Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
______________________________________________________________________________________ 31
To further optimize transient response, vary R
COMP
in
20% steps and C
COMP
in 50% steps while observing
transient response waveforms.
Charge-Pump Regulators
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest number
of charge-pump stages that meet the output requirement.
The number of positive charge-pump stages is given by:
GH DROPOUT AVDD
POS
SUPP D
V V - V
n
V - 2 V
+
=
×
where n
POS
is the number of positive charge-pump
stages, V
GH
is the output of the positive charge-pump
regulator, V
SUPP
is the supply voltage of the charge-
pump regulators, V
D
is the forward voltage drop of the
charge-pump diode, and V
DROPOUT
is the dropout
margin for the regulator. Use V
DROPOUT
= 300mV.
The number of negative charge-pump stages is given by:
GOFF DROPOUT
NEG
SUPN D
-V V
n
V - 2 V
+
=
×
where n
NEG
is the number of negative charge-pump
stages and V
GOFF
is the output of the negative charge-
pump regulator.
The above equations are derived based on the
assumption that the first stage of the positive charge
pump is connected to V
AVDD
and the first stage of
the negative charge pump is connected to ground.
Sometimes fractional stages are more desirable for
better efficiency. This can be done by connecting the
first stage to V
OUT
or another available supply. If the
first charge-pump stage is powered from V
OUT
, then the
above equations become:
GH DROPOUT OUT
POS
SUPP D
V V - V
n
V - 2 V
+
=
×
GOFF DROPOUT OUT
NEG
SUPN D
-V V V
n
V - 2 V
+ +
=
×
Flying Capacitors
Increasing the flying capacitor CX (connected to DRVP
and DRVN) value lowers the effective source impedance
and increases the output current capability. Increasing
the capacitance indefinitely has a negligible effect on
output current capability because the internal switch
resistance and the diode impedance place a lower limit
on the source impedance. A 0.1FF ceramic capacitor
works well in most low-current applications. The flying
capacitor’s voltage rating must exceed the following:
CX POS(NEG) SUPP(SUPN)
V n V> ×
where n
POS(NEG)
is the number of stages in which the
flying capacitor appears. It is the same as the number of
charge-pump stages.
Charge-Pump Output Capacitor
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:
LOAD_CP
OUT_CP
SW RIPPLE_CP
I
C
2 f V× ×
R
where C
OUT
_
CP
is the output capacitor of the charge
pump, I
LOAD
_
CP
is the load current of the charge pump,
and V
RIPPLE_CP
is the peak-to-peak value of the output
ripple.
Output Voltage Selection
Adjust the positive charge-pump regulator’s output
voltage by connecting a resistive voltage-divider from
VGH output to GND with the center tap connected to FBP
(Figure 1). Select the lower resistor of divider R4 in the
10kI to 30kI range. Calculate upper resistor R3 with the
following equation:
VGH
FBP
V
R3 R4 -1
V
 
= ×
 
 
where V
FBP
= 1.25V (typ).
Adjust the negative charge-pump regulator’s output
voltage by connecting a resistive voltage-divider from
V
GOFF
to REF with the center tap connected to FBN
(Figure 1). Select R6 in the 20kI to 68kI range. Calculate
R5 with the following equation:
FBN GOFF
REF FBN
V - V
R5 R6
V - V
= ×
where V
FBN
= 250mV, V
REF
= 1.25V. Note that REF
can only source up to 50FA, using a resistor less than
20kI, for R6 results in a higher bias current than REF
can supply.