Rainbow Electronics MAX8728 - Manual (Page 19)

MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
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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 4.
The error comparator compares the feedback signal
(FBN) with a 250mV internal reference. If the feedback
signal is above the reference, the charge-pump regula-
tor turns on N2 and turns off P2 when the rising edge of
the oscillator clock arrives, level shifting the flying
capacitor (C16). The falling edge of the oscillator clock
turns off N2 and turns on P2, charging the flying capac-
itor (C16) through the diode that connects it to the
reservoir capacitor (C1). If the feedback signal is below
the reference (output is in regulation) when the rising
edge of the oscillator comes, the regulator ignores this
clock edge and keeps P2 on and N2 off.
The negative charge-pump regulator is enabled when
SHDN and EN are logic high and the step-down regula-
tor reaches regulation. Each time it is enabled, the neg-
ative charge-pump regulator goes through a soft-start
routine by ramping down its internal reference voltage
from 2V to 250mV in 128 steps. The soft-start period is
3ms (typ) and FBN fault detection is disabled during
this period. The soft-start feature effectively limits the
inrush current during startup. The MAX8728 also moni-
tors the FBN voltage for undervoltage conditions. If
V
FBN is continuously above 600mV for approximately
50ms, the MAX8728 sets a fault latch, shutting down all
outputs except VL, REF, and the step-down regulator.
High-Voltage Switch Control
The MAX8728’s high-voltage switch control block
(Figure 5) consists of two high-voltage, p-channel
MOSFETs: Q1, between SRC and GON and Q2,
between GON and DRN. The switch control block is
enabled when V
DEL goes above V REF / 2. Q1 and Q2
are controlled by CTL and MODE. There are two differ-
ent modes of operation (see the Typical Operating
Characteristics section).
Activate the first mode by connecting MODE to REF.
When CTL is logic high, Q1 turns on and Q2 turns off,
connecting GON to SRC. When CTL is logic low, Q1
0.25V
Q
CLK
D
OSC
FF
ERROR
COMPARATOR
P2
N2
DRVN
GND1
IN
FBN
NEGATIVE CHARGE-PUMP REGULATOR
INPUT
SUPPLY
OUTPUT
REF
MAX8728
C17
C16
C1
Figure 4. Negative Charge-Pump Regulator Block Diagram