MAX1542 Datasheet, PDF (17/20 Pages) Maxim Integrated Products – TFT LCD DC-to-DC Converter with Operational Amplifiers

TFT LCD DC-to-DC Converter with
Operational Amplifiers
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 5. Positive Charge-Pump Output Voltage vs. V_MAIN
negative charge-pump output voltages for a given
V_MAIN for one-, two-, and three-stage charge pumps,
based on the following equations:
G_ ON = V_MAIN + n(V_MAIN − V_D)
G_ OFF = − n(V_MAIN − V_D)
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-
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 6. Negative Charge-Pump Output Voltage vs. V_MAIN
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:
C_OUT
≥
2
×
I_LOAD
F_OSC × V_RIPPLE
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:
I_{CP_IN} = I_{CP_ OUT} × n
where n is the number of charge-pump stages.
______________________________________________________________________________________ 17