Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
______________________________________________________________________________________ 25
Driving Pure Capacitive Load
The LCD backplane consists of a distributed series
capacitance and resistance, a load that can be easily
driven by the operational amplifier. However, if the
operational amplifier is used in an application with a pure
capacitive load, steps must be taken to ensure stable
operation. As the operational amplifier’s capacitive load
increases, the amplifier’s bandwidth decreases and gain
peaking increases. A 5I to 50I small resistor placed
between OPO and the capacitive load reduces peaking,
but also reduces the gain. An alternative method of
reducing peaking is to place a series RC network
(snubber) in parallel with the capacitive load. The RC
network does not continuously load the output or reduce
the gain. Typical values of the resistor are between 100I
and 200I, and the typical value of the capacitor is 10nF.
Linear Regulator (VL)
The MAX17126/MAX17126A include an internal linear
regulator. INVL is the input of the linear regulator. The
input voltage range is between 8V and 16.5V. The output
voltage is set to 5V. The regulator powers the internal
MOSFET drivers, PWM controllers, charge-pump
regulators, and logic circuitry. The total external load
capability is 25mA. Bypass VL to GND with a minimum
1FF ceramic capacitor.
Reference Voltage (REF)
The reference output is nominally 1.25V, and can source
at least 50FA (see Typical Operating Characteristics). VL
is the input of the internal reference block. Bypass REF
with a 0.22FF ceramic capacitor connected between
REF and GND.
High-Accuracy,
High-Voltage Gamma Reference
The LDO is typically used to drive gamma-correction
divider string. Its output voltage is adjustable through a
resistor-divider. This LDO features high output accuracy
(Q0.5%) and low-dropout voltage (0.25V typ) and can
supply at least 60mA.
PGOOD Function
PGOOD is an open-drain output that connects to GND
when VDET is below its detection threshold (1.25V typ).
PGOOD is active after VL rises above UVLO threshold.
Frequency Selection and
Out-of-Phase Operation (FSEL)
The step-down regulator and step-up regulator use
the same internal oscillator. The FSEL input selects
the switching frequency. Table 3 shows the switching
frequency based on the FSEL connection. High-frequency
(750kHz) operation optimizes the application for the
smallest component size, trading off efficiency due
to higher switching losses. Low-frequency (500kHz)
operation offers the best overall efficiency at the expense
of component size and board space.
To reduce the input RMS current, the step-down regulator
and the step-up regulator operate 180N out of phase
from each other. The feature allows the use of less input
capacitance.
Figure 6. Op Amp Input Clamp Structure
Table 3. Frequency Selection
MAX17126
MAX17126A
±4V 500
OPN
OPP
OPO
VOP
OGND
FSEL
SWITCHING FREQUENCY
(kHz)
VL, INVL, OR FLOAT 750
GND 500