MAX1997ETJ+T Maxim Integrated Products, MAX1997ETJ+T Datasheet - Page 18

• Manufacturer Part Number: MAX1997ETJ+T
• Description: IC PWR SUPPLY TFT LCD 32TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX1997ETJ+T

• Applications: Controller, TFT, LCD
• Voltage - Input: 2.7 ~ 5.5 V
• Number Of Outputs: 5
• Voltage - Output: 2.7 ~ 13 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Quintuple/Triple-Output TFT LCD Power Supplies
with Fault Protection and VCOM Buffer
Figure 6. External P-Channel MOSFET Input Switch Control
The linear-regulator controller REG 2 is an analog gain
block with an open-drain N-channel output. It drives an
external PNP pass transistor with a 2.2kΩ base-to-emitter
resistor (Figure 1). Its guaranteed base drive sink current
is at least 5mA. The regulator is designed to deliver
30mA with an output capacitor of 2.2µF.
REG 2 is enabled when SHDN is high, the gate to the
input P-channel MOSFET is low, ONDC is high, and
V
Inrush Current Control section.) Each time it is enabled,
the regulator goes through a soft-start routine that
ramps up its reference input. REG 2 is typically used to
provide the TFT LCD gamma reference voltage.
The MAX1997 includes a VCOM buffer, which is an oper-
ational transconductance amplifier that provides a current
output for driving the backplane of a TFT LCD panel. The
unity-gain bandwidth of this current-output buffer is:
where g
the ratio of the output current to the input voltage. The
VCOM buffer requires only a 0.47µF ceramic output
capacitor for stability. The bandwidth is inversely pro-
portional to the output capacitance. Thus, large capaci-
tive loads reduce the bandwidth of the buffer output.
In order to improve the transient response time, the ampli-
fier has nonlinear transconductance. The amplifier senses
the output current and increases the transconductance
as the output current increases. The effect is to provide
additional output current when the load demands it.
18
CT
______________________________________________________________________________________
> V
m
ON2
is the amplifier’s transconductance, which is
Linear-Regulator Controller REG 2
. (See the Power-Up Sequencing and
VCOM Buffer (MAX1997 Only)
GBW = g
m
/C
OUT
ENABLE
GATE
(MAX1997 Only)
V
REF
2
To ensure that the input voltage is high enough for reli-
able operation, the MAX1997/MAX1998 include an
undervoltage lockout (UVLO) circuit. The UVLO thresh-
old at the IN pin is 2.7V (typ) rising and 2.35V (typ)
falling. The 350mV (typ) hysteresis prevents supply tran-
sients from causing a restart. Once the input voltage
exceeds the UVLO rising threshold, the controller
enables the reference block. Once the reference is
above 1.05V, an internal 10µA current source pulls the
GATE pin low and turns on an external P-channel
MOSFET switch (P1, Figure 1) that connects the input
supply to the regulator. When the input voltage falls below
the UVLO falling threshold, the controller turns off the ref-
erence and all the regulator outputs, and pulls GATE high
with an internal 100Ω switch to turn off P1 (Figure 6).
The reference output is nominally 1.25V, and can
source up to 75µA. (See the Typical Operating
Characteristics.) Bypass REF with a 0.22µF ceramic
capacitor connected between REF and GND.
The internal oscillator frequency is adjustable using the
three-level FREQ input. Connect FREQ to ground for
375kHz operation, connect FREQ to V
operation, and leave FREQ unconnected for 750kHz
operation. When FREQ is left unconnected, bypass
FREQ to ground with a 1000pF to 0.1µF capacitor to
prevent switching noise from coupling into the pin’s
high input impedance. Note that the soft-start period
scales with the oscillator frequency. (See the Soft-Start
section.) The fault timer period does not scale with the
oscillator frequency.
10µA
GATE
IN
Oscillator Frequency Control (FREQ)
V
IN
EXTERNAL
PFET
INPUT
CAP
INDUCTOR
Undervoltage Lockout (UVLO)
INPUT
CAP
Reference Voltage (REF)
IN
for 1.5MHz