MP3378

– 4-CHANNEL WLED CONTROLLER PLUS BUCK CONVERTER

PRELIMINARY SPECIFICATIONS SUBJECT TO CHANGE

MP3378 Rev. 1.0

www.MonolithicPower.com

20

5/26/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2017 MPS. All Rights Reserved.

Buck Converter Section:

Setting the Output Voltage

The external resistor divider is used to set the

output voltage. Also, the feedback resistor (R1)

sets the feedback loop bandwidth with the

internal compensation capacitor. When R1 is

fixed, R2 is then given by Equation (10):

OUT2

R1

R2

V

1

0.8V





(10)

A T-type network is highly recommended (see

Figure 5).

FB

R1

R2

RT

12

VOUT2

Figure 5

—T-type network

Table 2 lists the recommended T-type resistor

values for a common 5 V output voltage.

Table 2

—Resistor selection for common 5 V

output voltage

(V)

R1

(kΩ)

R2

(kΩ)

RT

(kΩ)

L2

(µH)

Co

(µF)

5

40.2

7.5

51

10

66

Selecting the Inductor

A 4.7 µH to 10 µH inductor with a DC current

rating at least 25 percent higher than the

maximum load current is recommended for most

applications. For highest efficiency, the inductor

DC resistance should be less than 15

mΩ. For

most designs, the inductance value can be

derived from Equation (11):

OUT2

IN2

OUT2

IN2

L2

OSC

V

(V

V

)

L2

V

I

F





  

(11)

Where ΔI

Choose the inductor current to be approximately

30 percent of the maximum load current. The

maximum inductor peak current is calculated

using Equation (12):

L2

L2(MAX)

LOAD2

I

II

2





(12)

Under light-load conditions (below 100 mA), a

larger inductance is recommended for improved

efficiency.

Setting the AAM Voltage

The AAM voltage is used to set the transition

point from AAM to CCM. It should be chosen to

provide

the

best

combination

of

efficiency,

stability, ripple, and transient.

If the AAM voltage is set low, stability and ripple

improves, but efficiency during AAM mode and

transient degrades. Likewise, if the AAM voltage

is set high, then the efficiency during AAM and

transient

improves,

but

stability

and

ripple

degrade. Calculate the optimal balance point of

AAM voltage for good efficiency, stability, ripple,

and transient.

Adjust the AAM threshold by connecting a

resistor from AAM to ground. An internal 6.2 µA

current source charges the external resistor (see

Figure 6).

R5

75K

13

AAM

Figure 6

—AAM network

Generally, R5 is then given by Equation (13):

(13)

To optimize AAM, see Figure 7.

Figure 7

—AAM selection for common output