lt1308-based 1A Charger

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Snipeye
 
Posts: 198
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Re: lt1308-based 1A Charger

Post by Snipeye »

Update!

Parts and boards got here, assembled it, checked each connection, checked the output voltage aaaaaand.... it's just the same as the batteries. :?

I checked the reference pin on the chip, datasheet says it should be 1.22v, but it was the same as the batteries, too. When I checked the resistance of the two divider resistors used to set the output voltage, what should have been 309k ohms was more like 100k, and what should have been 100k was more like 76K. I'm guessing this is just because of the other parts in the system - where else should I check voltages/resistances? I've doubled checked all solder joints, and I'm pretty sure they're all good.

Advice?

EDIT: according to the datasheet, resistances like that would provide a voltage of 2.8ish volts, but it's not giving that either.

waltr
 
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Re: lt1308-based 1A Charger

Post by waltr »

I've been waiting for an update and hoping for really good news but not so yet.

Measuring resistors 'in circuit' can be difficult due to other parts in the same circuit. Typically resistor will measure lower in-circuit, other resistances are in parallel so the resistance is less. The measurements you made sound ok. If a resistor in circuit ever measures much higher then there is a problem.

Troubleshooting this circuit.
1- remove the battery and re-check all connections.
2- check that all components that have a polarity are correctly installed. diode, Elect caps and IC.
3- check that all ground connection are good every where on the board and connectors.
4- check between pins for shorts on the chip with the Ohmmeter.

Ref V pin (FB) should be 1.22V. Yes, without the battery re-check the resistor dividers with the Ohmmeter.
With the battery, the top end of the resistors should be at Vout and the bottom at ground (0V). The middle should be between Vout and ground. If at Vout then either, the bottom resistor is open or the chip has a short from Vout to the ref(FB) pin (pin2). Pin 3 is connected to the Vin so a short between pins 2 and 3 could be the problem.

Check and post back.

Snipeye
 
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Re: lt1308-based 1A Charger

Post by Snipeye »

I'm not at home to test right now, but I took a bunch of measurements after my post that I'm trying to remember now - I had in 3 batteries, their combined voltage was 4.83V. How do you recommend I measure the voltage at FB since it's soldered on and the path of least resistance will not be through the multimeter...?

waltr
 
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Re: lt1308-based 1A Charger

Post by waltr »

I really do not understand your question.
When you get home and time to examine the circuit please follow the troubleshooting steps I outlined. And pay attention to the last paragraph. From the info you posted this is very likely where to problem is (a short between pins 2 and 3 on the LT1308 chip).

Then post back what you find.

Snipeye
 
Posts: 198
Joined: Thu Feb 25, 2010 10:03 am

Re: lt1308-based 1A Charger

Post by Snipeye »

I checked every pin on the LT1308b without the battery, and the only ones my multimeter registered as connected were pins 3, 5 and 6. That is, VIN, SW, and SHDN - all of which should be connected. In the picture below, I had not installed the switch to turn on the device, but instead put one probe of my multimeter and point a, and the other at points b, c, and d - voltages were as such:

B. 4.83V (VIN)
C. 4.80V
D. 4.83V (VIN)
points
points
points.png (65.1 KiB) Viewed 571 times
Anything else I should I measure?

waltr
 
Posts: 306
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Re: lt1308-based 1A Charger

Post by waltr »

Ok, lets see if I followed what you did:

With your meter on Ohm scale:
You measured across each adjacent pins on the LT1308: 1-2, 2-3, 3-4, 4-5, 5-6, 6-7, 7-8
What other two points did you measure with the Ohmmeter?

With the battery connected to the Point a (negative) and the pin above (Inductor and LT1308 pin 6).
The switch connector at the bottom of the board in un-connected.
Meter of Volt scale with the black lead on point A and the red lead at points B, C, D.
The measurement at point D should be at the battery Voltage if chip is not powered, battery + connected to the inductor but - not to ground. This is due to the inductor is very low Ohms at DC, and the diode conductor to that point. the other two points would also be at or very close to that same Voltage since the cattery is not connected.


Is the LT1308 installed correctly? pin 1 at pin1? These SM parts can be easy to install backwards.
The plastic package has a slight bevel on the side with pins 1 to 4. This bevel should be towards the top.

With the Voltmeter black lead on point B and the battery/switch connected and on, what are the Voltages at points D, C?
C should be about 1.2V and D at about 5V. The battery must be connected between point B (neg terminal) and point D (pos terminal) for the resistive divider to work and provide about 1.2 V at point C (pin 2).

The LT1308 says this chip will run at light loads but an output load may help. So connect a 500 Ohm resistor across the output to provide a 10mA load.

This chip is a STEP-UP converter so to get ~5V output the input Voltage must be less. Else the DC input voltage is divided to pin 2 and the chip sees that the output Voltage to high enough so it doesn't need to run.
So try a lower Battery Voltage, like less than 4V.

I've been check this thread often so post back results.

Snipeye
 
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Re: lt1308-based 1A Charger

Post by Snipeye »

On the LT1308, I measured from 1 to 2, 3, 4, 5, 6, 7, and 8, from 2 to 3, 4, 5, 6, 7, 8, from 3 to 4, 5, 6, 7, 8, from 4 to 5, 6, 7, 8, from 5 to 6, 7, 8, from 6 to 7, 8, and from 7 to 8. The only ones connected were 3, 5, and 6. With the battery installed at the two contacts below point a, and a switch installed and turned on at the two contact at point a, I measured the voltage from point b to points c and d with a VIN of 3.2V (two AA). At point C, the voltage was 0.6ish volts, at at point d, it was 2.5ish volts. This was with a 3k resistor attached for a hypothetical load of 60ma. The LT1308B rapidly heats to the point where it is too hot to touch, so I only turn the device on for the shortest amount of time possible - maybe 10 seconds to get a reading. Where else do you recommend I measure?

waltr
 
Posts: 306
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Re: lt1308-based 1A Charger

Post by waltr »

At point C, the voltage was 0.6ish volts, at at point d, it was 2.5ish volts.
This means the resistive divider for the FB (pin 2) is correct. But the correct Vout is not being produced.
The LT1308B rapidly heats to the point where it is too hot to touch, so I only turn the device on for the shortest amount of time possible - maybe 10 seconds to get a reading.
Ouch. This isn't good. Not leaving it run is also a good idea. Does any other part get hot or even warm (diode, inductor, caps)?
Did you buy a few extra LT1308's, PCB's and other parts?
Can you take a clear picture (in focus with good lighting) of the top and bottom of the circuit.
Have you double checked the polarity of the input and output caps.
Did you wash all the flux off the board after soldering and dried the board?

Do you have access to an O'scope? This can be absolutely necessary to see what the switching waveform on SW (pin 5) looks like. Also at the Lt1308's Vin and Vout pins. The switching wave forms should look like the pictures in the data sheet.

Measure the Voltage at Vc (pin 1) to ground.

Does the Input Voltage (battery) sag when the Lt1308 is switched on?

It is possible that there is insufficient phase margin in the FB loop and the output is oscillating (O'scope is needed to verify this). You could try adding a 100pf to 330pF ceramic cap across the 309k FB resistor (Vout to FB).

Also try adding a ceramic caps of a few uF's across the output cap.
Only try one change at a time.

FYI: 3k Ohm load at 5V is 1.66mA.

Snipeye
 
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Re: lt1308-based 1A Charger

Post by Snipeye »

My bad on the resistor current for the dummy load, dunno why I decided to do ohm's law backward there.

OK, I bought parts to make three of them, so after spending a frustrating several hours on the one, I moved over to the next one and soldered that up instead - it went much more smoothly, and it works perfectly! Outputting 5.00 and occasionally 5.01V - even able to charge my (very finicky) iPod Touch 3G! I would still love to debug and figure out what went wrong with the first, but I now know that it must be some mistake of mine - which doesn't make me feel great, but is sure better than having to redesign the board again! ;)

As for the other parts getting warm - I don't know for sure since I've only been turning it on for 10 seconds tops to measure voltages, but I don't think so. Could it be I fried the chip? I can still get you those pictures if you want, but now that I have a working board to measure things against, debugging should be more easy. Things to revise for the next round: the inductor holes on my board were too small, I guess when I moved my design to the new inductor I didn't change the pin hole size - I had to dremel them out a little bit on each board, being very careful to not break the electrical connection to/from either side of the board. I also need to move the USB port forward a little bit so it can be flush with the outside edge of the altoids tin I'll be encasing it in - it's just a little to far in as it is now. Other than that, things are good!

Thanks for the help so far! Anxiously waiting to hear your opinions on debugging.

waltr
 
Posts: 306
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Re: lt1308-based 1A Charger

Post by waltr »

Wooo Hooo. One of them works. Good job.
I was getting worried I picked some wrong parts.
A bad chip was what I was thinking when asked if you bought extras. Re-check every connection and part on the first board. If everything looks good then first remove the LT1308 and apply power from the battery and feel if anything else gets hot. If not them put a new LT1308 on, cross your fingers then apply power.
I've had switching DC-DC chips fry or just go bad on prototypes so this would not be uncommon.

Snipeye
 
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Re: lt1308-based 1A Charger

Post by Snipeye »

I'll give that a try, thank you. Really glad I've got at least one working! Will keep you updated as I have time to work more on this project. :)

waltr
 
Posts: 306
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Re: lt1308-based 1A Charger

Post by waltr »

Yes, post a picture of the completed board. I and others here would like to see it.

Are you powering the working board with 2 cells or 3 cells?

Snipeye
 
Posts: 198
Joined: Thu Feb 25, 2010 10:03 am

Re: lt1308-based 1A Charger

Post by Snipeye »

Currently with 3 cells, though it also appears to work with 2 - I expect that's only for as long as the cells are pretty highly charged, though, as the datasheet shows the chip goes down to 75% efficiency at 2.5V with a load of 1A, and takes a dive around there. Not terrible efficiency, but certainly not the 85, 90+ % efficiency I can get with 3 cells. I'll see if I get time to put that picture up tomorrow.

Snipeye
 
Posts: 198
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Re: lt1308-based 1A Charger

Post by Snipeye »

Update: PICTURES!

These are just some really quick ones with my cheapo point 'n' shoot, but my brother (who is a professional photographer) will be coming sometime soon to help me get some great pictures of this for a kickstarter - if I can get over 100 ordered at once, the price per drops by almost half.

http://i40.tinypic.com/29h4b6.jpg

http://oi39.tinypic.com/27xf8dw.jpg

EDIT: Grr, the forums are telling me the image is invalid for some reason. Well, they'll be links, then.

waltr
 
Posts: 306
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Re: lt1308-based 1A Charger

Post by waltr »

The pictures are good and the project turned out looking great.
I see you cut the tab off the diode to get it to fit inside the tin. Since the diode is large enough this works.

Yes, cost really drops as the quantity increases.

Most forums have restrictions on file sizes and types to be posted. There should be some where here that gives the guidelines.

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