AP6508

500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6508

Document number: DS33437 Rev. 4 - 2

11 of 14

www.diodes.com

July 2011

© Diodes Incorporated

Inductor (cont.)

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 an 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:

2

out

V

2

)

out

V

V

(

Δ

2

)

2

inductor

ΔI

out

L(I

o

C

−

+

+

=

Where

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.

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.

AP6508 is exposed at the bottom of the package and must

be soldered directly to a well designed thermal pad on the

PCB. This will help to increase the power dissipation.

External Bootstrap Diode

It is recommended that an external bootstrap diode be

added when the input voltage is no greater than 5V or the

5V rail is available in the system. This helps to improve

the efficiency of the regulator. This solution is also

applicable for D > 65%. The bootstrap diode can be a low

cost one such as BAT54 or a schottky that has a low Vf.

AP6508

BST

SW

10nF

BOOST

DIODE

5V

6

2,3,4,5

Figure 3. External Bootstrap Diode

Input capacitor C1

must be placed as

close as possible to the

IC and to L1.

34mm

52mm

External feedback

resistor dividers

must be placed

close to the FB pin