Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
24 _____________________________________________________________________________________
The negative charge-pump regulator is enabled after
the step-down regulator finishes soft-start. Each time it
is enabled, the negative charge-pump regulator goes
through a soft-start routine by ramping down its internal
reference voltage from 1.25V to 250mV in 128 steps. The
soft-start period is 1.8ms (typ) and FBN fault detection
is disabled during this period. The soft-start feature
effectively limits the inrush current during startup.
High-Voltage Switch Control
The MAX17126/MAX17126As’ high-voltage switch control
block (Figure 5) consists of two high-voltage p-channel
MOSFETs: Q1, between VGH, and VGHM and Q2, between
VGHM and DRN. The switch control block is enabled when
V
DLY1
exceeds V
REF
. Q1 and Q2 are controlled by GVOFF.
When GVOFF is logic-high, Q1 turns on and Q2 turns
off, connecting VGHM to VGH. When GVOFF is logic-
low, Q1 turns off and Q2 turns on, connecting VGHM to
DRN. VGHM can then be discharged through a resistor
connected between DRN and GND or AVDD. Q2 turns
off and stops discharging VGHM when VGHM reaches
10 times the voltage on THR.
The switch control block is disabled and DLY1 is held low
when the LCD is shut down or in a fault state.
Operational Amplifier
The operational amplifier is typically used to drive the
LCD backplane (VCOM). It features Q200mA output
short-circuit current, 45V/Fs slew rate, and 20MHz/3dB
bandwidth. The rail-to-rail input and output capability
maximizes system flexibility.
Short-Circuit Current Limit and Input Clamp
The operational amplifier limits short-circuit current to
approximately Q200mA if the output is directly shorted
to VOP or to OGND. If the short-circuit condition persists,
the junction temperature of the IC rises until it reaches
the thermal-shutdown threshold (+160NC typ). Once
the junction temperature reaches the thermal-shutdown
threshold, an internal thermal sensor immediately sets
the thermal fault latch, shutting off all the IC’s outputs.
The device remains inactive until the input voltage is
cycled. The operational amplifiers have 4V input clamp
structures in series with a 500I resistance and a diode
(Figure 6).
Figure 5. Switch Control
MAX17126
MAX17126A
9R
V
REF
R
1kI
Q4
Q2
Q1
GD DONE
EN
SHDN
FAULT
REF
10µA
DLY1
GVOFF
VGHM
THR
VGH
DRN