MP8861 – 2.85V - 18V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8861 Rev. 1.1

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31

5/28/2020

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Figure 13: Optional External Bootstrap Diode to

Enhance Efficiency

The recommended external BST diode is

IN4148, and the recommended BST capacitor

value is 0.1μF to 1μF.

Connect VCC to VIN at a Low Input Voltage

VCC can be connected to VIN directly when

VIN is lower than 3.5V. This helps improve the

MP8861’s

low

input

voltage

efficiency

performance. To use this application set-up, the

VIN spike voltage must be limited below 4V;

otherwise, VCC may be damaged.

PCB Layout Guidelines (9)

Efficient PCB layout is critical for stable

operation. For best results, refer to Figure 14

and follow the guidelines below. A four-layer

layout is strongly recommended to achieve

better thermal performance.

1. Place the high-current paths (PGND, VIN,

and SW) very close to the device with short,

direct, and wide traces.

2. Keep the VIN and PGND pads connected

with large copper planes.

3. Use at least two layers for the VIN and

PGND trace to achieve better thermal

performance.

4. Add several vias close to the VIN and

PGND pads to help with thermal dissipation.

Place m than one via adjacent to the inner

edge of PGND pad which connects to

AGND pad through low impedance routing

or ground plane.

5. Place the input capacitors as close to VIN

and PGND as possible.

6. Place the decoupling capacitor as close to

VCC and AGND as possible.

7. Add several vias close the the AGND pad,

and connect to PGND plane with Kelvin

connection.

8. Place the external feedback resistors next

to FB.

9. Ensure that there is no via on the FB trace.

10. Keep the switching node SW short and

away from the feedback network.

11. Keep the BST voltage path (BST, C3, and

SW) as short as possible.

NOTE:

9)

The recommended layout is based on the Typical

Application Circuit on page 33-35.