MP2147 – 5.5V, 4A, 1.2MHz, COT, BUCK SWITCHER

MP2147 Rev. 1.1

www.MonolithicPower.com

17

6/8/2016

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Selecting the Output Capacitor

The output capacitor stabilizes the DC output

voltage. Ceramic capacitors are recommended.

Low ESR capacitors are preferred to limit the

output voltage ripple. Estimate the output

voltage ripple as:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

 

equivalent series resistance (ESR) value of the

output capacitor.

When

using

ceramic

capacitors,

the

capacitance dominates the impedance at the

switching frequency, and causes most of the

output voltage ripple. For simplification, the

output voltage ripple can be estimated as:

OUT

OUT

OUT

2

S1

IN

VV

ΔV1

8f

L

C2

V



 



 



For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated as:

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. For

MP2147, 2pcs 22uF Co can satisfy the most

application. Add Co can reduce DCM and CCM

output ripple effectively. However, a very large

Co may cause light group pulse in sleep mode.

PCB Layout Recommendation

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 MP2147, the high speed step-down

regulator, the input capacitor should be placed

as close as possible to the IC pins. As shown in

Figure 7, the 0805 size ceramic capacitor is

used, please make sure the two ends of the

ceramic capacitor be directly connected to PIN

8 (the Power Input Pin) and PIN 10/11/12 (the

Power GND Pin).

Figure 7: Two ends of Input decoupling

Capacitor close to Pin 8 and Pin 10/11/12