MAX8753ETI-T Maxim Integrated, MAX8753ETI-T Datasheet - Page 17

• Manufacturer Part Number: MAX8753ETI-T
• Description: LCD Drivers
• Manufacturer: Maxim Integrated

The MAX8753’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward voltage. In general, a 2A Schottky
diode complements the internal MOSFET well.
The output voltage of the main step-up regulator can be
adjusted by connecting a resistive voltage-divider from
the output (V
to FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ
range. Calculate R1 with the following equation:
where V
is 1.245V. Place R1 and R2 close to the IC.
For stability, add a pole-zero pair from FB to GND in the
form of a series resistor (R
(C
value of the R2 feedback resistor. To further optimize
transient response, vary R
C
response waveforms.
Adjust the positive charge-pump regulator output volt-
age by connecting a resistive voltage-divider from the
regulator output V
nected to FBP (Figure 1). Select the lower resistor of
the divider R6 in the 10kΩ to 50kΩ range. Calculate
upper resistor R5 with the following equation:
where V
positive charge-pump regulator.
Adjust the negative charge-pump regulator output volt-
age by connecting a resistive voltage-divider from the
negative charge-pump output V
center tap connected to FBN (Figure 1). Select R4 in
the 20kΩ to 100kΩ range. Calculate R3 with the follow-
ing equation:
COMP
COMP
FB
FBP
). R
in 50% steps while observing transient
, the step-up regulator’s feedback set point,
MAIN
COMP
= 1.25V (typ) is the regulation point of the
R
3
R
) to GND with the center tap connected
POS
R
5
=
______________________________________________________________________________________
1
should be approximately half the
=
R
=
R
4
R
to GND with the center tap con-
6
×
Charge-Pump Regulators
2
×
V
V
×
FBN



REF



V
V
V
POS
FBP
Output-Voltage Selection
Output Voltage Selection
COMP
MAIN
V
−
−
FB
COMP
V
V
NEG
FBN
−
Loop Compensation
NEG
1
−



in 20% steps and
1



) and capacitor
to REF with the
Rectifier Diode
TFT LCD DC-DC Converter with
Integrated Charge Pumps
where V
tion point of the negative charge-pump regulator.
Increasing the flying capacitor (C
effective source impedance and increases the output-
current capability of the charge pump. Increasing the
capacitance indefinitely has a negligible effect on out-
put-current capability because the internal switch resis-
tance and the diode impedance place a lower limit on
the source impedance. A 0.1µF ceramic capacitor
works well in most low-current applications. The flying
capacitor’s voltage rating must exceed the following:
Use an input capacitor on SUPP and SUPN with a value
equal to or greater than the flying capacitors on that
charge pump. Place the capacitors as close to SUPP
and SUPN as possible. Connect the capacitors directly
to PGND.
Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-to-
peak transient voltage. With ceramic capacitors, the
output-voltage ripple is dominated by the capacitance
value. Use the following equation to approximate the
required capacitor value:
where C
pump, I
pump, and V
value of the output ripple.
Use low-cost silicon switching diodes for D2 and D3
with a current rating equal to or greater than two times
the average charge-pump input current. If it helps
avoid an extra stage, some or all of the diodes can be
replaced with Schottky diodes with an equivalent cur-
rent rating.
REF
LOAD_CP
OUT_CP
C
= 1.25V, and V
OUT CP
RIPPLE_CP
is the output capacitor of the charge
_
is the load current of the charge
V
V
V
V
C
C
C
C
Charge-Pump Output Capacitor
1
2
3
4
≥
Charge-Pump Input Capacitor
>
>
>
>
2
Charge-Pump Rectifier Diode
f
is the desired peak-to-peak
OSC RIPPLE CP
V
2
2
V
SUPP
V
SUPN
V
FBN
I
SUPP
SUPN
LOAD CP
V
= 250mV is the regula-
,
_
if used
X
) value lowers the
Flying Capacitor
_
17