MAX16838B Datasheet, PDF (13/21 Pages) Maxim Integrated Products – Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller

MAX16838B
Integrated, 2-Channel, High-Brightness LED Driver
with High-Voltage Boost and SEPIC Controller
use large output capacitors. If the PWM dimming on-pulse
is less than 20 switching cycles, the feedback controls the
voltage on OV such that the converter output voltage is
regulated at 95% of the OV threshold. This mode ensures
that narrow PWM dimming pulses are not affected by the
response time of the converter. During this mode, the error
amplifier remains continuously connected to the COMP
output.
Adaptive LED Voltage Control
The device reduces power dissipation using an adaptive
LED voltage-control scheme. The adaptive LED voltage
control regulates the DC-DC converter output based on
the operating voltage of the LED strings.
The voltage at each of the current-sink outputs (OUT_)
is the difference between the DC-DC regulator output
voltage (V_LED) and the total forward voltage of the LED
string connected to the output (OUT_). The DC-DC con-
verter then adjusts V_LED until the output channel with the
lowest voltage at OUT_ is 1V relative to LEDGND. As
a result, the device minimizes power dissipation in the
current sinks and still maintains LED current regulation.
For efficient adaptive control functionality, use an equal
number of HB LEDs of the same forward-voltage rating
in each string.
Current Limit
The device includes a fast current-limit comparator to
terminate the on-cycle during an overload or a fault
condition. The current-sense resistor (R_CS) connected
between the source of the internal MOSFET and ground
sets the current limit. The CS input has a 0.3V voltage trip
level (V_CS). Use the following equation to calculate R_CS:
R_CS = (V_CS)/I_PEAK
where I_PEAK is the peak current that flows through the
MOSFET.
Undervoltage Lockout
The device features two undervoltage lockouts: UVLO_IN
and UVLO_VCC. The undervoltage-lockout threshold for
V_IN is 4.3V (typ) and the undervoltage-lockout threshold
for V_CC is 4V (typ).
Soft-Start
The device features a soft-start that activates during
power-up. The soft-start ramps up the output of the con-
verter in 64 steps in a period of 100ms (typ), unless both
strings reach regulation point, in which case the soft-start
would terminate to resume normal operation immediately.
Once the soft-start is over, the internal soft-start circuitry
is disabled and the normal operation begins.
Oscillator Frequency/External Synchronization
The device’s oscillator frequency is programmable
between 200kHz and 2MHz using one external resistor
(R_RT) connected between RT and SGND. The PWM
MOSFET driver output switching frequency is the same
as the oscillator frequency. The oscillator frequency is
determined using the following formula:
f_SW = (7.342 x 10⁹/R_RT)(Hz)
where R_RT is in Ω.
Synchronize the oscillator with an external clock by
AC-coupling the external clock to the RT input. The
capacitor used for the AC-coupling should satisfy the
following relation:
C _SYNC
≤


9.862
 _RRT
−
0.144
×
10
−3



(µF)
where R_RT is in Ω.
The pulse width for the synchronization signal should
satisfy the following relations:
t _PW
t _CLK
V_S
<
0.8

^0.8

−
t _PW
t _CLK

V_{S }

+
V_S
>
3.4
where t_PW is the synchronization source pulse width,
t_CLK is the synchronization clock time period, and V_S is
the synchronization pulse voltage level. See Figure 1.
5V LDO Regulator (DRV)
The internal LDO regulator converts the input voltage
at IN to a 5V output voltage at DRV. The LDO regulator
output supports up to 30mA current, enough to provide
power to the internal control circuitry and the gate driver.
VS
tPW
tCLK
Figure 1. Synchronizing External Clock Signal
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