High
MAX16812 fig11
I_LED
100mA/div
V_DIM
5V/div
0A, 0V
V_DRV
2V/div
0V
10µs/div
Figure 11. LED Current, DIM Signal, and DRV Waveforms when
DIM Signal Goes High
When the DIM signal goes high, the LED current is
gradually increased to the programmed value. The rise
time of the LED current is controlled to 5µs for the
MAX16812 by controlling the voltage on DGT. After the
rise time, an internal sensing circuit monitors the volt-
age across the drain to the source of the external dim-
ming MOSFET. The LED current is now controlled at the
programmed value by a linear current regulating cir-
cuit. Once the voltage across the drain to source of the
dimming MOSFET drops below 0.5V, the reference for
the linear current regulating circuit is increased to 1.1
times the programmed value. The gate drive (DRV) to
the internal switching MOSFET is enabled and the error
amplifier is returned to state A (see the Internal Error
Amplifier section and Figures 4 and 5).
Fault Protection
The MAX16812 features built-in overvoltage protection
and thermal shutdown. Connect a resistive voltage-
divider between HV, OV, and AGND to program the over-
voltage protection. In the case of a short circuit across
the LED string, the temperature of the external dimming
MOSFET could exceed the maximum allowable junction
temperature. This is due to excess power dissipation in
the MOSFET. Use the fault protection circuit shown in
Figure 12 to protect the external dimming MOSFET.
Internal thermal shutdown in the MAX16812 safely turns
off the IC when the junction temperature exceeds
+165^°C.
14 ______________________________________________________________________________________