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LED driver circuit doesnt work

Winsu

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Hi All,

I have designed a circuit using the ZLED7730-ZI1R to power up a few LEDs. The string current is to be meant 0.35A but I had to limit the PSU as it would draw whatever figure I allow on the PSU. I have used normal diodes as LEDs are very expensive ( this is the diode I have used: CD123D-B140LR ).

The dim 1 and dime 2 pins are to set a dim profile. I initially set the DIM to GND, but as it wasn't working and left DIM 1 floating ( it would mean 100% always).

I don't know why it is not working. I set the PSU ay 24V and 0.4A and the voltage drop to 4V and I draws the 0.4A. There are 3.5V dropping accros the diodes and 0.5V between Lx and GND. For some reason he internal mosfet is constantly on and it is not regulating at all. Below is the schematic. Any help would be very useful, thanks.


1593713409794.png
 

KlausST

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Hi,

You have a wrong pin numbering. Maybe this is the problem.

Did you strictly follow PCB layout guidelines?


Klaus
 

Winsu

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Hi Klaus

I noticed that and I fixed on the PCB, the right hand side on the chip is actually up side down, is that what you mean?

I didn't follow the layout guidelines but I don't think it would stop completely to regulate just because of that...
 

KlausST

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Hi,

So, you really think they write this just out of fun...
The following guidelines are strongly recommended when laying out application circuits:
Don't be surprised if it does not work.

Klaus
 

Winsu

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I didn't follow the guide lines consciously but the result is fairly close to what the guide suggest.....

I can't find the reason why it is not working
 

KlausST

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Hi,

Without your PCB layout ... we neither can't find it...


Klaus
 

treez

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your sense res is 0.27v
You clamped the current to 0.4a.
So your sense voltage is only 0.108v.......this is below the threshold of 15% above 100mv....so no wonder the fet is on all the time.

What voltage is across the leds....?

Set psu to say 2v above this, and set the psu supply current limit to 0.6a, and try again
 

Winsu

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your sense res is 0.27v
You clamped the current to 0.4a.
So your sense voltage is only 0.108v.......this is below the threshold of 15% above 100mv....so no wonder the fet is on all the time.

What voltage is across the leds....?

Set psu to say 2v above this, and set the psu supply current limit to 0.6a, and try again
I have just raised the current and it keeps open all the time. Across the LEDs are 3.5V and the rest of the voltage is dropping across drains and source of the FET.
 

treez

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What is the current rating of your inductor?

What is the voltage across the sense res...if it is flat dc, and above 120mV, then the fet is broken and on all the time....or you have a misconnection somewhere
 

Winsu

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Here you can see attached the layout. I didn't spend much time on the layout as I just wanted to verify that they circuit works.....

Track connected to pin 5 needs to be removed and a track need to be added between pin 8 and anode of D1.



1593770849440.png
--- Updated ---

What is the current rating of your inductor?

What is the voltage across the sense res...if it is flat dc, and above 120mV, then the fet is broken and on all the time....or you have a misconnection somewhere

The Farnell order code for the inductor is 2329187 and its current rating is 1.7A. The voltage on the RS sense is when the PSU is set to 0.6A is almost 2V , that means the FET is not switching off at all. It could indeed be broken, but I have just measured continuity between LX and GND and it is not shorted.

If there is voltage across the LED it means that the loop is properly soldered. It is just four pins on the IC that needs to be soldered and they look like they are.....Could it be something wrong in the design?
 
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treez

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check if is the chip the right way round?
Is r1 correct value.....if cant check, remove it and see what happens then.

Your input capitance looks way too low...increase to 220uf or more.
 
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Winsu

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The chip is good way around and I already increase the input capacitance and it didn't improve... I think I will have to order component again and populate the board again....if it keeps failing there must be something in the design....
 

wwfeldman

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I have used normal diodes as LEDs are very expensive ( this is the diode I have used: CD123D-B140LR ).
the forward drop on the CD123s is about 0.35 V, so 10 in series is the 3.5V you measured across the diodes.
the forward drop on LEDs is 2? 3? V.

you have essentially shorted the output
the regulator is trying to produce the desired output voltage and/or current, but your load is too small
add a series resistor consistent with the expected V and I for the real LEDs you want to use

or replace the schottkey diodes with plain cheap LEDs -
digikey has about 500 different LEDs, surface mount, less than $1 (American) in stock
 

Winsu

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the forward drop on the CD123s is about 0.35 V, so 10 in series is the 3.5V you measured across the diodes.
the forward drop on LEDs is 2? 3? V.

you have essentially shorted the output
the regulator is trying to produce the desired output voltage and/or current, but your load is too small
add a series resistor consistent with the expected V and I for the real LEDs you want to use

or replace the schottkey diodes with plain cheap LEDs -
digikey has about 500 different LEDs, surface mount, less than $1 (American) in stock
Indeed the output is kind of shorted. I then added more resistance to the string but the voltage across the string rises as I increase the resistance but if I allow more current to flow from the PSU the current keeps going up. The FET is not regulating anymore.

I don't know when it failed....., when I get more drivers I will test it adding more resistance to the string.....
 

treez

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Are you sure your diode is not dry joint?....because if it was then the fet may get overvoltaged when it switches off.
As you know, the inductor current always needs somewhere to flow to
 

Winsu

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Are you sure your diode is not dry joint?....because if it was then the fet may get overvoltaged when it switches off.
As you know, the inductor current always needs somewhere to flow to
I have just double checked and it is not that, unfortunately....
 

treez

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ok, can you take the inductor off, and just see if just connecting up vcc draws loads of current.....just to check if the chip really is dead
 

    Winsu

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KlausST

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Hi,

Still the pin numbering mistake.
* datasheet says: pin8 is output Lx
* your PCB says: pin5 is output Lx

Klaus
 

    Winsu

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Winsu

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Hi All,

Thanks Klausst, it was corrected previously in the PCB with flying wires and cutting a track.

I have noticed that the inductor was incorrectly soldered...., it was soldered in a way that the inductor was by passed and it was effectible like a piece of wire....

It is working fine now, the Rsense which is 0.27ohm has across 0.108V , so it is spot on, thanks!
 

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