Rainbow Electronics MAX17075 - Manual (Page 15)

MAX17075
Boost Regulator with Integrated Charge Pumps,
Switch Control, and High-Current Op Amp
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Negative Charge-Pump Regulator
The negative charge-pump regulator is typically used to
generate the negative supply rail for the TFT LCD gate
driver ICs. The output voltage is set with an external
resistive voltage-divider from its output to REF with the
midpoint connected to FBN. The number of charge-
pump stages and the setting of the feedback divider
determine the output of the negative charge-pump regu-
lator. The charge-pump controller includes a high-side p-
channel MOSFET (P2) and a low-side n-channel MOSFET
(N2) to control the power transfer as shown in Figure 5.
The error amplifier compares the feedback signal (FBN)
with a 250mV internal reference. If the feedback signal
is above the reference, the charge-pump regulator
turns on N2 and turns off P2 when the rising edge of the
oscillator clock arrives, level shifting C11. This connects
C11 in parallel with reservoir capacitor C10. If the volt-
age across C10 minus a diode drop (V
C10 - V DIODE ) is
higher than the level-shifted flying capacitor voltage
(-V
C11 ), charge flows from C10 to C11 until diode D4-2
turns off. The falling edge of the oscillator clock turns
off N2 and turns on P2, allowing V
SUP
to charge up fly-
ing capacitor C11 through diode D4-1. If the feedback
signal is below the reference when the rising edge of
the oscillator comes, the regulator ignores this clock
edge and keeps P2 on and N2 off.
The MAX17075 also monitors the FBN voltage for
undervoltage conditions. If the V
FBN is continuously
below 80% of the nominal regulation voltage (V REF -
V FBN ) for approximately 50ms, the MAX17075 sets a
fault latch, shutting down all outputs except REF. Once
the fault condition is removed, cycle the input voltage
(below the UVLO falling threshold) to clear the fault
latch and reactivate the device.
Operational Amplifiers
The MAX17075 has one operational amplifier. The oper-
ational amplifier is typically used to drive the LCD back-
plane (VCOM) or the gamma-correction divider string. It
features ±500mA output short-circuit current, 45V/µs
slew rate, and 20MHz, 3dB bandwidth.
Figure 5. Negative Charge-Pump Regulator Block Diagram
OSC
REF
0.25V
ERROR
AMPLIFIER
NEGATIVE CHARGE-PUMP REGULATOR
MAX17075
P2
N2
SUP
INPUT
SUPPLY
GOFF
REF
C10
C11
D4-1
D4-2
DRVN
FBN
R7
R6