MAX8739 Datasheet, PDF (14/19 Pages) Maxim Integrated Products – TFT, LCD, DC-DC Converter with Operational Amplifiers

TFT, LCD, DC-DC Converter with
Operational Amplifiers
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.
As the operational amplifier’s capacitive load increases,
the amplifier’s bandwidth decreases and gain peaking
increases. A 5Ω to 50Ω 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 10pF.
Switch Control and Delay
A capacitor C_DEL (C8 in Figure 1), from DEL to AGND
selects the switch-control block supply startup delay.
After the LDO voltage exceeds its undervoltage lockout
threshold (2.7V typ) and the soft-start routine for each
regulator is complete, a 5µA current source charges
C_DEL. Once the capacitor voltage exceeds V_REF (1.25V
typ), COM can be connected to SRC or DRN through
the internal p-channel switches, depending upon 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 to set the delay time using
the following equation:
C_DEL =
DELAY _ TIME
x
5µ A
1.25V
The switch-control input (CTL) is not activated until all
three of the following conditions are satisfied: the LDO
voltage exceeds its undervoltage lockout voltage, the
soft-start routine of all the regulators is complete, and
V_DEL exceeds its turn-on threshold. Once activated and
if CTL is high, the 15Ω internal p-channel switch
between COM and SRC (Q1) turns on and the 30Ω p-
channel switch between DRN and COM (Q2) turns off.
If CTL is low, Q1 turns off and Q2 turns on.
LDO
5µA
2.7V
DLP
REF
CTL
SRC
Q1
COM
Q2
DRN
Figure 4. Switch Control
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