Datasheet

Step-Up, Step-Down Regulator, Gate-On Charge Pump,
and Boost-Buck Regulator for TV TFT LCD Display
MAX17122
______________________________________________________________________________________ 31
regulator, V
AVDD
is the step-up regulator output and is
also the supply voltage of the charge-pump regulators,
V
D
is the forward voltage drop of charge-pump diode D4,
and V
PNP
is the voltage across the pnp transistor P1 emit-
ter and collector. For a doubler configuration, n
POS
= 1.
The previous equation is derived based on the assump-
tion that the first stage of the positive charge pump is
connected to V
AVDD
. Sometimes fractional stages are
more desirable for better efficiency. This can be done
by connecting the first stage to another available supply
V
INCP
. If the first charge-pump stage is powered from
V
INCP
, then the previous equation becomes:
GON PNP INCP
POS
AVDD D
V V - V
n
V - 2 V
+
=
×
Flying Capacitors
Increasing the flying capacitor C
FLY
(connected to
LX1) 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 diode impedance place a lower limit on the source
impedance. A 0.1FF ceramic capacitor works well in
most low-current applications. The flying capacitor’s volt-
age rating must exceed the following:
CFLY POS AVDD
V n V> ×
where n
POS
is the number of stages in which the
flying capacitor appears. It is the same as the number of
charge-pump stages.
Charge-Pump Output Capacitor
Increasing the output capacitance, or decreasing the
ESR, reduces the output-voltage ripple 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:
GON
GON
SW RIPPLE_GON
I
C
2 f V
≥
× ×
where C
GON
is the output capacitor of the charge pump, I
GON
is the load current of the charge pump, and V
RIPPLE_GON
is
the peak-to-peak value of the output ripple.
Output-Voltage Selection
Adjust the charge-pump regulator’s output voltage by
connecting a resistive voltage-divider from the V
GON
output to AGND with the center tap connected to FBP
(Figure 1). Select the lower resistor of divider R6 in the
10kI to 30kI range. Calculate upper resistor R5 with the
following equation:
GON
FBP
V
R5 R6 - 1
V
 
= ×
 
 
where V
FBP
= 1.25V (typical).
Charge-Pump Rectifier Diodes
Use low-cost silicon switching diodes with a current
rating 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. A
small resistor (R
P
) in series with charging diode D4 is
usually required to reduce the magnitude of the current
pulses into the step-up regulator switching node LX,
which can cause its current mode control to terminate
LX1 pulses too early. The value of this small resistor is
determined by the available charge-pump headroom
according to the following question:
HEADROOM CP P
V I R= ×
where I
CP
is the charging current and R
P
is the series
resistor. Normally, a 2I to 5I resistor is sufficient for this
purpose.
Pass-Transistor Selection
The pass transistor must meet specifications for current
gain (h
FEP
), input capacitance, collector-emitter saturation
voltage, and power dissipation. The transistor’s current
gain limits the guaranteed maximum output current to:
BEP
CP(MAX) DRVP FEP(MIN)
BEP
V
I (I - ) h
R
= ×
where I
DRVP
is the minimum guaranteed base-drive
current, V
BEP
is the pnp transistor’s base-to-emitter for-
ward-voltage drop, and R
BEP
is the pullup resistor con-
nected between the pnp transistor’s base and emitter.
Furthermore, the transistor’s current gain increases the
linear regulator’s DC loop gain so excessive gain desta-
bilizes the output. Therefore, transistors with current gain
over 100 at the maximum output current can be difficult
to stabilize and are not recommended unless the high
gain is needed to meet the load-current requirements.