MAX8795A Maxim Integrated Products, MAX8795A Datasheet - Page 18

• Manufacturer Part Number: MAX8795A
• Description: TFT-LCD DC-DC Converter
• Manufacturer: Maxim Integrated Products

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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.
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
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.
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).
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.
Gate-Off Linear-Regulator Controller, REG N
18
______________________________________________________________________________________
Short-Circuit Current Limit and Input Clamp
REF
Driving Pure Capacitive Load
to 250mV in 128 steps.
Operational Amplifiers
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.
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 voltage falls below the UVLO falling
threshold, the controller turns off the main step-up regu-
lator, turns off the linear-regulator outputs, and disables
the switch control block; the operational-amplifier out-
puts are high impedance.
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.
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-regulator 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 reference voltage. For the gate-off linear
regulator, the DAC output steps from the reference
down to 250mV in 128 steps. The soft-start duration is
14ms (typ) for all three regulators.
A capacitor (C
switch-control-block startup delay. After the input volt-
age exceeds the UVLO threshold (2.25V typ) and the
soft-start routine for each regulator is complete and
there is no fault detected, a 5µA current source starts
charging C
Power-Up Sequence and Soft-Start
DEL
DEL
. Once the capacitor voltage exceeds
Undervoltage Lockout (UVLO)
) from DEL to AGND determines the
Reference Voltage (REF)