AP5100

1.2A Step-Down Converter with 1.4MHz Switching

Frequency

AP5100

10 of 12

MARCH 2010

Document number: DS32130 Rev. 1 - 2

www.diodes.com

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.

External Diode

The external diode’s forward current must not exceed the

maximum output current. Since power dissipation is a

critical factor when choosing a diode, it can be calculated

from the equation below:

0.3V

out

I

)

IN

V

OUT

V

(1

diode

P

×

×

−

=

Note: 0.3V is the voltage drop across the schottky diode.

A diode that can withstand this power dissipation must

be chosen.

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. 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.

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 improve the

efficiency of the regulator. The bootstrap diode can be a

low cost one such as IN4148 or BAT54.

AP5100

BST

SW

10nF

BOOST

DIODE

5V

1

6

Figure 5.

External Bootstrap Diode