AP65111A

Document number: DS39760 Rev. 1 - 2

10 of 13

www.diodes.com

June 2017

© Diodes Incorporated

AP65111A

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

*

ΔI

Vout

inductor

An output capacitor with high 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

ΔV is the maximum output voltage overshoot.

PC Board Layout

The layout is very important in high frequency switching converter design. With power devices switching efficiently at 500kHz, the resulting current

transitions from one device to another cause voltage spikes across the interconnecting impedances and parasitic circuit elements. These voltage

spikes can degrade efficiency, radiate noise into the circuit, and lead to device overvoltage stress. Careful component layout and printed circuit

board design minimizes these voltage spikes. As an example, consider the turn-off transition of the HS MOSFET. Prior to turn-off, the HS

MOSFET is carrying the full load current. During turn-off, current stops flowing in the HS MOSFET and is picked up by the internal body diode.

Any parasitic inductance in the switched current path generates a large voltage spike during the switching interval. Careful component selection,

tight layout of the critical components and short, wide traces minimize the magnitude of voltage spikes. There are two sets of critical components

in the regulator switching converter. The switching components are the most critical because they switch large amounts of energy and therefore

tend to generate large amounts of noise. Next are the small signal components, which connect to sensitive nodes for controlling the regulator.

The switching components should be placed close to the regulator first. Minimize the length of the connections between the input capacitors and

the power switches by placing them nearby. Position both the ceramic and bulk input capacitors as close to the upper MOSFET drain as possible.

The critical small signal components include feedback components and BST capacitor. Place the compensation components close to the FB pin.

The feedback resistors should be located as close as possible to the FB pin with vias tied straight to the ground plane. See Figure 4 for reference.

Figure 4. PC Board Layout

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 device

See below for Diodes Incorporated‟s recommended diodes.

AP65111A

BST

SW

10nF

BOOST

DIODE

5V

Figure 5. External Bootstrap Compensation Components

Recommended Diodes:

Part Number

Voltage/Current

Rating

B130

30V, 1A

SK13

30V, 1A