MP1530 – TRIPLE OUTPUT STEP-UP PLUS LINEAR REGULATORS FOR TFT BIAS
APPLICATION INFORMATION
saturate under the worst-case load transient
and startup conditions.
COMPONENT SELECTION
Setting the Output Voltages
Set the output voltage on each output by
selecting the resistive voltage divider ratio. The
voltage divider drops the output voltage to the
feedback threshold voltage. Use 10kΩ to 50kΩ
for the low-side resistor RL of the voltage
divider.
Calculate the required inductance value by the
equation:
VIN × (VOUT - VIN )
L =
VOUT × fSW × ∆I
VOUT ×ILOAD
(MAX)
IIN(MAX)
=
For the step-up converter, determine the high-
side resistor RH by the equation:
VIN ×η
30% − 50% IIN(MAX)
∆I =
)
VMAIN − VFB1
RH
=
⎛
⎞
VFB1
RL
Where ILOAD(MAX) is the maximum load current, ∆I
is the peak-to-peak inductor ripple current, and η
is efficiency.
⎜
⎜
⎟
⎟
⎝
⎠
Where VMAIN is the output voltage of the step-up
converter.
Selecting the Input Capacitor
For the positive charge-pump, determine the
high-side resistor RH by the equation:
An input capacitor is required to supply the AC
ripple current to the inductor, while limiting
noise at the input source. A low ESR capacitor
is required to keep the noise at the IC to a
minimum. Since it absorbs the input switching
current it requires an adequate ripple current
rating. Use a capacitor with RMS current rating
greater than the inductor ripple current (see
Selecting The Inductor to determine the
inductor ripple current). One 10µF ceramic
capacitor is used in the application circuit of
Figure 3 because of the high source impedance
seen in typical lab setups. Actual applications
usually have much lower source impedance
since the step-up converter typically runs
directly from the output of another regulated
supply. Typically, the input capacitance can be
reduced below the value used in the typical
application circuit.
VGH − VFB3
RH
=
⎛
⎞
VFB3
RL
⎜
⎜
⎟
⎟
⎝
⎠
For the negative charge-pump, determine the
high-side resistor RH by the equation:
−VGL
RH
=
⎛
⎞
VREF
RL
⎜
⎜
⎟
⎟
⎝
⎠
Selecting the Inductor
The inductor is required to force the higher
output voltage while being driven by the input
voltage. A larger value inductor results in less
ripple current that results in lower peak inductor
current, reducing stress on the internal
N-Channel.switch. However, the larger value
inductor has a larger physical size, higher
series resistance, and/or lower saturation
current.
To insure stable operation place the input
capacitor as close to the IC as possible.
Alternately a smaller high quality 0.1µF ceramic
capacitor may be placed closer to the IC if the
larger capacitor is placed further away.
A 4.7µH inductor is recommended for most
applications. A good rule of thumb is to allow
the peak-to-peak ripple current to be
approximately 30-50% of the maximum input
current. Make sure that the peak inductor
current is below 75% of the current limit to
prevent loss of regulation due to the current
limit. Also make sure that the inductor does not
MP1530 Rev. 1.4
5/19/2006
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