2013/5/2

Rev.A.1

10

EM5301F/G

The equations below relate the compensation

network’s poles and zeros to the components (R1,

R2, R3, C1, C2 and C3).

1

2

Z1

C

*

R

*

2

1

F

π

=

)

2

1

2

1

2

P1

C

C

C

*

C

(

*

R

*

2

1

F

+

=

π

3

3

1

Z1

C

*

)

R

(R

*

2

1

F

+

=

π

3

3

P2

C

*

R

*

2

1

F

π

=

Fig.2 shows the Bode plot for the control loop. The

compensation

gain

uses

external

impedance

stable control loop has a gain crossing with

-20db/decade slope and phase margin greater than

45 degrees.

10

100

1K

10K

100K

1M

10M

-60

-40

-20

0

20

40

60

80

100

20Log(R2/R1)

Error Amp Open

Loop Gain

Modulator Gain

Close Loop Gain

Compensation

Gain

FREQUENCY(Hz)

Fig.2 Bode Plot of Voltage Mode Buck Converter

Output Inductor Selection

The output inductor is selected to meet the output

voltage ripple requirements and minimize the

response time to the load transient. The inductor

value determines the current ripple and voltage

ripple. The ripple current is approximately the

following equation:

SW

IN

OUT

OUT

IN

L

F

*

V

V

L

V

V

ΔI

∗

−

=

Output Capacitor Selection

An output capacitor is required to filter the output

and supply the load transient. The selection of

output capacitor depends on the output ripple

voltage. The output ripple voltage is approximately

bounded by the following equation:

)

OUT

SW

L

OUT

C

*

F

*

8

1

(ESR

*

ΔI

ΔV

+

=

Input Capacitor Selection

Use a mix of input bypass capacitors to control the

voltage overshoot across the MOSFET. Use small

ceramic capacitors for high frequency decoupling

and bulk capacitors to supply the current needed

each time the upper MOSFET turn on. Place the

small ceramic capacitors physically close to the

MOSFETs and between the drain of the upper

MOSFET and the source of the lower MOSFET. The

important parameters of the input capacitor are

the voltage rating and the RMS current rating.

The capacitor voltage rating should be at least 1.25

times greater than the maximum input voltage and

a voltage rating of 1.5 times is a conservative

guideline. The RMS current rating requirement can

be expressed as the following equation:

D)

-

D(1

I

I

OUT

RMS

=

For a through hole design, several electrolytic

capacitors may be needed. For surface mount

designs, solid tantalum capacitors can also be used

but caution must be exercised with regard to the

capacitor surge current rating. These capacitors

must be capable of handling the surge current at

power-up. Some capacitor series available from

reputable manufacturers are surge current tested.