AP6507

500 kHz 18V 3A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6507

Document number: DS33435 Rev. 2 - 2

11 of 13

www.diodes.com

May 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:

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

V

2

)

out

V

V

(

Δ

2

)

2

inductor

ΔI

out

L(I

o

C

−

+

+

=

Where ΔV is the maximum output voltage overshoot.

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.

AP6507 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.

AP6507

BST

SW

10nF

BOOST

DIODE

5V

4

3

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.