Datasheet

MAX1513/MAX1514
TFT-LCD Power-Supply Controllers
24 ______________________________________________________________________________________
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output volt-
age requirement. Figures 13 and 14 show the positive
and negative charge-pump output voltages for a given
V
MAIN
for one-, two-, and three-stage charge pumps.
The number of positive charge-pump stages is given by:
where n
POS
is the number of positive charge-pump
stages, V
GON
is the gate-on linear-regulator REG P out-
put, V
MAIN
is the main step-up regulator output, V
D
is
the forward-voltage drop of the charge-pump diode,
and V
DROPOUT
is the dropout margin for the linear reg-
ulator. Use V
DROPOUT
= 0.3V.
The number of negative charge-pump stages is given by:
where n
NEG
is the number of negative charge-pump
stages, V
GOFF
is the gate-off linear-regulator REG N
output, V
MAIN
is the main step-up regulator output, V
D
is the forward-voltage drop of the charge-pump diode,
and V
DROPOUT
is the dropout margin for the linear reg-
ulator. Use V
DROPOUT
= 0.3V.
The above equations are derived based on the
assumption that the first stage of the positive charge
pump is connected to V
MAIN
and the first stage of the
negative charge pump is connected to ground.
Sometimes fractional stages are more desirable for bet-
ter efficiency. This can be done by connecting the first
stage to V
IN
or another available supply. If the first
charge-pump stage is powered from V
IN
, then the
above equations become:
Flying Capacitors
Increasing the flying capacitor (C
X
) value lowers the
effective source impedance and increases the output-
current capability. Increasing the capacitance indefinite-
ly has a negligible effect on output-current capability
because the switch resistance and the diode impedance
place a lower limit on the source impedance. A 0.1µF
ceramic capacitor works well in most low-current appli-
cations. The flying capacitor’s voltage rating must
exceed the following:
where n is the stage number in which the flying capaci-
tor appears, and V
MAIN
is the output voltage of the
main step-up regulator.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-to-
peak voltage during load transients. With ceramic
VnV
CX MAIN
>×
n
VV V
VV
n
VV V
VV
POS
GON DROPOUT IN
MAIN D
NEG
GOFF DROPOUT IN
MAIN D
=
+
×
=
+
×
-
-
-+
-
2
2
n
VV
VV
NEG
GOFF DROPOUT
MAIN D
=
+
×
-
- 2
n
VV V
VV
POS
GON DROPOUT MAIN
MAIN D
=
+
×
-
- 2
POSITIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. V
MAIN
V
MAIN
(V)
G_ON (V)
1210864
10
20
30
40
50
60
0
214
2-STAGE CHARGE-PUMP
3-STAGE CHARGE-PUMP
V
D
= 0.3V TO 1V
1-STAGE CHARGE-PUMP
Figure 13. Positive Charge-Pump Output Voltage vs. V
MAIN
NEGATIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. V
MAIN
V
MAIN
(V)
G_OFF (V)
1210864
-40
-35
-30
-25
-20
-15
-10
-5
-0
-45
214
1-STAGE
CHARGE-PUMP
2-STAGE
CHARGE-PUMP
3-STAGE
CHARGE-PUMP
V
D
= 0.3V TO 1V
Figure 14. Negative Charge-Pump Output Voltage vs. V
MAIN