MP2158 – 1A, 6V, 1.5MHz SYNCHRONOUS STEP-DOWN SWITCHER

MP2158 Rev. 1.0

www.MonolithicPower.com

11

10/10/2012

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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 by:

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.

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.

PCB Layout

Proper layout of the switching power supplies is

very important, and sometimes critical for

proper

function.

For

the

high-frequency

switching converter, poor layout design can

result in poor line or load regulation and stability

issues.

The high current paths (GND, IN and SW)

should be placed very close to the device with

short, direct and wide traces. The input

capacitor needs to be as close as possible to

the IN and GND pins. The external feedback

resistors should be placed next to the FB pin.

Keep the switching node SW short and away

from the feedback network.

EN

AGND

OUT

GND

SW

VIN

1

2

3

4

8

7

6

5

PG

C2

R2

C1

Figure 3: PCB Layout Recommendation

Design Example

Below is a design example following the

application guidelines for the specifications:

Table 2: Design Example

5V

1.2V

1500kHz

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.