DER337 Design Example Questions

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Hello,

We build the DER337 and it works even better then expected!

Now we added a potmeter to set the output current.

As result we see that the driver makes steps up/down (about 50mA) to create the output current.

Question 1: How can we modify the step-size to 10mA?

When we change the input voltage we see the driver starts to correct the output current by making steps up/down. As result we see a litle change in LED-light. Also on the LED-current we see a short over-/under shoot.

Question 2:  What is the best way to minimize this over-/undershoot?

Thanks inadvance for all usefull respons.

Hi,

 

DER337 is using direct surrent sensing so potmeter is usually not recommended. Using potmeter will require a very low value potmeter (2ohms) to get good accuracy which i believe is very hard to get.

 

Regarding the overshoot and undershoot, the low bandwidth of the system that makes the pf high causes this overshoot and undershoot scenarios. LEDS must withsatnd these undershoot problems. If its not acceptable, you can always add a current sense/active preload circuit  that loads the output everytime the output current exceeds a threshold value. This could be a transistor that robs the output current only when the output exceeds a threshold value. 

 

Regards

 

Hi,

We use a 10E potmeter with a serie resistor of 2E.

It is NOT liniear but works OK for us.

My question was also: is it possible to influence the step-size of corrections?

Part 2 the over/undershout is now clear for me.

Thanks for your reaction.

Hi,

Try doubling the capacitance of C5 (RC filter from the current sense from DER337 schematic) from 1uF to 2.2uF.

Regards