MP3378E

– 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER

MP3378E Rev. 1.02

www.MonolithicPower.com

20

5/26/2017

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© 2017 MPS. All Rights Reserved.

Under light-load conditions below 100mA, a

larger inductance is recommended for improved

efficiency.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, and 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 22µF

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 by Equation (16):

OUT2

OUT2

C4

LOAD2

IN2

IN2

VV

I

I

1

VV

















(16)

shown in Equation (17):

LOAD2

C4

I

I

2



(17)

For simplification, choose an input capacitor with

an 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. 0.1μF) should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide

a sufficient

enough

charge to prevent excessive voltage ripple at the

input.

The

input

voltage

ripple

caused

by

capacitance can be estimated with Equation (18):

LOAD2

OUT2

OUT2

IN2

SW2

IN2

IN2

I

V

V

ΔV

(1

)

F

C4

V

V





 



(18)

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 (19):

OUT2

OUT2

OUT2

ESR

SW2

IN2

SW2

VV

1

ΔV

(1

) (R

)

F

L2

V

8 F

C5



 











(19)

equivalent series resistance (ESR) value of the

output capacitor.

When using ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance, which mainly causes the output

voltage ripple. For simplification, the output

voltage ripple can be estimated with Equation

(20):

OUT2

OUT2

OUT2

2

SW2

IN2

VV

ΔV

(1

)

8 F

L2 C5

V



 







(20)

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 (21):

OUT2

OUT2

OUT2

ESR

SW2

IN2

VV

ΔV

(1

) R

F

L2

V



 





(21)

The characteristics of the output capacitor affect

the stability of the regulation system.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency

of

the

regulator.

The

applicable

conditions of the external BST diode are:

IN2

OUT2

V

V

>65%

In these cases, an external BST diode from

VCC2 to BST is recommended (see Figure 6).

The recommended external BST diode is IN4148,

and the BST cap is 0.1─1μF.

+

VCC2

SW

BST

MP3378E

0.1-1

μF

L2

External BST Diode

1N4148

Figure 6: Add Optional External Bootstrap Diode

to Enhance Efficiency