Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
22 _____________________________________________________________________________________
PWM Controller Block
An error amplifier compares the signal at FB1 to 1.25V
and changes the COMP output. The voltage at COMP
sets the peak inductor current. As the load varies, the
error amplifier sources or sinks current to the COMP
output accordingly to produce the inductor peak cur-
rent necessary to service the load. To maintain stabil-
ity at high duty cycles, a slope compensation signal is
summed with the current-sense signal.
On the rising edge of the internal clock, the controller
sets a flip-flop, turning on the n-channel MOSFET and
applying the input voltage across the inductor. The
current through the inductor ramps up linearly, storing
energy in its magnetic field. Once the sum of the current-
feedback signal and the slope compensation exceed
the COMP voltage, the controller resets the flip-flop
and turns off the MOSFET. Since the inductor current is
continuous, a transverse potential develops across the
inductor that turns on diode D1. The voltage across the
inductor then becomes the difference between the out-
put voltage and the input voltage. This discharge condi-
tion forces the current through the inductor to ramp back
down, transferring the energy stored in the magnetic
field to the output capacitor and the load. The MOSFET
remains off for the rest of the clock cycle.
Step-Up Regulator External pMOS Pass Switch
As shown in Figure 1, a series external p-channel
MOSFET can be installed between the cathode of the
step-up regulator Schottky catch diode and the V
AVDD
filter capacitors. This feature is used to sequence power
to AVDD after the MAX17126/MAX17126A have pro-
ceeded through normal startup to limit input surge current
during the output capacitor initial charge, and to provide
true shutdown when the step-up regulator is disabled.
When EN is low, GD is internally pulled up to the GD_I
through a 25I resistor. Once EN is high and the negative
charge-pump regulator is in regulation, the GD starts pull-
ing down with a 10FA (typ) internal current source. The
external p-channel MOSFET turns on and connects the
cathode of the step-up regulator Schottky catch diode
to the step-up regulator load capacitors when GD falls
below the turn-on threshold of the MOSFET. When V
GD
reaches V
GD_I
- 6V(GD done), the step-up regulator is
enabled and initiates a soft-start routine.
When not using this feature, leave GD high impedance,
and connect GD_I to the output of the step-up converter.
Soft-Start
The step-up regulator achieves soft-start by linearly
ramping up its internal current limit. The soft-start is
either done internally when the capacitance on pin SS is
< 200pF or externally when capacitance on pin SS is >
200pF. The internal soft-start ramps up the current limit
in 128 steps in 12ms. The external soft-start terminates
when the SS pin voltage reaches 1.25V. The soft-start
feature effectively limits the inrush current during startup
(see the Step-Up Regulator Soft-Start Waveforms in the
Typical Operating Characteristics).
Positive Charge-Pump Regulator
The positive charge-pump regulator (Figure 3) is typically
used to generate the positive supply rail for the TFT LCD
gate driver ICs. The output voltage is set with an external
resistive voltage-divider from its output to GND with the
midpoint connected to FBP. The number of charge-pump
stages and the setting of the feedback divider determine
Figure 3. Positive Charge-Pump Regulator Block Diagram
MAX17126
MAX17126A
D3
C14
C15
VGH
REF
1.25V
GD_I
ERROR
AMPLIFIER
OSC
POSITIVE CHARGE-PUMP REGULATOR
C13
C12
D5
P1
N1
DRVP
FBP
CPGND
SUPP