MP28266 – 21V, 3A, 500kHz, SYNCHRONOUS, STEP-DOWN CONVERTER

MP28266 Rev. 1.0

www.MonolithicPower.com

12

3/16/2012

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Refer to Figure 5 to select an optimal voltage and

then use the following equation to determine the

value of R4. Assume R3 to be 10kΩ:

VCC

R4 R3

1

AAM









0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10 1112

Figure 5: AAM Selection for Common Output

External Bootstrap Diode

An external bootstrap diode can enhance the

efficiency of the regulator given the applicable

conditions:





Duty cycle is high: D=

IN

OUT

V

V

>65%

The external BST diode connects from the VCC

pin to the BST pin, as shown in Figure 6.

MP28266

SW

BST

C

L

VCC

BST

External BST Diode

IN4148

+

Figure 6: Optional External Bootstrap Diode

for Enhanced Efficiency

The recommended external BST diode is

IN4148, and the BST capacitor is 0.1µF to 1μF.

PC Board Layout

Place the high current paths (GND, IN and SW)

very close to the device with short, direct, and

wide traces. Place the input capacitor as close as

possible to the IN and GND pins. Place the

external feedback resistors next to the FB pin.

Keep the switching node SW short and away

from the feedback network.