Rainbow Electronics MAX8728 - Manual (Page 18)

MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
18 ______________________________________________________________________________________
The soft-start feature effectively limits the inrush current
during startup (see the Step-Up Regulator Soft-Start
Waveforms in the Typical Operating Characteristics).
The MAX8728 monitors FB2 for undervoltage condi-
tions. If the voltage is continuously below 90% of the
nominal regulation point for approximately 50ms, the
MAX8728 sets a fault latch, shutting down all outputs
except VL, REF, and the step-down regulator.
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 output voltage is set with an external
resistive voltage-divider from its output to GND with the
midpoint connected to FBP. The number of charge-
pump stages and the setting of the feedback divider
determine the output voltage of the positive charge-
pump regulator. The charge pump includes a high-
side, p-channel MOSFET (P1) and a low-side,
n-channel MOSFET (N1) to control the power transfer
as shown in Figure 3.
The error comparator compares the feedback signal
(FBP) with a 2.0V internal reference. If the feedback
signal is below the reference, the charge-pump regula-
tor turns on P1 and turns off N1 when the rising edge of
the oscillator clock arrives, level shifting the flying
capacitors (C18 and C19) by V
SUPP volts. If the result-
ing voltage on C18 and C19 is greater than their asso-
ciated reservoir capacitors (C20 and C15), charge
flows until the diode connecting each flying capacitor
to its reservoir capacitor turns off. The falling edge of the
oscillator clock turns off P1 and turns on N1, charging
the flying capacitors (C18 and C19) through the diodes
that connect them to the reservoir capacitors (C21 and
C20). If the feedback signal is above the reference
when the rising edge of the oscillator comes, the regula-
tor ignores this clock edge and keeps N1 on and P1 off.
The positive charge-pump regulator’s startup can be
delayed by connecting an external capacitor from DEL to
GND. An internal constant-current source begins charg-
ing the DEL capacitor when EN and
SHDN are logic high,
the negative charge pump reaches regulation, and GATE
has gone low. When the DEL voltage exceeds V
REF /2,
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 ref-
erence voltage from 0 to 2V 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 MAX8728 also
monitors the FBP voltage for undervoltage conditions. If
VFBP is continuously below 80% of its nominal regulation
point for approximately 50ms, the MAX8728 sets a fault
latch, shutting down all outputs except VL, REF, and the
step-down regulator.
LEVEL
SHIFT
REF
Q
CLK
D
OSC
FF
ERROR
COMPARATOR
P1
N1
DRVP
C21
C20
C15
C28
C19
C18
GNDP
SUPP
FBP
POSITIVE CHARGE-PUMP REGULATOR
INPUT
SUPPLY
OUTPUT
MAX8728
Figure 3. Positive Charge-Pump Regulator Block Diagram