help with LCD power

[bob_s]

I've been trying to power an LCD module using a UA78M05CKTPR regulator (power-flex package) but the regulator gets very hot. I measured it with a temperature probe and its more than 155F which is too high. The max operating temp is 150F and the recommended operating temp is 125F. The LCD LED backlight requires 5v at 210ma and I also 5 other LED's (10ma each). I am supplying the regulator with a regulated 12v. I thought at 250ma a heat sink would not be required as it only needs to dissipate 3 watts of power, 12v x 250ma = 3W (although I have soldered it on a 1 inch square on an FR4 0.062" copper PCB) . I had this same circuit working just fine when I powered it with an unregulated 8v at 2 watts of power but I needed the 12v for another addition to the circuit. I have two other regulators off of the same 12v. One at 5v (50ma) and the other at 3.3v (100ma) and neither of them even get warm. So why would the extra 4v cause such a problem? I've attached the datasheet for the regulator.

Thanks for any help.

Bob

UA78M00_Series.pdf

 

[audioguru2]

Hi Bob,
Your calculation for power dissipation of the regulator is incorrect.
If it is supplied with 12V and is passing 250mA, the source is loaded with 3W (the regulator plus the LCD dissipations). The regulator's pass transistor has only 7V (12-5=7) across it and 250mA through it which is only 1.75W dissipation. Without a heatsink it should be only 74 degrees C.

Since it worked OK with unregulated 8V but gets very hot with 12V regulated supplies, the difference could be that it is now oscillating because it needs the input cap that was provided by the unregulated supply. Perhaps the 12V regulated supply doesn't have a suitable output cap that is close enough to the input of this 5V regulator.
Try it with a 0.33uf cap very close to its input pin and ground. Also on the datasheet a 0.1uF output cap is recommended.
Does the backlighting LED have a current limiting resistor?

 

[bob_s]

audioguru,

Thanks, you were right. It did need both 0.33uf and 0.1uf cap. Brought the temp down to 126 and stable. It must have been oscillatint. Thanks also for correcting me in my calculation. I'll try to remember that for next time.

Bob

 

[audioguru2]

Hi Bob,
Good, you've almost fixed it. I'm glad to have helped.
It is still much hotter than expected, dissipating more than 3.5W, so it must have a load of more than 510mA.
Your 126 degrees C measurement is on its external heatsink so the actual temperature of its internal chip is much higher, probably beyond its maximum rating. It won't be reliable operating that way.

It would be interesting to discover why this regulator has such a high load current but in the meantime adding a proper small heatsink to it will cool it better. Can you measure the load current?

 

[bob_s]

audioguru,

The temp is 126F which is about 52C which works out to 52C/28C = 1.85W. That's pretty close to what we were looking for. Sorry about the confusion with the temperature unit.

Thanks again.

Bob

 

[MP1]

Bob,
I use a lot of LCD displays. They are current hogs. Which one are you using?
Without seeing your setup, I can only comment in general terms:

One thing you should note is that your regulator needs 2 volts more than the output. Anything above this is just dumped in the form of heat. The additional voltage is NOT converted to current or anything useable. When your output needs higher current, you need to put in LESS voltage (within the limits of the spec) or figure on a heat sink. By the way, your pcb pad heat sink is not going to do much for you.
Most likely, your setup is using more current than you figured. You will see a noticeable difference by changing from a 12 volt supply to a 9 volt supply of the same current specification.

MP

 

[bob_s]

MP,

Thanks for the reply. However the problem has been solved (except for the imperial/metric problem ) The regulator had gone into oscillation and required some filtering.

Bob

 

[MP1]

Conversion problem, huh? ok, Still some good advice in there if you care to use it.

MP

 

[audioguru2]

Bob,
One thing you should note is that your regulator needs 2 volts more than the output. Anything above this is just dumped in the form of heat. The additional voltage is NOT converted to current or anything useable.
MP

Correction:
This regulator works much better with 3V more input than the output, like most other regulators, especially ones with higher output current. Its typical Input Voltage Regulation spec is 3 times better, and its Ripple Rejection is not even spec'd with only a 2V difference. It is not a low-dropout-voltage regulator.
ALL spec's of most other regulators are only stated with a 3V difference.

 

[MP1]

Wrong. I stand behind my last post. There is nothing there to correct. Audioguru is either mistaken or he does not accept the manufacturer's specs in the data sheet.

Below I have posted an excerpt from the data sheet. Also, I use this regulator, so I have had the opportunity to find this out first hand with micros and LCD displays. 7V is fine by the manufacturer and by the many tests that I have put on it.

MP

View attachment 36119

 

[bob_s]

Sorry MP, I have to disagree with you. I know from experience that this regulator will not work well with less than 8v. This is why my previous circuit provided 8v for it to regulate properly. With any less the regulator was not consistent. Besides the problem was I needed 12v for my circuit not 8v. After adding the filtering caps the regulator ran well within the acceptable limits. Thanks for your input though.

Bob

 

[MP1]

I would guess that your problem was in the ripple. Of course you need more voltage if you do not take care of ripple properly with the correct value of capacitor. It is a simple rule of design.
I design microprocessor projects with this regulator all the time and have been for years. I have no idea about what your design needs because you did not bother to post a schematic. But I do know that this regulator can run with 7 volts into it if you design properly. Aside from that, the spec sheet also says that you can.

MP

 

[audioguru2]

MP,
Sorry to disagree, but the 2V dropout voltage is when it doesn't regulate any more.
You didn't read the spec's details:

View attachment 36120

 

[MP1]

Sorry audioguru, but I know how to read a data sheet. Pour through books of them in a typical week. Those are not minimums. Those are the specs they used for traceability. See where it says "Test Conditions"? That means this is the voltage they used to test it. It does not state or mean that these are minimum voltages. Now look at where it says "recommended operating conditions" in my last picture post. That means recommended operating conditions. See where it says Min.? That means minimum voltage at recommended operating conditions is 7 volts.

I have been through this with you in the past several times.
Data sheets seem to be your weak point.

MP

 

[audioguru2]

MP,
You seem to operate regulators right at their minimum limit. You had the same problem with the LM317.

The datasheet details that I posted clearly show that it has a typical Input Voltage Regulation spec that is 3 times better with a 3V differential, and they don't even guarantee a Ripple Rejection spec unless it has a 3V differential.

 

[bob_s]

MP,

Here is some more data that you might be interested in. It has to do with power dissipation of the KTP package I'm using. The one inch square pcb copper improves the power dissipation greatly if done correctly. Take a look at page 6, I'm using a pattern similar to #4. If the power dissipation is calculated as (Tj - Ta)/Ja = (125C - 25C)/(25.8C/W) = 3.876W which is more than double what I need. I would like it if the package was cooler but it is well within it's operating parameters. The issue of input voltage was defined at the start, I'm using 12v. I did't see the purpose in dropping the voltage down just to regulate it down once more.

Bob

Power_Flex_KTP.pdf

 

[MP1]

MP,
You seem to operate regulators right at their minimum limit. You had the same problem with the LM317.

The datasheet details that I posted clearly show that it has a typical Input Voltage Regulation spec that is 3 times better with a 3V differential, and they don't even guarantee a Ripple Rejection spec unless it has a 3V differential.

Audioguru: You are reading the data sheet wrong.
..BTW, I did not have a problem with the LM317. Worked on paper and worked well in real life.
Applications engineer at National agreed with me on that one, by the way.
Since you were so sure of yourself, I posed the question in an unrelated conversation with the guy. Was nice to get the backup.
You should ease off arguing with people so much.

bob: Yes, KTP package will benefit from this. I missed in the thread that you were using that package type. Thanks for pointing it out.

MP