MP8845C – 5A HIGH EFFICIENT SYNC. STEP DOWN SWITCHER WITH I2C

MP8845C Rev. 1.02

www.MonolithicPower.com

18

4/25/2014

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APPLICATION INFORMATION

COMPONENT SELECTION

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low

ESR capacitors for the best performance.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients. For

most applications, a 10µF capacitor is sufficient.

For higher output voltage, 47µF may be needed

for more stable system.

Since the input capacitor absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1

For simplification, choose the input capacitor

whose RMS current rating greater than half of the

maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, a small and high quality ceramic

capacitor, i.e. 0.1μF, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Selecting the Output Capacitor

The output capacitor (C2) is required to maintain

the DC output voltage.

Low ESR ceramic capacitors can be used with

MP8845C to keep the output ripple low.

Generally, 22μF output ceramic capacitor is

enough for most of the cases. In higher output

voltage condition, 47μF might be needed for a

stable system. Larger output capacitor can get

smaller output voltage ripple.

Using ceramic capacitors, the impedance at the

switching frequency is dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated by:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the output

ripple can be approximated to:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the stability of the regulation system.

Layout Recommendation of MP8845C

Proper layout of the switching power supplies is

very important, and sometimes critical to make it

work properly. Especially, for the high switching

converter, if the layout is not carefully done, the

regulator could show poor line or load regulation,

stability issues.

For MP8845C, the high speed step-down

regulator, the input capacitor should be placed as

close as possible to the IC pins. As shown in

Figure 6, the 0805 size ceramic capacitor is used,

please make sure the two ends of the ceramic

capacitor be directly connected to VIN pin and

GND pin. A 0603 size decouple ceramic

capacitor is strongly recommended here.