MP4470/4470A–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4470/4470A Rev. 1.1

www.MonolithicPower.com

14

8/26/2013

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© 2013 MPS. All Rights Reserved.

Output Capacitor

The output capacitor maintains the DC output

voltage. Use ceramic or POSCAP capacitors.

The output voltage ripple can be estimated as:

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

FL

V

8 F

C

Δ=

× −

×

+

××

×

(16)

the output capacitor.

For ceramic capacitors, capacitance dominates

the impedance at the switching frequency, can is

the primary cause of the output-voltage ripple.

For simplification, estimate the output voltage

ripple as:

OUT

OUT

OUT

2

SW

OUT

IN

VV

V(1

)

8F

L C

V

Δ=

× −

×× ×

(17)

The output voltage ripple caused by ESR is very

small and therefore requires an external ramp to

stabilize the system. The voltage ramp is ~30mV.

IN

OUT

ON

RAMP

(V

V

) T

V

R4

C4

−×

=

×

(18)

SW

R1 R2

11

()

2F

C4

5

R1 R2

×

<×

π×

×

+

(19)

For POSCAP capacitors, the ESR dominates the

impedance at the switching frequency. The ramp

voltage generated from the ESR is high enough

to stabilize the system. Therefore, an external

ramp is not needed. A minimum ESR value of

12mΩ is required to ensure stable operation of

the converter. For simplification, the output ripple

can be approximated as:

OUT

OUT

OUT

ESR

SW

IN

VV

V(1

) R

FL

V

Δ=

× −

×

×

(20)

Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switching input voltage. A larger inductance

will result in less ripple current and a lower output

ripple voltage. However, a larger inductance

resultsin a larger inductor, which will physically

larger, and have a higher series resistance

and/or lower saturation current. A good rule for

determining the inductor value is to allow the

peak-to-peak ripple current in the inductor to be

approximately 30% to 40% of the maximum

switch current limit. Ensure that the peak inductor

current is below the maximum switch current limit.

The inductance value can be calculated as:

OUT

OUT

SW

L

IN

VV

L(1

)

FI

V

=× −

×Δ

(21)

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated as:

OUT

OUT

LP

OUT

SW

IN

VV

II

(1

)

2F

L

V

=+

× −

×

(22)