Datasheet

13Maxim Integrated
Integrated, 2-Channel, High-Brightness LED Driver
with High-Voltage Boost and SEPIC Controller
MAX16838
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 MAX16838 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 MAX16838 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 MAX16838 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 MAX16838 oscillator frequency is programmable
between 200kHz and 2MHz using one external resis-
tor (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
= (7.342X10
9
/R
RT
)(Hz)
where R
RT
is in ω.
Synchronize the oscillator with an external clock by
AC-coupling the external clock to the R
RT
input. The
capacitor used for the AC-coupling should satisfy the
following relation:
3
SYNC
T
9.862
C 0.144 10 ( F)
R
−


≤ −× µ


where R
RT
is in I.
The pulse width for the synchronization signal should
satisfy the following relations:
PW
S
CLK
PW
SS
CLK
t
V 0.8
t
t
0.8 V V 3.4
t
<

− +>


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.
Figure 1. Synchronizing External Clock Signal
V
S
t
PW
t
CLK