Blue LED Premature Failure

[Justin1269]

Hi All,

We are using a blue LED in our design From Rohm and We have been having premature LED failure within a month from the field.
I am not sure what's going with the LED driver.

I have attached the driver circuit/datasheet for your review and help would be greatly appreciated.

Thanks,
WJ

 

[danadakk]

Did you send them in to Rohm for failure analysis ?


Regards, Dana.

 

[betwixt]

The LED is rated for 30mA absolute maximum continuous current but you can let it pass (7.5 - 3.3) / 121 = 34mA and that assumes the voltage doesn't exceed 7.5V but if the IC is powered from a higher supply it can force extra current through the LED.

Brian.

 

[danadakk]

What is the power applied to the OpAmp pins, the - and + supply pins ?

Any chance during power sequencing DAC output spikes >> 5V ? Using
your DSO, edge trigger, Vtrig set to say 6V, single shot, timebase around
1 mS or so.....you would confirm if there is any triggers.


Regards, Dana.

 

[pjmelect]

I suspect that you are overdriving the LED, what is the maximum ambient temperature that it is operating at?

 

[FvM]

A special property of your current regulator is that it starts with a current peak when you switch on the enable transistor. This should be fixed.

 

[D.A.(Tony)Stewart]

There are many reasons for LED field failures that in my business I have successfully identified and corrected with various factories, having supplied about 1 million parts from many of the best factories of the brightest parts before I retired.

What symptoms and sample size do you have?
Can you show us the product and electrical test results of failures?
Can you send an optical microscope image?

 

[KlausST]

A special property of your current regulator is that it starts with a current peak when you switch on the enable transistor. This should be fixed.

Really?
This was my first thought, too.

Then I checked the circuit.
When you switch ON the transistor, then it was OFF before. So high ohmic on the DRAIN. This means high voltage at the DRAIN, thus high FEEDBACK voltage.
Thus I expect the OPAMP output to be LOW.

Klaus

 

[danadakk]

I simulated power on transient to the OpAmp as well, no indication there
would be excess current. But then OpAmp Spice models are not exactly known
for incorporating their non linear behaviour into the model......


Regards, Dana.

 

[stenzer]

Hi,

as asked by @danadakk already, what is the supply voltage of the opamp? By having a look on the datasheet of the LM324 [1], one can see that a higher VCC than 5 V to is required to controle the BJT in a sufficient way (Vout = ~3.5 V @ VCC = 5 V). In case the opamp supply voltage is sufficient high, and the DAC voltage is set to a high voltage, the opamp will act as current source. Also here, the opamp might not be able to provide the current (over full temperature range) according to the datasheet. If the opamp is supplied from a high voltage e.g. 15 V and it has to provide the current e.g. 25 mA, this would lead to quite some thermal stress of the opamp. Also the orientation of your electrolytic capacitor is questionable as the output of the opamp will have the higher potential. But I assume it's just the wrong symbol, and you are using a ceramic type one, as the capacitance is only 100nF. And as pointed out already, have a look on the derating curve of the diode, where you can see the maximum current depends on the ambient temperature.

So here my questions are

• Can you ensure the diode is failing and not an other component in the shown circuitry i.e. the opamp?
• What's the supply voltage of the opamp?
• What is the (expected) ambient operating temperature?

[1] https://www.onsemi.com/download/data-sheet/pdf/lm324-d.pdf

BR

 

[KlausST]

Hi,

what could be the problem?
* missing power suppply capacitors for the Opamp and LED supply
* huge (stray) inductance on the LED supply
* unstable VS at all. In case there is a short VS outage, the OPAMP output will saturate HIGH ... and on immediate coming back of VS it may cause a current spike on the LED
*??

***
I think I´d rather connect the LED on the collector side of the BJT. This way the resistor in the emitter path acts as a fast local feedback.
Also the OPAMP output swing is reduced.

Klaus

 

[D.A.(Tony)Stewart]

I could suggest a few more scenarios of LED failure but the bottom line is that it is an engineer’s responsibility to find the root cause from :
ESD mishandling, storage mishandling unpackaged , unbaked , design review flaws, lack of DVT, design flaw from lack of active current sensing or limiting excess soldering time in liquidus state, if THT, leads too short or bent wrong, etc, etc

The designer is fully responsible to find the root cause.

 

[FvM]

Then I checked the circuit.
When you switch ON the transistor, then it was OFF before. So high ohmic on the DRAIN. This means high voltage at the DRAIN, thus high FEEDBACK voltage.
Thus I expect the OPAMP output to be LOW.

You are right. The current sense connection is so that the controller starts from minimal current.

 

[D.A.(Tony)Stewart]

My review of your design is as follows;

If you need a brighter source , use two in parallel then 125 is nominal 121 is K
Never design using Absolute maximum Specs.
This compromises reliability expectations.

 

[Easy peasy]

Where did you buy the Blue LEDS from ?

 

[D.A.(Tony)Stewart]

Any 5 mm LEDs with 5V on DAC and draw 41 mA will slowly burn out LED, regardless of supplier

This is a design fault.

 

[Justin1269]

Any 5 mm LEDs with 5V on DAC and draw 41 mA will slowly burn out LED, regardless of supplier

This is a design fault.

Ok I got it, what would you suggest?

 

[D.A.(Tony)Stewart]

change as I marked up schematic according to Vin , Vout , Vcc max
what kind of batteries?

If you know for sure that the current never exceeded 20 mA from Vinput =2.4 max then you have an ESD handling problem.

 

[pjmelect]

I would increase the value of R59 to 220 ohms.

 

[D.A.(Tony)Stewart]

Hi All,

We are using a blue LED in our design From Rohm and We have been having premature LED failure within a month from the field.
I am not sure what's going with the LED driver.

I have attached the driver circuit/datasheet for your review and help would be greatly appreciated.

Thanks,
WJ

View attachment 188932

Here is another approach.

http://tinyurl.com/26a4r5tv SIM

Designed for 0 to %v for 0 to 20 mA out <5% tolerance using 1% resistors.

Here lowering Vin with an attenuator allows Ve to reduce for 20 mA while using gain ratios to match attenuation ratio and 50 ohm current sensing R.

or this raise the output Vo even more.