MP1541 – 1.3MHz BOOST CONVERTER
MP1541 Rev.1.5 www.MonolithicPower.com 7
9/5/2017 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2017 MPS. All Rights Reserved.
APPLICATIONS INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. Use 11.8k for the low-
side resistor R2 of the voltage divider.
Determine the high-side resistor R1 by the
equation:
FB
FBOUT
V
V - V2R
1R
where V
OUT
is the output voltage.
For R2 = 11.8k and V
FB
= 1.25V, then
R1 (k) = 9.44k (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 4.7F 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 4.7F to 10F 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:
SW
UT
O
IN
UT
OLOAD
RIPPLE
f2CV
VVI
V
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.
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:
IfV
) V- (VV
L
SWOUT
INOUTIN
IN
)MAX(
LOADOUT
)MAX(IN
V
IV
I
)MAX(IN
I%50%30I
Where I
LOAD(MAX)
is the maximum load current, I
is the peak-to-peak inductor ripple current, and
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.
