MAX1542/MAX1543
TFT LCD DC-to-DC Converter with
Operational Amplifiers
10 ______________________________________________________________________________________
Pin Description (continued)
PIN
MAX1542 MAX1543
NAME FUNCTION
14 14 IN Supply Voltage. IN can range from 2.6V to 5.5V.
— 15 FREQ
Oscillator Frequency Select Input. Pull FREQ low or leave it unconnected for 640kHz
operation. Connect FREQ high for 1.2MHz operation. This input has a 5µA pulldown
current.
16 16 FB
Step-Up Converter Feedback Input. Regulates to 1.24V (nominal). Connect a resistor-
divider from the output (V
MAIN
) to FB to analog ground (AGND). Place the resistor-divider
within 5mm of FB.
17 17 COMP
Step-Up Regulator Error Amplifier Compensation Point. Connect a series RC from COMP
to AGND. See the Loop Compensation section for component selection guidelines.
18 18 DEL
High-Voltage Switch Delay Input. Connect a capacitor from DEL to AGND to set the high-
voltage switch startup delay. A 5µA current source charges C
DEL
.
For the MAX1542, the high-voltage switch between SRC and COM is disabled until V
DEL
exceeds 1.24V. Following the delay period, CTL controls the state of the high-voltage
switch.
For the MAX1543, the switches between SRC, COM, and DRN are disabled and a 1kΩ
pulldown between COM and PGND is enabled until V
DEL
exceeds 1.24V. Following the
delay period, the 1kΩ pulldown is released and CTL controls the state of the high-voltage
switches (see the Delay Control Circuit section).
19 19 CTL
High-Voltage Switch Control Input. When CTL is high, the high-voltage switch between
COM and SRC is on and the high-voltage switches between COM and DRN (MAX1543)
are off. When CTL is low, the high-voltage switch between COM and SRC is off and the
high-voltage switches between COM and DRN (MAX1543) are on. CTL is inhibited by the
undervoltage lockout and when V
DEL
is less than 1.24V.
— 20 DRN
Switch Input. Drain of the internal high-voltage back-to-back P-channel MOSFETs
connected to COM.
Typical Application Circuits
The MAX1542 typical application circuit (Figure 1) and
the MAX1543 typical application circuit (Figure 2) gen-
erate an +8V source driver supply and approximately
+22V and -7V gate driver supplies for TFT displays. The
input voltage is from +2.6V to +5.5V. Table 1 lists rec-
ommended components and Table 2 lists contact infor-
mation for component suppliers.
Detailed Description
The MAX1542/MAX1543 include a high-performance
step-up regulator, two high-current operational ampli-
fiers, and startup timing and level-shifting functionality
useful for active matrix TFT LCDs. Figure 3 shows the
MAX1542/MAX1543 functional diagram.
Main Step-Up Converter
The MAX1542/MAX1543 main step-up converter
switches at 1.2MHz or 640kHz (MAX1543 only) (see the
Oscillator Frequency (FREQ) section). The devices
employ a current-mode, fixed-frequency, pulse-width
modulation (PWM) architecture to maximize loop band-
width providing fast transient response to pulsed loads
found in source drivers for TFT LCD panels. The high-
switching frequency also allows the use of low-profile
inductors and capacitors to minimize the thickness of
LCD panel designs. The integrated high-efficiency
MOSFET and the IC’s built-in digital soft-start function
reduce the number of external components required
while controlling inrush current. The output voltage of
the main step-up converter (V
MAIN
) can be set from V
IN
to 13V with an external resistive voltage-divider at FB.
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