MP8865

– SYNCHRONOUS STEP-DOWN CONVERTER

MP8865 Rev. 1.03

www.MonolithicPower.com

24

1/18/2018

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External Bootstrap Diode

An external bootstrap diode enhances the

efficiency of the regulator given the following

conditions:

IN

OUT

V

V

>65%

In these cases, add an external BST diode from

VCC to BST (see Fig. 12).

Figure 12. Optional External Bootstrap Diode to

Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST capacitor value is 0.1µF

to 1μF.

Efficient PCB layout is critical to achieve stable

operation, especially for VCC capacitor and

input capacitor placement. For best results,

refer to Fig. 13 and follow the guidelines below:

1)

Use

a

large

ground

plane

connected

directly to GND. If bottom layer is ground

plane, add vias near GND.

2)

Place as close as possible the VCC

capacitor to VCC and GND. Make the trace

length of VCC to the VCC capacitor anode

to the VCC capacitor cathode chip to GND

as short as possible.

3)

Place the ceramic input capacitor close to

IN and GND. Keep the connection of the

input capacitor and IN as short and wide as

possible.

4)

Route SW and BST away from sensitive

analog areas (such as FB). It is NOT

recommended to route SW and BST traces

under the

chip’s bottom layer.

5)

Place the T-type feedback resistor R3 close

to the chip to ensure the trace (which

connects to FB) is as short as possible.

Notes:

11) The recommended layout is based on Fig. 15

in the “Typical

Application Circuit

s” section on page 26.

Top Layer

Bottom Layer

Figure 13. Recommended PCB Layout