MAX16831 Datasheet, PDF (11/20 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
Reference Voltage Output
The MAX16831 includes a 5% accurate 3V (typ)
buffered reference output, REF. REF is a push-pull out-
put capable of sourcing/sinking 100µA of current and
can drive a maximum load capacitance of 100pF.
Connect REF to DIM through a resistive voltage-divider
to supply an analog signal for dimming. See the
Dimming Input (DIM) section.
Dimming MOSFET Driver (DDR)
The MAX16831 requires an external n-channel
MOSFET for PWM dimming. Connect the MOSFET to
the output of the DDR dimming driver, DGT, for normal
operation. V_DGT swings between V_LO and V_CLMP. The
DDR dimming driver is capable of sinking or sourcing
up to 20mA of current. The average current required to
drive the dimming MOSFET (I_{DRIVE_DIM}) depends on
the MOSFET’s total gate charge (Q_{G_DIM}) and the dim-
ming frequency of the converter, f_DIM. Use the follow-
ing equation to calculate the average gate drive current
for the n-channel dimming FET.
I_{DRIVE_DIM} = Q_{G_DIM} x f_DIM
n-Channel MOSFET Switch Driver (DRV)
The MAX16831 drives an external n-channel MOSFET.
Use an external supply or connect REG2 to DRI to
power the MOSFET driver. The driver output, V_DRV,
swings between ground and V_DRI. Ensure that V_DRI
remains below the absolute maximum V_GS rating of the
external MOSFET. DRV is capable of sinking 2.5A or
sourcing 1.4A of peak current, allowing the MAX16831
to switch MOSFETs in high-power applications. The
average current sourced to drive the external MOSFET
depends on the total gate charge (Q_G) and operating
frequency of the converter, f_SW. The power dissipation
in the MAX16831 is a function of the average output
drive current (I_DRIVE). Use the following equations to
calculate the power dissipation in the gate driver sec-
tion of the MAX16831 due to I_DRIVE:
I_DRIVE = Q_G x f_SW
P_D = (I_DRIVE + I_CC) x V_DRI
where V_DRI is the supply voltage to the gate driver and
I_CC is the operating supply current. I_DRIVE should not
exceed 20mA.
Dimming Input (DIM)
The dimming input, DIM, functions with either analog or
PWM control signals. Once the internal pulse detector
detects three successive edges of a PWM signal with a
frequency between 80Hz and 2kHz, the MAX16831 syn-
chronizes to the external signal and pulse-width-modu-
lates the LED current at the external DIM input frequency
with the same duty cycle as the DIM input. If an analog
REF
R₃
MAX16831
DIM
AGND
R₄
Figure 2. Creating a DIM Input Signal from REF
control signal is applied to DIM, the MAX16831 com-
pares the DC input to an internally generated 200Hz
ramp to pulse-width-modulate the LED current (f_DIM =
200Hz). The output current duty cycle is linearly
adjustable from 0 to 100% (0.2V < V_DIM < 2.8V).
Use the following formula to calculate the voltage, V_DIM,
necessary for a given output-current duty cycle, D:
V_DIM = (D x 2.6) + 0.2V
where V_DIM is the voltage applied to DIM in volts.
Connect DIM to REF through a resistive voltage-divider
to apply a DC DIM control signal (Figure 2). Use the
required dimming input voltage, V_DIM, calculated
above and select appropriate resistor values using the
following equation:
R₄ = R₃ x V_DIM / (V_REF - V_DIM)
where V_REF is the 3V reference output voltage and
30kΩ ≤ R₃ + R₄ ≤ 150kΩ.
For proper operation at startup or after toggling ENABLE,
the controller needs three clock edges or an analog volt-
age greater than 0.3V on the DIM input.
Oscillator, Clock, and Synchronization
The MAX16831 is capable of stand-alone operation or
synchronizing to an external clock, and driving external
devices in SYNC mode. For stand-alone operation, pro-
gram the switching frequency by connecting a single
external resistor, R_T, between RTSYNC and ground.
Select the switching frequency, f_SW, from 125kHz to
600kHz and calculate R_T using the following formula:
R_T
=
500kHz
f_SW
×
25kΩ
where the switching frequency is in kHz and RT is in kΩ.
The MAX16831 is also capable of synchronizing to an
external clock signal ranging from 125kHz to 600kHz.
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