MP4050 – NON-ISOLATED, HIGH BRIGHTNESS, LED DRIVER

MP4050 Rev.1.02

www.MonolithicPower.com

13

1/18/2016

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APPLICATION INFORMATION

Component Selection

Input Capacitor

The input capacitor is used to supply the DC

input voltage for the converter. Figure 6 shows

the typical DC bus voltage waveform of full

bridge rectifier.

t

DC input voltage

AC input voltage

0

Figure 6: Input voltage waveform

When the full-bridge rectifier is used, the input

capacitor is usually set as 2μF/W for the

universal input condition. And when small

power output, the half-bridge rectifier could also

be used with a bigger capacitor.

Very low DC input voltage could cause thermal

problem in LED application in Buck topology

application. The minimum DC voltage is limited

by the maximum duty cycle of MP4050 as

following expression:

OON _ MAX

OFF _ MIN

DC(min)

ON _ MAX

V(t

t

)

V

t





Inductor

MP4050 has a minimum off time limit and

maximum on time limit. Both time limits affect

the inductance value. The maximum inductance

value and minimum inductance value can be

obtained as follows:

DC(min)

O

ON _ MAX

mMAX

Peak

(V

V ) t

LL

I





OOFF _ MIN

mMIN

Peak

Vt

LL

I





If the inductance value is too large, the

converter enters CCM when the frequency

reaches the minimum operating frequency. In

such case, the reverse recovery of freewheeling

diode results more power loss. Normally, it’s

better to make the converter operate in DCM.

The following expression shows the limit of the

minimum operating frequency.

O

m

2

SMIN

Peak

DC(min)

O

O

2I

L

11

f(

) I

VV

V











Freewheeling Diode

The diode should have a maximum reverse

voltage rating which is greater than the

maximum input voltage. The current rating of

diode is determined by the output current which

should be larger than 1.5~2 times output

current.

Slow diodes cause excessive leading edge

current spikes during start-up which is not

acceptable. Long reverse recovery time of

freewheeling diode can also affect the efficiency

and the circuit operation. So ultrafast diode

(Trr<75ns) such as WUGC10JH or EGC10GH

are recommended.

Output Capacitor

The output capacitor is required to filter the

inductance current and maintain the DC output

voltage.

The output current ripple is reduced by using a

bigger output capacitor. A low ESR capacitor is

necessary in low temperature application.

If the output voltage ripple is limited, the

ceramic, tantalum or low ESR electrolytic

capacitors are recommends to use. The output

voltage ripple can be estimated by:

CCM_Ripple

ESR

SO

i

Vi R

8f C



  

CCM

2

OPeak

O

DCM_Ripple

Peak

ESR

SO

Peak

II

I

V(

)

I

R

fC

I









DCM