MAX16812 Datasheet, PDF (10/21 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
Undervoltage Lockout
The MAX16812 features an adjustable UVLO through
the enable input (EN). Connect EN directly to IN to use
the 5V default UVLO. Connect EN to IN through a resis-
tive divider to ground to set the UVLO threshold. The
MAX16812 is enabled when V_EN exceeds the 1.38V
(typ) threshold.
Calculate the EN UVLO resistor-divider values as fol-
lows (see Figure 2):
R _UV1
=
R _UV2
x
⎛
⎝⎜
V_EN
V_UVLO -
V_EN
⎞
⎠⎟
where R_UV1 is in the 20kΩ range, V_EN is the 1.38V (typ)
EN threshold voltage, and V_UVLO is the desired input-
voltage UVLO threshold in volts. Due to the 100mV hys-
teresis of the UVLO threshold, capacitor C_EN is
required to prevent chattering at the UVLO threshold
due to line impedance drops at power-up and during
dimming. If the undervoltage setting is very close to the
required minimum operating voltage, there can be
jumps in the voltage at IN while dimming. C_EN should
be large enough to limit the ripple on EN to less than
100mV (EN hysteresis) under these conditions so that it
does not turn on and off due to the ripple on IN.
Soft-Start
The soft-start feature of the MAX16812 allows the LED
string current to ramp up in a controlled manner, thus
minimizing output-voltage overshoot. While the part is in
UVLO, C_REF is discharged (Figure 3). Upon coming out
of UVLO, an internal current source starts charging C_REF
during the soft-start cycle. Use the following equation to
calculate total soft-start time:
t_ST
=
C_REF
×
1.238
I_REF
where I_REF is 40µA, C_REF is in µF, and t_ST is in sec-
onds. Operation begins when REF ramps above 0.6V.
Once the soft-start is complete, REF is regulated to
1.238V, the internal voltage reference.
Low-Side Internal
Switching MOSFET Driver Supply (L_REG)
L_REG is the regulated (5.2V) internal supply voltage
capable of delivering 20mA. L_REG provides power to
the gate drive of the internal switching power MOSFET.
V_{L_REG} is referenced to AGND. Connect a 3.3µF
ceramic capacitor from L_REG to AGND.
High-Side Regulator (H_REG)
H_REG is a low-dropout linear regulator referenced to
LV. H_REG provides the gate drive for the external
n-channel dimming MOSFET and also powers up the
MAX16812’s LED current-sense circuitry. Bypass
H_REG to LV with a 1µF ceramic capacitor.
V_IN
IN
R_UV2
MAX16812
EN
C_EN
R_UV1
AGND
V_IN
C_REF
IN
MAX16812
REF
AGND
Figure 2. UVLO Threshold Setting
Figure 3. Soft-Start Setting
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