TM
MP1540 – 1.3MHz, 18V STEP-UP CONVERTER
APPLICATIONS INFORMATION
Selecting the Inductor
COMPONENT SELECTION
The inductor is required to force the output
voltage higher while being driven by the lower
input voltage. Choose an inductor that does not
saturate at the SW current limit. A good rule for
determining the inductance 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 typical current limit at the duty cycle used to
prevent loss of regulation due to the current
limit variation.
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. Use 11.8kΩ for the low-
side resistor R2 of the voltage divider.
Determine the high-side resistor R1 by the
equation:
R2
(
VOUT - VFB
)
R1=
VFB
Where VOUT is the output voltage and VFB is the
feedback voltage.
Calculate the required inductance value L using
the equations:
For R2 = 11.8kΩ and VFB = 1.25V, then
R1 (kΩ) = 9.44kΩ (VOUT – 1.25V).
VIN(VOUT - VIN )
L =
VOUT × fSW × ∆I
Selecting the Input Capacitor
An input capacitor is required to supply the AC
ripple current to the inductor, while limiting noise
at the input source. This capacitor must have low
ESR, so ceramic is the best choice.
VOUT ×ILOAD
(MAX)
IIN(MAX)
=
VIN ×η
30% − 50% IIN(MAX)
∆I =
)
Use an input capacitor value of 4.7µF or
greater. This capacitor must be placed
physically close to the IN pin. Since it reduces
the voltage ripple seen at IN, it also reduces the
amount of EMI passed back along that line to
the other circuitry.
Where ILOAD(MAX) is the maximum load current, ∆I
is the peak-to-peak inductor ripple current and η
is efficiency. For the MP1540, 4.7µH is
recommended for input voltages less than 3.3V
and 10µH for inputs greater than 3.3V.
Selecting the Diode
Selecting the Output Capacitor
The output rectifier diode supplies current to the
inductor when the internal MOSFET is off. To
reduce losses due to diode forward voltage and
recovery time, use a Schottky diode. Choose a
diode whose maximum reverse voltage rating is
greater than the maximum output voltage. It is
recommended to choose the MBR0520 for most
applications. This diode is used for load currents
less than 500mA. If the average current is more
than 500mA the Microsemi UPS5817 is a good
choice.
A single 4.7µF to 10µF ceramic capacitor
usually provides sufficient output capacitance
for most applications. If larger amounts of
capacitance are desired for improved line
support and transient response, tantalum
capacitors can be used in parallel with the
ceramic. The impedance of the ceramic 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 VRIPPLE is calculated as:
ILOAD
UT
VO − VIN
UT
VRIPPLE
=
VO × C2× fSW
Where VIN is the input voltage, ILOAD is the load
current, C2 is the capacitance of the output
capacitor, and fSW is the 1.3MHz switching
frequency.
MP1540 Rev. 1.1
2/14/2006
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