MP3221

–2.7A, 1.2MHZ, HIGH-EFFICIENCY STEP UP CONVERTER WITH INPUT DISCONNECT

MP3221 Rev. 1.0

www.MonolithicPower.com

11

9/23/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Input Capacitor Selection

Low

ESR

input

capacitors

reduce

input

switching noise and reduce the peak current

drawn from the battery. Ceramic capacitors are

good choice for input decoupling and it should

be located as close as possible to the IN and

GND pin. Add a ceramic capacitor larger than

10μF close to the IC.

Selecting the Output Capacitor

The output capacitor requires a minimum

capacitance value of 22μF at the programmed

output voltage to ensure stability over the full

operating range. A higher capacitance value

may be required to lower the output ripple and

also

the

transient

response.

Low

ESR

capacitors, such as X5R- or X7R-type ceramic

capacitors, are recommended. Assuming that

the ESR is zero, estimate the minimum output

capacitance to support the ripple in the CCM

mode as:

O

OUT

F

IN(MIN)

O

S

OUT

F

I

(V

V

V

)

C

f

(V

V )

V













 

Where:

ΔV= Acceptable output ripple

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-

inductor has a larger physical size, higher

series

resistance,

and/or

lower

saturation

current.

A 2.2μH inductor is recommended for most

applications. However, a more exact inductance

value can be calculated. 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 at the

operating

duty

cycle

to

prevent

loss

of

regulation due to the current limit. Also make

sure that the inductor does not saturate under

the worst-case load transient and startup

conditions. Calculate the required inductance

value by the equation:

IN

OUT

F

IN

OUT

F

SW

V

(V

V

V )

L

(V

V ) f

I













 

OUT

LOAD(MAX)

IN(MAX)

IN

VI

I

V







IN(MAX)

I

(30% 50%)I

 



ΔI is the peak-to-peak inductor ripple current,

and η is efficiency.

Selecting the Diode

The output rectifier diode supplies current to the

inductor when the internal MOSFET is off. To

reduce power loss due to diode forward voltage

and recovery current, use a Schottky diode with

the MP3221. The diode should be rated for a

reverse voltage equal to or higher than the

output voltage. The average current rating must

be higher than the maximum load current, and

the peak current rating must be higher than the

peak inductor current.

Selecting the PMOS

The MP3221 is capable of driving P-Channel

power MOSFETS to disconnect input and

output. The critical parameter selection of a

MOSFET are:

Ideally,

the

off-state

voltage

across

the