Datasheet

MAX8795A
TFT-LCD DC-DC Converter with
Operational Amplifiers
22 ______________________________________________________________________________________
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
, 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 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 imped-
ance. A 0.1µF ceramic capacitor works well in most
low-current applications. 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.
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 8. 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 9. Negative Charge-Pump Output Voltage vs. V
MAIN