MAX1516 Datasheet, PDF (21/26 Pages) Maxim Integrated Products – TFT-LCD DC-DC Converters with Operational Amplifiers

TFT-LCD DC-DC Converters with
Operational Amplifiers
V_IN
V_MAIN
>13V
STEP-UP
CONTROLLER
MAX1516
MAX1517
MAX1518
LX
FB
PGND
Figure 8. Operation with Output Voltages >13V Using
Cascoded MOSFET
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.
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
Flying Capacitors
Increasing the flying-capacitor (C_X) value lowers the
effective source impedance and increases the output-
current capability. Increasing the capacitance indefi-
nitely has a negligible effect on output-current capabili-
ty 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:
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 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 _ CP}
≥
I_{LOAD _ CP}
2f_OSC V_{RIPPLE _ CP}
where C_{OUT_CP} is the output capacitor of the charge
pump, I_{LOAD_CP} is the load current of the charge
pump, and V_{RIPPLE_CP} is the peak-to-peak value of the
output ripple.
Charge-Pump Rectifier Diodes
Use low-cost silicon switching diodes with a current rat-
ing equal to or greater than two times the average
charge-pump input current. If it helps avoid an extra
stage, some or all of the diodes can be replaced with
Schottky diodes with an equivalent current rating.
Linear-Regulator Controllers
Output-Voltage Selection
Adjust the gate-on linear-regulator (REG P) output volt-
age by connecting a resistive voltage-divider from the
REG P output to AGND with the center tap connected
to FBP (Figure 1). Select the lower resistor of the divider
R5 in the range of 10kΩ to 30kΩ. Calculate the upper
resistor R4 with the following equation:
R4
= R5
×


V_GON
V_FBP

− 1_
where V_FBP = 1.25V (typ).
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