MP2321 Rev. 1.0

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18

10/26/2016

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A larger output capacitor can also achieve a

better

load

transient

response,

but

the

maximum output capacitor limitation should

also be considered in the design application. If

the output capacitor value is too high, the

output voltage cannot reach the design value

during the soft-start time and fails to regulate.

can be limited approximately with Equation (23):

O_ MAX

LIM _ AVG

OUT

ss

OUT

C(I

I

) T / V





(23)

start time.

External Bootstrap Diode

The BST voltage may become insufficient in

particular conditions. In this case, an external

bootstrap diode can be added to enhance the

efficiency of the regulator and help prevent BST

voltage insufficiency in light-load PFM operation.

BST voltage insufficiency is more likely to occur

at either of the following conditions:



VIN is low



Duty cycle is large: D =

IN

OUT

V

V

> 65%

In these cases, if the BST voltage is insufficient,

the

output

ripple

voltage

may

become

extremely large at light-load condition or show

poor efficiency at heavy-load condition. Add an

external BST diode from VCC to BST (see

Figure 11).

Figure 11: Optional External Bootstrap Diode

The recommended external BST diode is

IN4148.

PCB Layout Guidelines

Efficient layout of the switching power supplies

is critical for stable operation. A poor layout

design can result in poor line or load regulation

and stability issues. For best results, refer to

Figure 12 and follow the guidelines below.

1. Place the high current paths (GND, IN, and

SW) very close to the device with short,

direct, and wide traces.

2. Place the input capacitor as close to IN and

GND as possible.

3. Place the mode/frequency circuit close to

the part.

4. Place the external feedback resistors next

to FB.

5. Keep the switching node (SW) short and

away from the feedback network.

For better performances, it is recommended to

use four-layer boards. Figure 12 shows the top

and bottom layers (Inner 1 and Inner 2 are both

GND).

sw

VOUT

GND

VIN

R1

R2

GND

C1A

C1B

C2A

C2B

R5

R7

R6

C3

C4

Rb

Cb

R3

L

C1