MP1494 – SYNCHRONOUS STEP-DOWN CONVERTER

MP1494 Rev. 1.04

www.MonolithicPower.com

13

12/26/2012

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External Bootstrap Diode

An external bootstrap diode can enhance the

efficiency of the regulator given the following

conditions:

Duty cycle is high: D=

IN

OUT

V

V

>65%

In these cases, add an external BST diode from

the VCC pin to BST pin, as shown in Figure 7.

SW

BST

MP1494

C

L

BST

External BST Diode

VCC

IN4148

Figure 7: Optional External Bootstrap Diode to

Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST capacitor value is 0.1µF

to 1μF.

PCB layout is very important to achieve stable

operation especially for VCC capacitor and

input capacitor placement. For best results,

follow these guidelines:

1) Use large ground plane directly connect to

GND pin. Add vias near the GND pin if bottom

layer is ground plane.

2) Place the VCC capacitor to VCC pin and

GND pin as close as possible. Make the trace

length of VCC pin-VCC capacitor anode-VCC

capacitor cathode-chip GND pin as short as

possible.

3) Place the ceramic input capacitor close to IN

and GND pins. Keep the connection of input

capacitor and IN pin as short and wide as

possible.

4) Route SW, BST away from sensitive analog

areas such as FB. It’s not recommended to

route SW, BST trace under chip’s bottom side.

5) Place the T-type feedback resistor R9 close

to chip to ensure the trace which connects to

FB pin as short as possible

Notes:

8) The recommended layout is based on the Figure 8 Typical

Application circuit on the next page.

L1

C2

C2A

C1

C1A

R5

R6

R7

R3

C3

C4

C6

Vin

GND

Vout

SW

GND

GND

SW

GND

EN/SYNC

BST

VCC