MP1531 – LOW POWER, TRIPLE OUTPUT STEP-UP PLUS CHARGE PUMP FOR TFT BIAS
To insure stable operation place the input
capacitor as close to the IC as possible.
Alternately a smaller high quality 0.1µF ceramic
capacitor may be placed closer to the IC if the
larger capacitor is placed further away.
Selecting the Rectifier Diodes
Schottky diodes are recommended for most
applications because of their fast recovery time
and low forward voltage. Use Schottky diodes
with a current rating equal to or greater than 4
times the average output current, and a voltage
rating at least 1.5 times V
GH
for the positive
charge-pump and V
GL
for the negative charge-
pump. 100mA Schottky diodes such as Central
Semiconductor CMPSH-3 are recommended
for low current charge-pump circuits.
Selecting the Output Capacitor of the Step-
Up Converter
The output capacitor is required to maintain the
DC output voltage. Low ESR capacitors are
preferred to keep the output voltage ripple to a
minimum. The characteristics of the output
capacitor also affect the stability of the
regulation control system. A 10-22µF ceramic
capacitor works well in most applications. In the
case of ceramic capacitors, the impedance of
the capacitor at the switching frequency is
dominated by the capacitance, and so the
output voltage ripple is mostly independent of
the ESR. The output voltage ripple is estimated
to be:
V
RIPPLE
≅
( V
MAIN
−
V
IN
)
×
I
LOAD
V
MAIN
×
C2
×
f
SW
dropout margin for the linear regulator. The
positive charge-pump can also be configured
based on V
IN
for better efficiency. Then the
equation will be:
N
POS
≅
(
V
GH
+
V
DROPOUT
−
V
IN
)
V
MAIN
−
2
×
V
D
The number of negative charge-pump stages
N
NEG
is approximately given by:
N
NEG
≅
−
V
GL
+
V
DROPOUT
V
MAIN
−
2
×
V
D
Use V
DROPOUT
= 0.5V for positive charge-pump
and V
DROPOUT
= 0.15V for negative charge-pump.
Selecting the Flying Capacitor in Charge-
Pump Stages
Increasing the flying capacitor C
X
values
increases the output current capability. A
0.33µF ceramic capacitor works well in most
low current applications. The flying capacitor’s
voltage rating must exceed the following:
V
CX
>
N
×
V
MAIN
Where N is the stage number in which the flying
capacitor appears.
Step-Up Converter Compensation
The MP1531 uses current-mode control which
unlike voltage mode has only a single pole roll
off due to the output filter. The DC gain (A
V
DC
) is
equated from the product of current control to
output gain (A
V
CSCONTROL
), error amplifier gain
(A
V
EA
), and the feedback divider.
Av
DC
=
A
CSCONTROL
×
Av
EA
×
A
FB1
A
CSCONTROL
=
4
×
A
FB1
=
Av
DC
=
V
FB1
V
MAIN
V
IN
I
LOAD
Where V
RIPPLE
is the output ripple voltage, I
LOAD
is the load current, and C2 is the capacitance of
the output capacitor of the step-up converter.
Selecting the Number of Charge-Pump
Stages
For highest efficiency, always choose the
lowest number of charge-pump stages that
meets the output requirement.
The number of positive charge-pump stages
N
POS
is approximately given by:
N
POS
≅
1600
×
V
IN
×
V
FB1
I
LOAD
×
V
MAIN
The output filter pole is given in hertz by:
f
FILTERPOLE
=
I
LOAD
π ×
V
MAIN
×
C2
(
V
GH
+
V
DROPOUT
V
MAIN
−
V
MAIN
)
−
2
×
V
D
Where V
D
is the forward voltage drop of the
charge-pump diode, and V
DROPOUT
is the
MP1531 Rev. 1.2
5/22/2006
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9
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