Positive Charge-Pump Regulator
The positive charge-pump regulator is typically used to
generate the positive supply rail for the TFT LCD gate
driver ICs. The positive charge pump is a two-stage
charge pump with external pump and reservoir capaci-
tors. The output voltage is set with an external resistive
voltage-divider from its output to GND with the midpoint
connected to FBP. The charge pump includes internal
switches with drivers to control the power transfer.
Figure 4 shows the block diagram of the positive
charge pump.
The controller regulates the positive charge-pump output
voltage so that V
FBP
= V
REF
. If V
FBP
goes below the refer-
ence, P1 and P3 are turned on when the rising edge of
the oscillator arrives, while the drivers pull C1N to GND
and C2N to SUP. The first stage flying capacitor C6
(Figure 4) is charged by the charge-pump supply input
and the second stage flying capacitor C7 is discharged
into the reservoir capacitor C8. After a fixed period, P2 is
turned on and P1 and P3 are turned off while C1N is
pulled to SUP and C2N is pulled to GND. C7 is charged
by C6 in preparation for the next pump cycle.
The MAX8784 implements a digital variable-resistance
algorithm to control the current delivered to the output.
The algorithm sets the on-resistance of the positive
charge-pump drivers according to the load current. The
on-resistance of the drivers is set by counting the num-
ber of charging pulses in the previous 12 oscillator
cycles. As the number of charging pulses in the previ-
ous 12 oscillator cycles increases, the on-resistance of
the switch is reduced. The number of charging pulses in
the previous 12 oscillator cycles is a measure of the load
current. The period of C1N and C2N switching is 1.66µs.
The positive charge-pump regulator’s startup can be
delayed by connecting an external capacitor from
GDEL to GND. An internal constant-current source
begins charging the GDEL capacitor when the negative
charge pump reaches regulation. When the GDEL volt-
age exceeds V
REF
, the positive charge-pump regulator
is enabled. Each time it is enabled, the positive charge-
pump regulator goes through a soft-start routine by
ramping up its internal reference voltage from 0 to V
REF
in 128 steps. The soft-start period is 3ms (typ) and FBP
fault detection is disabled during this period. The soft-
start feature effectively limits the inrush current during
startup.
The MAX8784 monitors the FBP and SUP voltage to
detect fault conditions. If V
FBP
is continuously below
80% of its nominal regulation point for approximately
55ms, the MAX8784 sets a fault latch, shutting down all
outputs except the reference and operational amplifiers.
If SUP exceeds the SUP overvoltage faut threshold (20V,
typ), LX switching is inhibited until SUP decreases.
Furthermore, if SUP exceeds its overvoltage charge-
pump inhibit level (21V, typ), positive charge-pump
switching is inhibited until SUP decreases to prevent
damage to the charge pump.
MAX8784
Step-Up Regulator, Internal Charge Pumps, Switch
Control, and Operational Amplifier for TFT LCDs
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