MP4473–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4473 Rev. 1.0

www.MonolithicPower.com

17

12/15/2014

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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)

where,

output capacitor.

For ceramic capacitors, capacitance dominates

the impedance at the switching frequency. This is

the primary cause of the output-voltage ripple. To

simplify calculations, 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, requiring an external ramp to stabilize the

system. The voltage ramp is ~30mV. The

using the following equation:

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. To simplify calculations, 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

results in less ripple current and a lower output-

ripple voltage. However, a larger inductance

results in a larger inductor, which is not only

physically larger, but has 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)

where,

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)