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Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Fixed-Frequency Step-Up Controller
The MAX8790’s fixed-frequency, current-mode, step-up
controller automatically chooses the lowest active FB_
voltage to regulate the feedback voltage. Specifically,
the difference between the lowest FB_ voltage and the
current source-control signal plus an offset (V SAT ) is
integrated at the CCV output. The resulting error signal
is compared to the external switch current plus slope
compensation to terminate the switch on-time. As the
load changes, the error amplifier sources or sinks cur-
rent to the CCV output to adjust the required peak
inductor current. The slope-compensation signal is
added to the current-sense signal to improve stability at
high duty cycles.
At light loads, the MAX8790 automatically skips pulses
to improve efficiency and prevent overcharging the out-
put capacitor. In SKIP mode, the inductor current ramps
up for a minimum on-time of approximately 150ns, then
discharges the stored energy to the output. The switch
remains off until another pulse is needed to boost the
output voltage.
Internal 5V Linear Regulator
V CC and UVLO
The MAX8790 includes an internal low-dropout linear
regulator (V CC ). When V IN is higher than 5.5V and
SHDN is high, this linear regulator generates a 5V sup-
ply to power an internal PWM controller, control logic,
and MOSFET driver. This linear regulator can deliver at
least 10mA of total additional load current. If V IN is less
than or equal to 5.5V, V CC and IN can be connected
together and powered from an external 5V supply.
There is an internal diode from V CC to IN, so V IN must
be greater than V CC (see Figure 2).
The MAX8790 includes UVLO protection. The controller
is disabled until V CC exceeds the UVLO threshold of
4.25V (typ). Hysteresis on UVLO is approximately 20mV.
The V CC pin should be bypassed to GND with a 1μF or
greater ceramic capacitor.
V IN
2.8V TO 5.5V
C IN
L1
0.9 μ H
D1
C OUT
V OUT
UP TO 22V
SHDN
IN
EXT
N1
EXTERNAL
5V SUPPLY
511k Ω
1 μ F
N.C.
V CC
ENA
ISET
BRT
FSET
OSC
CS
GND
R S
30m Ω
R1
1M Ω
OV
1.2k Ω
CCV
MAX8790
R2
59k Ω
0.1 μ F
FB1
CPLL
0.1 μ F
EP
Figure 3. Low-Input-Voltage Application Circuit
FB2
FB3
FB4
FB5
FB6
12
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