MP1530 – TRIPLE OUTPUT STEP-UP PLUS LINEAR REGULATORS FOR TFT BIAS
Selecting the Rectifier Diodes
The number of negative charge-pump stages
The MP1530’s high switching frequency
demands high-speed rectifiers. Schottky diodes
are recommended for most applications
because of their fast recovery time and low
forward voltage. Typically, a 1A Schottky diode
is recommended for the step-up converter.
100mA Schottky diodes such as Central
Semiconductor CMPSH-3 are recommended
for low current charge-pump circuits.
N
NEG is given by:
−VGL + VDROPOUT
VMAIN − 2VD
NNEG
=
Use VDROPOUT = 1V for positive charge-pump
and VDROPOUT = 0.3V for negative charge-pump.
Selecting the Flying Capacitor in Charge-
Pump Stages
Increasing the flying capacitor CX values
increases the output current capability. A 0.1µF
ceramic capacitor works well in most low
current applications. The flying capacitor’s
voltage rating must exceed the following:
Selecting the Output Capacitor of the
Step-Up Converter
The output capacitor is required to maintain the
DC output voltage. Low ESR capacitors are
preferred to keep the output voltage ripple to a
minimum. The characteristics of the output
capacitor also affect the stability of the
regulation control system. A 10µF ceramic
capacitor works well in most applications. In the
case of ceramic capacitors, the impedance of
the capacitor at the switching frequency is
dominated by the capacitance, and so the
output voltage ripple is mostly independent of
the ESR. The output voltage ripple is estimated
to be:
VCX > N× VMAIN
Where N is the stage number in which the flying
capacitor appears.
Step-Up Converter Compensation
The MP1530 uses current mode control which
unlike voltage mode has only a single pole roll
off due to the output filter. The DC gain (AV ) is
equated from the product of current control to
output gain (AVCSCONTROL), error amplifier gain
DC
(AV ), and the feedback divider.
EA
⎛
⎜
⎝
⎞
⎟
⎟
⎠
VIN
ILOAD
⎜
VRIPPLE ≅ 1−
×
AvDC = ACSCONTROL × AvEA × AFB1
VMAIN
C2× fSW
VIN
ACSCONTROL = 4 ×
ILOAD
Where VRIPPLE is the output ripple voltage, ILOAD
is the load current, and C2 is the capacitance of
the output capacitor of the step-up converter.
VFB1
AFB1
=
Selecting the Number of Charge-Pump
Stages
For highest efficiency, always choose the
lowest number of charge-pump stages that
meets the output requirement.
VMAIN
1600 × VIN × VFB1
ILOAD × VMAIN
AvDC
=
The output filter pole is given in hertz by:
ILOAD
The number of positive charge-pump stages
NPOS is given by:
fFILTERPOLE
=
π × VMAIN × C2
VGH − VDROPOUT − VMAIN
VMAIN − 2VD
NPOS
=
The output filter zero is given in hertz by:
1
Where VD is the forward voltage drop of the
charge-pump diode, and VDROPOUT is the
dropout margin for the linear regulator.
fFILTERZERO
=
2π × RESR × C2
Where RESR is the output capacitor’s equivalent
series resistance.
MP1530 Rev. 1.4
5/19/2006
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