MP2159A – 1 A, 6 V, 1.5 MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2159A Rev. 1.02

www.MonolithicPower.com

13

6/30/2016

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Selecting the Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic

capacitors

are

recommended.

Low

ESR

capacitors are preferred to keep the output

voltage ripple low. The output voltage ripple can

be estimated using Equation (8):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

 

(8)

equivalent series resistance (ESR) value of the

output capacitor.

When using 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 using Equation (9):

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2



 



 



(9)

When using 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

ΔV1

R

fL

V



 









(10)

The characteristics of the output capacitor

affect the stability of the regulation system.

PCB Layout Guidelines

Efficient PCB layout is critical for stable

operation. For the high-frequency switching

converter, a poor layout design can result in

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,

and SW) very close to the device with

short, direct, and wide traces.

2. Place the input capacitor as close as

possible to VIN and GND.

3. Place the external feedback resistors

next to FB.

4. Keep the switching node (SW) short and

away from the feedback network.

8

7

6

5

L1

R4

R3

1

2

3

4

OUT

VIN

GND

SW

C1

C1A

Figure 3—Recommended PCB layout

Design Example

Table 2 shows a design example following the

application

guidelines

for

the

given

specifications:

Table 2—Design example

5 V

1.2 V

1500 kHz

The detailed application schematic is shown in

Figure 4. The typical performance and circuit

waveforms have been shown in the “Typical

Performance Characteristics” section. For more

device applications, please refer to the related

evaluation board datasheets.