MAX1542/MAX1543
TFT LCD DC-to-DC Converter with
Operational Amplifiers
______________________________________________________________________________________ 17
negative charge-pump output voltages for a given
V
MAIN
for one-, two-, and three-stage charge pumps,
based on the following equations:
where G_ON is the positive charge-pump output volt-
age, G_OFF is the negative charge-pump output volt-
age, n is the number of charge-pump stages, and V
D
is
the voltage drop across each diode.
V
D
is the forward voltage drop of the charge-pump
diodes.
Flying Capacitors
Increasing the flying capacitor (C3, C4, and C5) value
increases the output current capability. Increasing the
capacitance indefinitely has a negligible effect on out-
put current capability because the internal switch resis-
tance and the diode impedance limit the source
impedance. A 0.1µF 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 capaci-
tor appears, and V
MAIN
is the main output voltage. For
example, the two-stage positive charge pump in the
Typical Application Circuits (Figures 1 and 2) where
V
MAIN
= 8V contains two flying capacitors. The flying
capacitor in the first stage (C5) requires a voltage rat-
ing greater than 8V. The flying capacitor in the second
stage (C4) requires a voltage rating greater than 16V.
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:
where V
RIPPLE
is the acceptable peak-to-peak output-
voltage ripple.
Charge-Pump Rectifier Diodes
To maximize the available output voltage, use Schottky
diodes with a current rating equal to or greater than two
times the average charge-pump input current. If the
loaded charge-pump output voltage is greater than
required, some or all of the Schottky diodes can be
replaced with low-cost silicon switching diodes with an
equivalent current rating. The charge-pump input cur-
rent is:
where n is the number of charge-pump stages.
OUTPUT VOLTAGE vs. V
OUTPUT VOLTAGE vs. V
(25 paginas)
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Comentarios a estos manuales