MP3418 – 400mA, 1.2MHz, SYNCHRONOUS, STEP-UP CONVERTER WITH OUTPUT DISCONNECT

MP3418 Rev 1.0

www.MonolithicPower.com

11

2/7/2013

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APPLICATION INFORMATION

COMPONENT SELECTION

Input Capacitor Selection

Low

ESR

input

capacitors

reduce

input

switching noise and reduce the peak current

drawn from the battery. It follows that ceramic

capacitors are also a good choice for input

decoupling and should be located as close as

possible to the device. Add a ceramic capacitor

larger than 4.7µF in parallel with a 100nF

ceramic capacitor close to the IC.

Output Capacitor Selection

The output capacitor requires a minimum

capacitance value of 10µF 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 PWM

mode as.

O

OUT(MAX)

IN(MIN)

O

S

OUT(MAX)

I

(V

V

)

C

f

V

∆V

×

−

≥

×

×

(2)

Where,

∆V= Acceptable output ripple

Additional output capacitance may also be

reduce ripple in PSM mode and to ensure

stability in PWM mode, especially at higher

output load currents.

Inductor Selection

The MP3418 can use small surface-mount

inductors due to its 1.2MHz switching frequency.

Inductor values between 4.7µH and 10µH are

suitable for most applications. Larger values of

inductance will allow slightly greater output

current capability (and lower the PSM threshold)

by

reducing

the

inductor

ripple

current.

Increasing the inductance above 10µH will

increase component size while providing little

improvement in output current capability. The

minimum inductance value is given by:

IN(MIN)

OUT(MAX)

IN(MIN)

OUT(MAX)

L

S

V

(V

V

)

L

2 V

∆I

f

×

−

≥

×

×

×

(3)

ripple

The inductor current ripple is typically set at

30% to 40% of the maximum inductor current.

High-frequency ferrite-core inductor materials

reduce frequency-dependent power losses and

improve

efficiency

compared

to

cheaper

powdered-iron cores. The inductor should have

low DCR (inductor series resistance without

saturated windings) to reduce the resistive

power loss; further reducing the DCR will

significantly

improve

efficiency

when

The device enters PSM at a load that borders

continuous and discontinuous PWM operation,

which means the averaged inductor current

lower PSM enter level.