Datasheet

MAX1997/MAX1998
Quintuple/Triple-Output TFT LCD Power Supplies
with Fault Protection and VCOM Buffer
26 ______________________________________________________________________________________
Flying Capacitors
Increasing the flying capacitor (C
X
) value 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 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 circuit (Figure 1) where V
MAIN
= 9V
contains two flying capacitors. The flying capacitor in
the first stage (C25) requires a voltage rating greater
than 9V. The flying capacitor in the second stage (C24)
requires a voltage rating greater than 18V.
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 peak-to-peak value of the output
ripple.
Charge-Pump Rectifier Diodes
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 current is:
I
CP
_
IN
= I
CP
_
OUT
✕
N
where N is the number of charge-pump stages.
Linear-Regulator Controllers
Output Voltage Selection
Adjust the positive linear-regulator (REG P) output volt-
age by connecting a resistive voltage-divider from
V
G_ON
to GND with the center tap connected to FBP
(Figure 1). Select R20 in the range of 10kΩ to 30kΩ.
Calculate R19 with the following equation:
R19 = R20 [(V
G_ON
/ V
FBP
) - 1]
where V
FBP
= 1.25V.
The output voltages of linear regulators REG 1 and
REG 2 can be similarly adjusted.
Adjust the negative linear-regulator (REG N) output
voltage by connecting a resistive voltage-divider from
V
G_OFF
to REF with the center tap connected to FBN
(Figure 1). Select R17 in the range of 10kΩ to 30kΩ.
Calculate R16 with the following equation:
R16 = R17 [(V
FBN
- V
G_OFF
) / (V
REF
- V
FBN
)]
where V
FBN
= 125mV, V
REF
= 1.25V. REF can source
up to 75µA, using a resistor greater than 17kΩ for R17
leaves at least 10µA for other uses.
Pass Transistor Selection
The pass transistor must meet specifications for current
gain (β), input capacitance, collector-emitter saturation
voltage, and power dissipation. The transistor’s current
gain limits the guaranteed maximum output current to:
where I
DRV
is the minimum guaranteed base drive cur-
rent, V
BE
is the base-to-emitter voltage of the transistor,
and R
BE
is the pullup resistor connected between the
transistor’s base and emitter. Furthermore, the transis-
tor’s current gain increases the linear regulator’s DC
loop gain (see the Stability Requirements section), so
excessive gain destabilizes the output. Therefore, tran-
sistors with current gain over 100 at the maximum out-
put current can be difficult to stabilize and are not
recommended. The transistor’s input capacitance and
input resistance also create a second pole, which
could be low enough to make the output unstable when
heavily loaded.
The transistor’s saturation voltage at the maximum out-
put current determines the minimum input-to-output
voltage differential that the linear regulator supports.
Alternatively, the package’s power dissipation could
limit the usable maximum input-to-output voltage differ-
ential. The maximum power dissipation capability of the
transistor’s package and mounting must exceed the
actual power dissipation in the device. The power dissi-
pation equals the maximum load current times the max-
imum input-to-output voltage differential:
P = I
LOAD(MAX)
(V
LDOIN
- V
LDOOUT
) = I
LOAD(MAX)
V
CE
II
V
R
LOAD(MAX) DRV
BE
BE
MIN
=






− β
C
I
V
OUT
LOAD
OSC RIPPLE
≥
2f