MP3378E

– 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER

MP3378E Rev. 1.02

www.MonolithicPower.com

19

5/26/2017

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Table 1: The Range of PWM Dimming Duty

100<f

200

0.30%

100%

200<f

500

0.75%

100%

500<f

1k

1.50%

100%

1k<f

2k

3.00%

100%

2k<f

5k

7.50%

100%

5k<f

10k

15.00%

100%

10k<f

20k

30.00%

100%

2. Analog Dimming

For analog dimming, apply a PWM signal or a

DC voltage signal to ADIM. An internal RC filter

(10M

Ω

resistor

and

100pF

capacitor)

is

integrated to ADIM. If a PWM signal is applied to

ADIM, a >20kHz frequency is recommended for

better PWM signal filtering performance and to

ensure the amplitude voltage is higher than 1.5V,

and the low-level voltage is less than 0.4V. For

the DC signal input, apply a DC input signal

ranging from 0.41V to 1.49V to set the LED

current from minimum to full scale linearly.

BUCK CONVERTER SECTION:

Setting the Output Voltage

The external resistor divider is used to set the

output voltage. The feedback resistor R1 also

sets the feedback loop bandwidth with the

internal compensation capacitor. When R1 is

fixed, R2 is then given by Equation (13):

OUT2

R1

R2

V

1

0.8V





(13)

The T-type network is highly recommended (see

Figure 5).

FB

R1

R2

RT

12

VOUT2

Figure 5: T-type Network

Table 2 below lists the recommended T-type

resistor values for the common 5V output voltage.

Table 2: Resistor Selection for Common Output

Voltages

(V)

R1

(kΩ)

R2

(kΩ)

RT

(kΩ)

L2

(µH)

Co

(µF)

5

40.2

7.5

51

10

66

Selecting the Inductor

A 4.7µH to 10µH inductor with a DC current

rating at least 25% higher than the maximum

load

current

is

recommended

for

most

applications. For the highest efficiency, the

inductor DC resistance should be less than

15mΩ. For most designs, the inductance value

can be derived from Equation (14):

OUT2

IN2

OUT2

IN2

L2

OSC

V

(V

V

)

L2

V

I

F





  

(14)

Where ΔI

Set the inductor current to be approximately 30%

of the maximum load current. The maximum

inductor peak current is calculated with Equation

(15):

L2

L2(MAX)

LOAD2

I

II

2





(15)