MP4050A – NON-ISOLATED, HIGH-BRIGHTNESS LED DRIVER, WITH ENHANCED THERMAL FEATURE

MP4050A Rev. 1.01

www.MonolithicPower.com

14

1/14/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

Component Selection

Input Capacitor

The input capacitor is used to supply DC input

voltage to the converter. Fig. 6 shows the

typical DC bus voltage waveform of a full-bridge

rectifier.

FIGURE 6. Input Voltage Waveform

When a full-bridge rectifier is used, the input

capacitor is set usually as 2μF/W for the

universal input range. With a low-power output,

the half-bridge rectifier can be used with a

bigger capacitor.

Very low DC input voltage causes thermal

problems in LED applications with buck

topology. The minimum DC voltage is limited by

the maximum duty cycle of the MP4050A as

follows:

OON _ MAX

OFF _ MIN

DC(MIN)

ON _ MAX

V(t

t

)

V

t





Inductor

The MP4050A has a minimum off-time limit and

maximum on-time limit. Both time limits affect

the inductance. The maximum and minimum

inductance values can be obtained as follows:

DC(MIN)

O

ON _ MAX

mMAX

PK

(V

V ) t

LL

I





OOFF _ MIN

mMIN

PK

Vt

LL

I





If the inductance is too large, the converter

enters CCM, and the frequency drops to the

minimum operating frequency. If this occurs,

the reverse recovery of the freewheeling diode

results in more power loss. Normally, it’s better

to have the converter operate in DCM. The

following expression calculates the limit of the

minimum operating frequency:

O

m

2

SW _ MIN

PK

DC(MIN)

O

O

2I

L

11

f(

) I

VV

V











Freewheeling Diode

The diode’s maximum reverse-voltage rating is

higher than the maximum input voltage. The

current rating of the diode is determined by the

output current (which is larger than 1.5 to 2

times the output current).

Slow recovery diodes cause excessive leading

edge current spikes during start-up. The long

reverse-recovery time of the freewheeling diode

affects efficiency and the operation of the

WUGC10JH or EGC10GH, is recommended.

Output Capacitor

An 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 applications.

If the output-voltage ripple is limited, ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended. The output-voltage ripple can

be estimated by the following equations:

CCM_Ripple

ESR

SW

O

i

Vi R

8f C



  

in CCM

2

OPK

O

DCM_Ripple

PK

ESR

SW

O

PK

II

I

V(

)

I

R

fC

I









in DCM