MP92265

– 18V, 5A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP92265 Rev. 1.0

www.MonolithicPower.com

13

11/29/2016

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The input capacitor can be electrolytic, tantalum,

or ceramic. When using electrolytic or tantalum

capacitors, add a small, high-quality ceramic

capacitor (e.g.:

0.1μF) as close to the IC as

possible.

When

using

ceramic

capacitors,

ensure that they have enough capacitance to

provide sufficient charge to prevent excessive

voltage ripple at the input. The input voltage

ripple caused by the capacitance can be

estimated with Equation (7):

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V1

f

C1

V

V









 







(7)

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors. For best results,

use low ESR capacitors to keep the output

voltage ripple low. The output voltage ripple can

be estimated with Equation (8):

OUT

OUT

OUT

ESR

S

1

IN

S

VV

1

V

1

R

f

L

V

8 f

C2









 











 

 

(8)

equivalent series resistance (ESR) value of the

output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency, and the capacitance causes the

majority of the output voltage ripple. For

simplification, the output voltage ripple can be

estimated with Equation (9):

OUT

OUT

OUT

2

IN

S1

VV

ΔV

1

V

8 f

L

C2





 





 



(9)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (10):

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV

1

R

f

L

V





 









(10)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP92265 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

The BST voltage may become insufficient in

some particular conditions. In these cases, an

external bootstrap diode can enhance the

efficiency of the regulator and help prevent

output

ripple

caused

by

BST

voltage

insufficiency during PFM operation at light load.

For better efficiency, a diode is needed if the

following condition occurs :



Duty cycle is large: Duty =

IN

OUT

V

V

> 75%

To avoid ripple caused by a BST refresh, a

diode is needed if the following condition

occurs:



In these cases, it is recommended to add an

external BST diode from VCC to BST (see

Figure 5).

Figure 5: Optional External Bootstrap Diode

The recommended external BST diode is

IN4148, and the recommended BST capacitor

value is 0.1µF to 1μF.