MAX1542 Datasheet, PDF (12/20 Pages) Maxim Integrated Products – TFT LCD DC-to-DC Converter with Operational Amplifiers

TFT LCD DC-to-DC Converter with
Operational Amplifiers
G_OFF
-7V AT 20mA
V_IN
2.6V TO 5.5V
C1
10µF
R8
100kΩ
C11
220pF
C10
33nF
C3
0.1µF
C4
0.1µF
D4
G_ON
+22V AT 20mA
C2
D2
0.1µF
L1
4.7µF
C5
0.1µF
D1
C6
C7
D3
0.1µF _0.1µF
R1
75kΩ
C8
4.7µF
V_MAIN
+8V AT 250mA
C9
4.7µF
IN
COMP
LX
FB
SUP
MAX1542
POS1
POS2
SRC
CTL
COM
DEL
PGND
NEG1
OUT1
NEG2
OUT2
AGND
R5
40kΩ
R2
13.7kΩ
R3
40kΩ
R6
40kΩ
R4
40kΩ
TO VCOM
BACKPLANE
Figure 1. MAX1542 Typical Application Circuit
Delay Control Circuit
A capacitor from DEL to AGND selects the switch control
block supply startup delay. After the input voltage
exceeds V_UVLO, a 5µA current source charges C_DEL.
Once the capacitor voltage exceeds the turn-on thresh-
old (1.24V) COM can be connected to SRC, depending
on the state of CTL. Before startup and when IN is less
than V_UVLO, DEL is internally connected to AGND to dis-
charge C_DEL. Select C_DEL using the following equation:
C_DEL
=
(DELAY
TIME)
×
5µA
1.24 V
MAX1542 Control Block Switch
The switch control input (CTL) is not activated until
V_DEL exceeds the turn-on voltage (1.24V) and the input
voltage (V_IN) exceeds V_UVLO (2.5V). Once activated,
CTL controls the P-channel MOSFET, between COM
and SRC. A high at CTL turns on Q1 between SRC and
COM, and a low at CTL turns Q1 off (Figure 4).
MAX1543 Control Block Switch
The switch control input (CTL) is not activated until the
input voltage (V_IN) exceeds V_UVLO (2.5V) and V_DEL
exceeds the turn-on voltage (1.24V). During UVLO or
when DEL is below the turn-on threshold, COM is
pulled low to PGND through Q3 and a 1kΩ resistance.
Once activated, CTL controls the COM MOSFETs,
switching COM between SRC and DRN. A high at CTL
turns on Q1 and disables Q2. A low at CTL turns on Q2
and turns off Q1 (Figure 4).
Undervoltage Lockout (UVLO)
The UVLO comparator of the MAX1542/MAX1543 com-
pares the input voltage at IN with the UVLO threshold
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