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78M05 Voltage Regulator and Switching LED Driver Question

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Batur

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Hi,

I am trying to implement a LED driver using TPS61500 for a project. However I have come across a particular problem. I am using an atmega328p to control the driver. My VCC is 12 V and the uP is fed by a 78M05 12V to 5V voltage regulator, TPS61500 is a switching driver and when it is active it drops the 5V output of 78M05 to 1.6V since they are both connected to same supply.

I guessed the switching of led driver affects the regulator so I placed a 47uF capacitor parallel to output of 78M05 and the issue seems to be solved, but since I am not sure what is really happening I am not sure if there is a better solution.

I would greatly appreciate any advice on this issue.

Best Regards
 

Supplying the LED driver from linear voltage regulator is a bad idea in terms of efficiency. TPS61500 has 18 V input range, so it can be operated from 12V input as well.

It sounds that switcher input peak current is exceeding 7805 current limit. If a 47 µF capacitor solves the problem, it's a reasonable solution. I presume there will be still switching transients on the 5V node, depends on your application if it can be tolerated. ADC accuracy of ATmega might be affected.
 

Even if it "appears" to have solved the problem, unless you can check the Vcc with an oscilloscope you really don't know.
Perhaps the voltage is still dropping, not as far as 1.6 volts but dropping nevertheless. Perhaps 1/4 or 1/2 volt.

Depending whether you have the brownout detector enabled on the processor, you may find that it mysteriously resets or crashes.
Feed the LED driver directly from the 12V supply and use the 78M05 only for the microcontroller and other sensitive circuits.
 

Have you used the "soft start" feature? Without it, switching on the TPS61500 will generate a very large current spike. Also, as schmitt trigger already pointed out, this kind of chip works best on the 12V supply, and you avoid a lot of trouble when you move it away from the 5V regulator output. Even the conversion efficiency is also higher at 12V in (see the curves on the data sheet, page 7, figure 2)!
 

give separate supply for LED driver(common ground).
how many LED did you connect?? refer datasheet. use all parameters.
for removing ripple prefer tantalum capacitors after and before 7805 circuit. use TO-220 package it can give more current. see the image
Untitled.png
Thanks
 

I am almost sure that the original poster is now AWOL.
Most likely this was a school project which either he has already solved or abandoned.

All the tell-tale signs are there: Less than 10 posts, a vague original description, never replied to the other members requests for additional info, and the thread is almost 2 months old.
 

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