MAX17061
8-String White LED Driver with
SMBus for LCD Panel Applications
______________________________________________________________________________________ 11
Pin Description (continued)
PIN NAME FUNCTION
22 V
DD
Boost Reg ul ator M OS FE T G ate D r i ve S up p l y. Byp ass V
D D
to G N D w i th a cer am i c cap aci tor of 1µF or g r eater .
23 V
CC
5V Linear Regulator Output. V
CC
provides power to the MAX17061. Bypass V
CC
to GND with a ceramic
capacitor of 1µF or greater. If V
IN
is less than or equal to 5.5V, tie V
CC
to IN to disable internal LDO and use
external 5V supply to V
CC
.
24 CCV
Step-Up Converter Compensation Pin. Connect a 0.022µF ceramic capacitor and 5.1kΩ resistor from CCV to
GND. When the MAX17061 shuts down, CCV is discharged to 0V through an internal 20kΩ resistor.
25 OV
Overvoltage Sense. Connect OV to the center tap of a resistive voltage-divider from V
OUT
to ground. The
detection threshold for voltage limiting at OV is 1.236V (typ).
26 ISET
Full-Scale LED Current Adjustment Pin. The resistance from ISET to GND controls the full-scale current in
each LED string:
I
LEDMAX
= 20mA x 200kΩ/R
ISET
The acceptable resistance range is 133kΩ < R
ISET
< 266kΩ, which corresponds to full-scale LED current of
30mA > I
LEDMAX
> 15mA. Connect ISET to V
CC
for a default full-scale LED current of 25mA.
27 FB1
LED String 1 Cathode Connection. FB1 is the open-drain output of an internal regulator, which controls
current through FB1. FB1 can sink up to 30mA. If unused, connect FB1 to V
CC
.
28 FB2
LED String 2 Cathode Connection. FB2 is the open-drain output of an internal regulator, which controls
current through FB2. FB2 can sink up to 30mA. If unused, connect FB2 to V
CC
.
—EP
Exposed Backside Pad. Solder to the circuit board ground plane with sufficient copper connection to ensure
low thermal resistance. See the PCB Layout Guidelines section.
Detailed Description
The MAX17061 is a high-efficiency driver for arrays of
white LEDs. It contains a fixed-frequency current-
mode PWM step-up controller, a 5V linear regulator,
dimming control circuit, SMBus interface, internal
power MOSFET, and eight regulated current sources
(see Figure 2). When enabled, the step-up controller
boosts the output voltage to provide sufficient head-
room for the current sources to regulate their respec-
tive string currents. The MAX17061 features selectable
switching frequency (500kHz, 750kHz, or 1MHz), which
allows trade-offs between external component size and
operating efficiency. The control architecture automati-
cally skips pulses at light loads to improve efficiency
and prevents overcharging the output capacitor.
WLED brightness is controlled by turning the WLEDs
on and off with a DPWM signal. The DPWM frequency
can be accurately adjusted with a resistor. The bright-
ness of the LEDs is proportional to the duty cycle of the
DPWM signal, which is controlled externally through
either a PWM or 2-wire SMBus-compatible interface, or
both. When both interfaces are used at the same time,
the product of the PWM duty cycle and SMBus com-
mand value is used for the dimming control. This
DPWM control scheme provides a full dimming range
with 8-bit resolution.
The MAX17061 has multiple features to protect the con-
troller from fault conditions. Separate feedback loops limit
the output voltage in all circumstances. The MAX17061
checks each FB_ voltage during the operation. If one or
more strings are open, the corresponding FB_ voltages
are pulled below 175mV (typ), and open-circuit fault is
detected. As a result, the respective current sources are
disabled. When one or more LEDs are shorted and the
FB_ voltage exceeds 1.1 x V
CC
, short fault is detected
and the respective current source is disabled. In either
LED open or short conditions, the fault strings are dis-
abled while other strings can still operate normally. The
controller features cycle-by-cycle current limit to provide
stable operation and soft-start protection. In a current-
limit condition, the controller shuts down after a 128µs
overcurrent fault timer expires. A thermal-shutdown cir-
cuit provides another level of protection.
