MAX8728ETJ+T Maxim Integrated Products, MAX8728ETJ+T Datasheet - Page 18

• Manufacturer Part Number: MAX8728ETJ+T
• Description: Display Drivers Low-Cost Multiple-Ou tput Power Supply fo
• Manufacturer: Maxim Integrated Products

Specifications of MAX8728ETJ+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
The soft-start feature effectively limits the inrush current
during startup (see the Step-Down Regulator Soft-Start
(Heavy Load) waveforms in the Typical Operating
Characteristics). The MAX8728 monitors FB2 for under-
voltage conditions. 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.
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
ing voltage on C18 and C19 is greater than their asso-
Figure 3. Positive Charge-Pump Regulator Block Diagram
18
______________________________________________________________________________________
Positive Charge-Pump Regulator
REF
COMPARATOR
ERROR
OSC
POSITIVE CHARGE-PUMP REGULATOR
MAX8728
SUPP
D
CLK
FF
volts. If the result-
Q
LEVEL
SHIFT
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
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
V
point for approximately 50ms, the MAX8728 sets a fault
latch, shutting down all outputs except VL, REF, and the
step-down regulator.
FBP
is continuously below 80% of its nominal regulation
N1
P1
SUPP
GNDP
DRVP
FBP
SUPPLY
INPUT
C28
C21
C18
C19
C20
C15
OUTPUT
REF
/2,