Datasheet

MAX1997/MAX1998
Quintuple/Triple-Output TFT LCD Power Supplies
with Fault Protection and VCOM Buffer
28 ______________________________________________________________________________________
6) To ensure stability, choose CLDO large enough so
that the crossover occurs well before the poles and
zero calculated in steps 2 to 5. The poles in steps 3
and 4 generally occur at several megahertz and
using ceramic capacitors ensures the ESR zero
occurs at several megahertz as well. Placing the
crossover below 500kHz is sufficient to avoid the
amplifier-delay pole and generally works well,
unless unusual component choices or extra capaci-
tances move the other poles or zero below 1MHz.
A capacitor connected between the regulator output and
the feedback node can improve the transient response
and reduce the noise coupled into the feedback loop
(Figure 1).
If a low-dropout solution is needed, an external P-channel
MOS pass transistor can be used for REG 1. However, a
PMOS-based linear regulator requires higher output
capacitance to stabilize the loop. The high gate capaci-
tance of the P-channel MOSFET lowers f
POLE(CIN)
and
can cause instability. A large output capacitor must be
used to reduce the unity-gain bandwidth and ensure that
the pole is well above the unity-gain crossover frequency.
A ceramic capacitor of at least 30µF is recommended for
V
IN
= 2.7V, V
OUT
= 2.5V, and I
LOAD
= 250mA.
VCOM Buffer
Connect the inverting input FBNB directly to the output
OUTB to configure the buffer as a voltage follower.
Adjust the buffer’s output voltage by connecting a volt-
age-divider from the gamma reference V
GAMMA
to
GND with the center tap connected to the noninverting
input FBPB (Figure 1). Select R14 in the 10kΩ to 100kΩ
range. Calculate R13 with the following equation:
The VCOM buffer is designed to be stable with a
0.47µF capacitor from OUTB to GND. The charge and
discharge currents of the VCOM buffer output can be
optimized by adding resistor R
B
in series with the LCD
backplane load and a ceramic capacitor C
B
(1µF or
larger) in parallel with the backplane load (Figure 11).
Start with a 10Ω resistor, then gradually increase the
value of R
B
, balancing between display quality and
buffer power dissipation. Increasing the value of C
B
improves the efficiency of the VCOM buffer.
R13 = R14
V
GAMMA
FBPB
V














−1
Figure 11. Optimizing VCOM Buffer Drive Current
V
GAMMA
C
B
R
B
TO TFT LCD
BACKPLANE
VCOM BUFFER
0.47µF
Figure 12. High-Current Loops of Step-Up Regulator
V
OUT
V
IN
C
OUT
C
IN
D
GROUND IMPEDANCE
L
Figure 13. Operation with Output Voltage >13V Using
Cascoded MOSFET
MAX1997
MAX1998
STEP-UP
REGULATOR
PGND
V
P
V
MAIN
15V
FB
LX
V
IN
V
N