MP1541 – 1.3MHZ BOOST CONVERTER
APPLICATIONS INFORMATION
COMPONENT SELECTION
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:
R1
=
R2
(
V
OUT
- V
FB
)
V
FB
where V
OUT
is the output voltage.
For R2 = 11.8kΩ and V
FB
= 1.25V, then
R1 (kΩ) = 9.44kΩ (V
OUT
– 1.25V).
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.
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.
Selecting the Output Capacitor
A single 4.7µF to 10µF ceramic capacitor
usually provides sufficient output capacitance
for most applications. If larger amounts of
capacitance is 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 V
RIPPLE
is calculated as:
V
RIPPLE
=
I
LOAD
(
V
O UT
−
V
IN
)
V
O UT
×
C2
×
f
SW
Selecting the Inductor
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.
Calculate the required inductance value L using
the equations:
L
=
V
IN
(V
OUT
- V
IN
)
V
OUT
×
f
SW
× ∆
I
I
IN(MAX )
=
V
OUT
×
I
LOAD (MAX )
V
IN
× η
∆
I
=
(
30%
−
50%
)
I
IN(MAX )
Where I
LOAD(MAX)
is the maximum load current,
∆I
is the peak-to-peak inductor ripple current, and
η
is efficiency. For the MP1541, 4.7µH is
recommended for input voltages less than 3.3V
and 10µH for inputs greater than 3.3V.
Selecting the Diode
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. For
output voltage less than 20V, 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.
Where V
IN
is the input voltage, I
LOAD
is the load
current, C2 is the capacitance of the output
capacitor, and f
SW
is the 1.3MHz switching
frequency.
MP1541Rev.1.2
3/20/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
5
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