MAX1513 Datasheet, PDF (24/28 Pages) Maxim Integrated Products – TFT-LCD Power-Supply Controllers

TFT-LCD Power-Supply Controllers
POSITIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. V_MAIN
60
V_D = 0.3V TO 1V ^{3-STAGE CHARGE-PUMP}
50
40
2-STAGE CHARGE-PUMP
30
20
10
1-STAGE CHARGE-PUMP
0
2 4 6 8 10 12 14
V_MAIN (V)
Figure 13. Positive Charge-Pump Output Voltage vs. VMAIN
NEGATIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. V_MAIN
-0
1-STAGE
-5
CHARGE-PUMP
-10
-15
-20
2-STAGE
-25 ^CHARGE-PUMP
-30
3-STAGE
-35
CHARGE-PUMP
-40 _{VD = 0.3V TO 1V}
-45
2468
10 12 14
V_MAIN (V)
Figure 14. Negative Charge-Pump Output Voltage vs. VMAIN
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:
n_POS
=
V_GON + V_DROPOUT - V_MAIN
V_MAIN - 2 × V_D
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:
n_POS
=
V_GON + V_DROPOUT -
V_MAIN - 2 × V_D
V_IN
n_NEG
=
-V_GOFF + V_DROPOUT +
V_MAIN - 2 × V_D
V_IN
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:
n_NEG
=
-V_GOFF + V_DROPOUT
V_MAIN - 2 × V_D
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.
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:
V_CX > n × V_MAIN
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
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