AP6502

340kHz 23V 2A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6502

Document Number: DS35423 Rev. 2 - 2

9 of 12

www.diodes.com

September 2011

© Diodes Incorporated

Compensation Components (cont.)

2. Choose the compensation capacitor (C3) to achieve the

below one fourth of the crossover frequency to provide

sufficient phase margin. Determine the C3 value by the

following equation:

fc

3

R

2

3

C

×

×

π

>

Where R3 is the compensation resistor value.

(V)

Cin/C1

(µF)

Cout/C2

(µF)

Rc/R3

(k

Ω)

Cc/C3

(nF)

L1

(µH)

1.2

22

47

3.24

6.8

3.3

1.8

22

47

6.8

6.8

3.3

2.5

22

47

6.8

6.8

10

3.3

22

47

6.8

6.8

10

5

22

47

6.8

6.8

10

12

22

47

6.8

6.8

15

Table 2—Resistor

Component Selection

Inductor

Calculating the inductor value is a critical factor in

designing a buck converter. For most designs, the

following equation can be used to calculate the inductor

value;

SW

f

L

ΔI

IN

V

)

OUT

V

IN

(V

OUT

V

L

⋅

⋅

−

⋅

=

Where

L

And

SW

f

is the buck converter switching frequency.

Choose the inductor ripple current to be 30% of the

maximum load current. The maximum inductor peak

current is calculated from:

2

L

ΔI

LOAD

I

L(MAX)

I

+

=

Peak current determines the required saturation current

rating, which influences the size of the inductor. Saturating

the inductor decreases the converter efficiency while

increasing the temperatures of the inductor and the

internal MOSFETs. Hence choosing an inductor with

appropriate saturation current rating is important.

A 1µH to 10µH inductor with a DC current rating of at least

25% percent higher than the maximum load current is

recommended for most applications.

For highest efficiency, the inductor’s DC resistance

should be less than 200m

Ω. Use a larger inductance

for improved efficiency under light load conditions.

Input Capacitor

The input capacitor reduces the surge current drawn from

the input supply and the switching noise from the device.

The input capacitor has to sustain the ripple current

produced during the on time on the upper MOSFET. It

must hence have a low ESR to minimize the losses.

The RMS current rating of the input capacitor is a critical

parameter that must be higher than the RMS input

current. As a rule of thumb, select an input capacitor

which has RMs rating that is greater than half of the

maximum load current.

Due

to

large

dI/dt

through

the

input

capacitors,

electrolytic or ceramics should be used. If a tantalum

must be used, it must be surge protected. Otherwise,

capacitor failure could occur. For most applications, a

4.7µF ceramic capacitor is sufficient.

Output Capacitor

The output capacitor keeps the output voltage ripple

small, ensures feedback loop stability and reduces the

overshoot of the output voltage. The output capacitor is a

basic component for the fast response of the power

supply. In fact, during load transient, for the first few

microseconds it supplies the current to the load. The

converter recognizes the load transient and sets the duty

cycle to maximum, but the current slope is limited by the

inductor value.

Maximum capacitance required can be calculated from

the following equation:

ESR of the output capacitor dominates the output voltage

ripple. The amount of ripple can be calculated from the

equation below:

ESR

*

inductor

ΔI

capacitor

Vout

=

An output capacitor with ample capacitance and low ESR

is the best option. For most applications, a 22µF ceramic

capacitor will be sufficient.

2

out

2

out

2

inductor

out

o

V

)

V

V

(

Δ

)

2

ΔI

L(I

C

−

+

+

=

Where

PC Board Layout

This is a high switching frequency converter. Hence

attention

must

be

paid

to

the

switching

currents

interference in the layout. Switching current from one

power device to another can generate voltage transients

across the impedances of the interconnecting bond wires

and circuit traces. These interconnecting impedances

should be minimized by using wide, short printed circuit

traces.