ZXLD383

Issue 1 - September 2008

6

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© Diodes Incorporated

www.diodes.com

Low ripple LED current mode

It is possible that the peak LED current required to achieve a given average current is either too

high for the LED of choice or it leads to some loss of efficiency (due to LED resistance losses). In

these cases, just two extra low cost components can be added to provide a low ripple current

supply for the LED. The Schottky diode D1 and capacitor C1 rectifies and smoothes the output of

the ZXLD383 giving a low ripple current supply to the load LED. Of course, this circuit could also

be used to power loads other than LEDs.

Buck-boost mode

Simple boost converters can run into problems when the input supply voltage is similar to or

exceeds the intended load voltage as there is usually a direct current path from the power source

through to the load via the boost inductor. This path does not require switching action and so is

uncontrolled. When using the ZXLD383, this problem can be avoided by wiring the cathode of the

load LED to Vcc rather than ground. Without switching action, the LED is reverse-biased and so

no current can flow. When switching, the anode of the LED is driven to Vcc + Vf(led). The higher

than normal output voltage reduces the available output current as described earlier and this is

shown in the typical data provided.

L

(u H )

(m A )

47

7.5

22

15.5

10

31

6.8

46

4.7

58

D1

LE D

C1

Z XLD 383

3

2

1

4

EN A

GN D

VO U T

5

VC C

L1

1. 5V

NC

L

(u H )

(m A )

47

5.5

22

10.3

10

23.2

6.8

36.7

4.7

46.2

ZXL D 3 8 3

3

2

1

4

EN A

GN D

VO U T

5

VC C

L1

LE D

NC

3V