MAX8795A_1106 Datasheet, PDF (18/31 Pages) Maxim Integrated Products – TFT-LCD DC-DC Converter with Operational Amplifiers 2.5V to 5.5V Input Supply Range

TFT-LCD DC-DC Converter with
Operational Amplifiers
REG P is enabled after the REF voltage exceeds 1.0V.
Each time it is enabled, the controller goes through a
soft-start routine that ramps up its internal reference
DAC in 128 steps.
Gate-Off Linear-Regulator Controller, REG N
The gate-off linear-regulator controller (REG N) is an
analog gain block with an open-drain p-channel output.
It drives an external npn pass transistor with a 6.8kΩ
base-to-emitter resistor (Figure 1). Its guaranteed base-
drive source current is at least 1mA. The regulator
including Q2 in Figure 1 uses a 0.47µF ceramic output
capacitor and is designed to deliver 50mA at -10V. Other
output voltages and currents are possible with the proper
pass transistor and output capacitor (see the Pass-
Transistor Selection and Stability Requirements sections).
REG N is typically used to provide the TFT-LCD gate
drivers’ gate-off voltage. A negative voltage can be
produced using a charge-pump circuit as shown in
Figure 1. REG N is enabled after the voltage on REF
exceeds 1.0V. Each time it is enabled, the control goes
through a soft-start routine that ramps down its internal
reference DAC from V_REF to 250mV in 128 steps.
Operational Amplifiers
The MAX8795A has five operational amplifiers. The opera-
tional amplifiers are typically used to drive the LCD back-
plane (VCOM) or the gamma-correction divider string.
They feature ±130mA output short-circuit current, 45V/µs
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 amplifiers limit short-circuit current to
approximately ±130mA if the output is directly shorted to
SUP or to BGND. If the short-circuit condition persists, the
junction temperature of the IC rises until it reaches the
thermal-shutdown threshold (+160°C typ). Once the junc-
tion 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 500Ω resistance and a diode (Figure 2).
Driving Pure Capacitive Load
The operational amplifiers are typically used to drive
the LCD backplane (VCOM) or the gamma-correction
divider string. The LCD backplane consists of a distrib-
uted 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 5Ω to 50Ω small resistor placed between
OUT_ 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 par-
allel with the capacitive load. The RC network does not
continuously load the output or reduce the gain. Typical
values of the resistor are between 100Ω and 200Ω, and
the typical value of the capacitor is 10nF.
Undervoltage Lockout (UVLO)
The UVLO circuit compares the input voltage at IN with the
UVLO threshold (2.25V rising, 2.20V falling, typ) to ensure
the input voltage is high enough for reliable operation. The
50mV (typ) hysteresis prevents supply transients from
causing a restart. Once the input voltage exceeds the
UVLO rising threshold, startup begins. When the input volt-
age falls below the UVLO falling threshold, the controller
turns off the main step-up regulator, turns off the linear-
regulator outputs, and disables the switch control block;
the operational-amplifier outputs are high impedance.
Reference Voltage (REF)
The reference output is nominally 1.25V and can
source at least 50µA (see the Typical Operating
Characteristics). Bypass REF with a 0.22µF ceramic
capacitor connected between REF and AGND.
Power-Up Sequence and Soft-Start
Once the voltage on IN exceeds approximately 2.25V, the
reference turns on. With a 0.22µF REF bypass capacitor,
the reference reaches its regulation voltage of 1.25V in
approximately 1ms. When the reference voltage exceeds
1.0V, the IC enables the main step-up regulator, the gate-
on linear-regulator controller, and the gate-off linear-regu-
lator controller simultaneously.
The IC employs soft-start for each regulator to minimize
inrush current and voltage overshoot and to ensure a well-
defined startup behavior. Each output uses a 7-bit soft-start
DAC. For the step-up and the gate-on linear regulator, the
DAC output is stepped in 128 steps from zero up to the ref-
erence voltage. For the gate-off linear regulator, the DAC
output steps from the reference down to 250mV in 128
steps from zero up to the reference voltage. For the gate-
off linear regulator’s voltage ramp soft-start, the DAC output
steps from the reference down to 250mV in 128 steps. The
soft-start duration is 14ms (typ) for all three regulators, and
DEL remains pulled down to AGND during the soft start
period.
Once the main step-up regulator, the gate-on linear-regula-
tor controller, and the gate-off linear-regulator controller
reach regulation, a 5µA current source starts charging
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