MAX16831_09 Datasheet, PDF (13/19 Pages) Maxim Integrated Products – High-Voltage, High-Power LED Driver with Analog and PWM Dimming Control

High-Voltage, High-Power LED Driver with
Analog and PWM Dimming Control
capacitor to hold the charge when the DIM signal has
turned off the gate drive. When DIM is high again, the
voltage on the compensation capacitors, C1 and C2,
will force the converter into steady-state instantaneously.
PWM Dimming
PWM dimming is achieved by driving DIM with either a
PWM signal or a DC signal. The PWM signal is internal-
ly connected to the error amplifier, the dimming
MOSFET gate driver, and the switching MOSFET gate
driver. When the DIM signal is high, the dimming
MOSFET and the switching MOSFET drivers are
enabled and the output of the voltage-error amplifier is
connected to the external compensation network. Also,
the buffered current-sense signal is connected to CS.
Preventing discharge of the compensation capacitor
when the DIM signal is low will allow the control loop to
return the LED current to its original value almost
instantaneously.
When the DIM signal goes low, the output of the error
amplifier is disconnected from the compensation net-
work and the voltage of compensation capacitors, C1
and C2 is preserved. Choose low-leakage capacitors
for C1 and C2. The drivers for the external dimming
and switching MOSFETs are disabled, and the convert-
er stops switching. The inductor energy is now trans-
ferred to the output capacitors.
When the DIM signal goes high and the gate drivers are
enabled, the additional voltage on the output capacitor
may cause a current spike on the LED string. A larger
output capacitor will result in a smaller current spike. The
MAX16831 thus achieves fast PWM dimming response.
Fault Protection
The MAX16831 features built-in overvoltage protection,
overcurrent protection, HICCUP mode current-limit pro-
tection, and thermal shutdown. Overvoltage protection
is achieved by connecting OV to HI through a resistive
voltage-divider. HICCUP mode limits the power dissi-
pation in the external MOSFETs during severe fault
conditions. Internal thermal shutdown protection safely
turns off the converter when the junction temperature
exceeds +165°C.
Overvoltage Protection
The overvoltage protection (OVP) comparator com-
pares the voltage at OV with a 1.235V (typ) internal ref-
erence. When the voltage at OV exceeds the internal
reference, the OVP comparator terminates PWM
switching and no further energy is transferred to the
V_LED+
R_OV1
R_OV2
MAX16831
OV
AGND
Figure 4. Setting the Overvoltage Threshold
load. The MAX16831 re-initiates soft-start once the
overvoltage condition is removed. Connect OV to HI
through a resistive voltage-divider to set the overvolt-
age threshold at the output.
Setting the Overvoltage Threshold
Connect OV to HI or to the high-side of the LEDs
through a resistive voltage-divider to set the overvolt-
age threshold at the output (Figure 4). The overvoltage
protection (OVP) comparator compares the voltage at
OV with a 1.235V (typ) internal reference. Use the fol-
lowing equation to calculate resistor values:
R_OV1
=
R_OV2
×
⎛
⎝⎜
V_OV
_ LIM ^-VOV
V_OV
⎞
⎠⎟
where V_OV is the 1.235V OV threshold. Choose R_OV1
and R_OV2 to be reasonably high-value resistors to pre-
vent discharge of filter capacitors. This will prevent
unnecessary undervoltage and overvoltage conditions
during dimming.
Load-Dump Protection
The MAX16831 features load-dump protection up to
80V. LED drivers using the MAX16831 can sustain sin-
gle fault load dump events. Repeated load dump events
within very short time intervals can cause damage to the
dimming MOSFET due to excess power dissipation.
Thermal Shutdown
The MAX16831 contains an internal temperature sensor
that turns off all outputs when the die temperature
exceeds +165°C. Outputs are enabled again when the
die temperature drops below +145°C.
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