MP2172 – 5.5V, 2A, 2.6MHz, SYNCHRONOUS STEP-DOWN SWITCHER

MP2172 Rev. 1.0

www.MonolithicPower.com

15

4/24/2019

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Selecting the Output Capacitor

The output capacitor (C2) maintains the output

DC voltage. Use low ESR, ceramic capacitors

to keep the output voltage ripple low. The

output voltage ripple can be estimated with

Equation (8):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

 

(8)

Where L1 is the inductor value, and RESR is the

equivalent series resistance of the output

capacitor.

For ceramic capacitors, the impedance at the

switching frequency is dominated by the

capacitance. The output voltage ripple is

caused mainly by

the

capacitance.

For

simplification, the output voltage ripple can be

estimated with Equation (9):

OUT

OUT

OUT

2

IN

S1

VV

∆V1

V

8f

L

C2



 



 



(9)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (10):

OUT

OUT

OUT

ESR

IN

S1

VV

∆V1

R

fL

V



 









(10)

The characteristics of the output capacitor also

affect the stability of the regulation system.

PCB Layout Guidelines

Proper layout of the switching power supplies is

critical for stable operation. For high-frequency

switching converters, a poor layout can lead to

poor line or load regulation and stability issues.

For best results, refer to Figure 3 and follow the

guidelines below.

1. Place the high-current paths (GND, VIN,

SW) very close to the device using short,

direct, and wide traces.

2. Place the input capacitor as close to VIN

and GND as possible.

3. Place the external feedback resistors next

to FB.

4. Keep the switching node SW short and

away from the feedback network.

Figure 3: Layout Recommendation