White LED driver debugging

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I'm using the following circuit to drive the backlight LEDs of an TFT LCD module:

 

 

Please note that the input voltage was changed to 3.3V from 5V (should not matter).

The FB pin is supposed to be regulated to 300mV so the current delivered to the LEDs is given by i = 0.3 / R

Since the LCD datasheet mentioned a max current of 27.5mA I've decided to go with the 11 Ohm resistor which limits the current to just a bit under that.

 

The driver datasheet mentioned it can drive up to 5 LEDs in series. Recommended max voltage is 24V. The LCD actually has 6 LEDs with a suggested max. voltage of about 21V so I figure that although there are 6 LEDs, electrically this seems to be all within spec.

When turning on the device (with SHDN on high) I get the LCDs going + current draw adds another 400mA and power supply goes into standby.

 

Is there anything that can be wrong in this part of the circuit or should I look for the faults elsewhere?

 

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looks good...assume you  connect your led to R3 (pin3) & not gnd.    Ensure that your pins 1-5 match the physical chip layout, since pin numbering can vary (if you are using a PCB model).

You are super close to the current limit (you say 27.5, but datasheet seems to show higher) ..try 15 ohms for starters.   Try driving 2 leds (jumper out 3 of them)...does that work?  

When you drive pin4 SHDN high is it really hi?  Make sure it is not 5V when the chip runs at 3.3V (though datasheet seems to allow that).

Are you leds wired backward (a common error), or even one led out of the gang is backwards (quite aggrevating).  

Is your supply solid? 1uF might not be enough--pulsating droop could confuse the chip.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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What are the spec's for the LED backlight?  max current?   You may have blown one out in the string of LED's.   I would start with a lower current (10-12ma) on a new display and see what happens.

 

Jim

 

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The circuit is quite simple. I don't see any obvious place where there could be a problem.

 

400mA is the approximate internal switch current limit. Hmmmm... Are you seeing 0.3V on the FB pin of U4?

 

What happens if you run it with D1 removed? U4 should go into over-voltage protection, and you should get a current draw of about 2mA.

 

If that doesn't happen, I would suspect either the internal switch has gone, or a solder short somewhere.

 

- S

 

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What happens if you run it with D1 removed? U4 should go into over-voltage protection, and you should get a current draw of about 2mA.

That mean's nothing...no D1, no output--boosted or otherwise!!!

 

No D1, no boosted voltage possible, hence probably no overvoltage detection either.

 

Time for a bowl of solder soup.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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What does the datasheet say?

Your circuit looks like a standard example implementation from the datasheet.

Is the inductor within recomended range?

Always, never asume your hardware is OK. Chips fail for no apparent reason, especially while experimenting with them.

Do you have more of these chips?

Also, if the datasheet recommends 5 leds max, you've put 6 on them and it don't work, then try a new chip with only 3 LED's.

 

It also seems to be a pretty whimpy chip.

I've experimented a (very little) bit with LED drivers myself, and it's not uncommon for these things to happily switch 1A and boost upto 70V.

 

Also:

Go buy an Oscilloscope. These things used to be expensive, but nowaday's you can get a pretty decent new scope for below EUR300.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com