MP3216 –36V, 1.3MHz STEP-UP CONVERTER WITH A 0.75A SWITCH
MP3216 Rev. 1.02 www.MonolithicPower.com 9
2/8/2017 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
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APPLICATION INFORMATION
Components referenced below apply to
Typical Application Circuit in figure 2.
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. Use 5.1k for the low-side
resistor R2 of the voltage divider. Determine the
high-side resistor R1 by the equation:
Where, V
OUT
is the output voltage, V
FB
= 1.24V.
For R2 = 5.1k and V
OUT
= 33V, then
R1 = 130.6k. There choose a 133k standard
1% value.
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.
To minimize the ripple to the inductor, use a
smaller ceramic capacitor value of 0.1µF placed
physically close to the L pin.
Selecting the Output Capacitor
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 characteristic of the output
capacitor also affects the stability of the
regulation control system. Ceramic, tantalum, or
low ESR electrolytic capacitors are
recommended. 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:
Where V
RIPPLE
is the output ripple voltage, V
IN
and
V
OUT
are the DC input and output voltages
respectively, I
LOAD
is the load current, f
SW
is the
switching frequency, and C2 is the capacitance
of the output capacitor.
Selecting the Inductor
The inductor is required to force the higher output
voltage while being driven by the lower input
voltage. A larger value inductor results in less
ripple current that results in lower peak inductor
current, reducing stress on the internal power
switch. However, the larger value inductor has a
larger physical size, higher series resistance,
and/or lower saturation current.
Inductance from 2.2µH to 33µH is a good choice
for high efficiency and small size. To prevent
saturation, use an inductor with a saturation
current rating that is higher than the device
current limit.
Selecting the Diode
The output rectifier diode supplies current to the
inductor when the internal power MOSFET is off.
To reduce losses due to diode forward voltage
and recovery time, use a Schottky diode with the
MP3216. The diode should be rated for a reverse
voltage equal to or greater than the output
voltage used. The average current rating must be
greater than the maximum load current
expected, and the peak current rating must be
greater than the peak inductor current.
Layout Consideration
High frequency switching regulators require very
careful layout for stable operation and low noise. All
components must be placed as close to the IC as
possible. Keep the path between the SW pin,
output diode, output capacitor and GND pin
extremely short for minimal noise and ringing. The
input capacitor must be placed close to the IN pin
for best decoupling. All feedback components must
be kept close to the FB pin to prevent noise
injection on the FB pin trace. The ground return of
the input and output capacitors should be tied close
to the GND pin. See the MP3216 demo board
layout for reference.
