MP3378

– 4-CHANNEL WLED CONTROLLER PLUS BUCK CONVERTER

MP3378 Rev. 1.01

www.MonolithicPower.com

21

5/26/2017

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© 2017 MPS. All Rights Reserved.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low

ESR

capacitors

for

the

best

performance.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients. For

most applications, a 22 µF capacitor is sufficient.

Since the input capacitor (C4) absorbs the input

switching current, it requires an adequate ripple-

current rating. The RMS current in the input

capacitor can be estimated with Equation (14):

OUT2

OUT2

C4

LOAD2

IN2

IN2

VV

I

I

1

VV

















(14)

See Equation (15):

LOAD2

C4

I

I

2



(15)

For simplification, choose an input capacitor with

a RMS current rating greater than half of the

maximum load current.

The input capacitor can be electrolytic, tantalum,

or

ceramic.

When

electrolytic

or

tantalum

capacitors are used, a small high-quality ceramic

capacitor (i.e. 0.1

μF) should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge in order

to prevent excessive voltage ripple at the input.

The input voltage ripple caused by capacitance

can be estimated with Equation (16):

LOAD2

OUT2

OUT2

IN2

SW2

IN2

IN2

I

V

V

ΔV

(1

)

F

C4

V

V





 



(16)

Selecting the Output Capacitor

The output capacitor (C5) is required to maintain

the DC output voltage. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred to keep the

output voltage ripple low. The output voltage

ripple can be estimated with Equation (17):

)

5

F

8

1

(R

)

V

V

(1

L2

F

V

ΔV

SW2

ESR

IN2

OUT2

SW2

OUT2

OUT2

C

















(17)

equivalent series resistance (ESR) value of the

output capacitor.

When using ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance. The output voltage ripple is caused

mainly by the capacitance. For simplification, the

output voltage ripple can be estimated with

Equation (18):

OUT2

OUT2

OUT2

2

SW2

IN2

VV

ΔV

(1

)

8 F

L2 C5

V



 







(18)

When using tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the output

ripple can be approximated with Equation (19):

OUT2

OUT2

OUT2

ESR

SW2

IN2

VV

ΔV

(1

) R

F

L2

V



 





(19)

The characteristics of the output capacitor affect

the stability of the regulation system.

External Bootstrap Diode (BST)

An external bootstrap diode may enhance the

efficiency

of

the

regulator,

the

applicable

conditions for an external BST diode are:

IN2

OUT2

V

V

>65%

In these cases, an external BST diode is

recommended from VCC2 to BST (see Figure 8).

The recommended external BST diode is IN4148,

and the BST capacitor is 0.1

μF ─1μF.

+

VCC2

SW

BST

MP3378

0.1µF-1μF

L2

External BST Diode

1N4148

Figure 8

—Add optional external bootstrap diode

to enhance efficiency