MP92325 – 24V, 3A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP92325 Rev. 1.0

www.MonolithicPower.com

12

3/24/2016

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Selecting the Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended.

Low

ESR

capacitors

are

preferred for keeping the output voltage ripple

low. The output voltage ripple can be estimated

with Equation (7):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

equivalent series resistance (ESR) value of the

output capacitor.

For ceramic capacitors, the impedance at the

switching frequency is dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification,

the output voltage ripple can be estimated with

Equation (8):

OUT

OUT

OUT

2

IN

S1

VV

∆V1

V

8f

L

C2



 



 



(8)

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple can be approximated with

Equation (9):

OUT

OUT

OUT

ESR

IN

S1

VV

∆V1

R

fL

V



 









(9)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP92325 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator, the applicable

conditions of the external BST diode are:





Duty cycle is high: D =

IN

OUT

V

V

> 65%

In these cases, an external BST diode is

recommended to be placed between VCC and

BST (see Figure 6).

Figure 6: Optional External Bootstrap Diode

Added to Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST cap is recommended to

be 0.1μF - 1μF.