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I have a voltage regulator TPS788 from TI. The output voltage is 3.3v, and the output current is 150mA. I connected a LED to it, and I found that the LED is blinking really fast. Does that mean the Regulator is not working properly, or the LED it self is not standing this amount of current ?
Note: I am using another regulator TPS789 (1.8V,100mA), and The LED is working fine.

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    \$\begingroup\$ Are appropriate capacitors part of the circuit? \$\endgroup\$ Sep 10, 2016 at 5:13
  • \$\begingroup\$ @IgnacioVazquez-Abrams: Yes. I did add the recommended capacitors to the circuit, as the datasheet mentioned exactly. By the way I am using TPS789 with an output of 1.8v, and 100mA current, and It's working fine. \$\endgroup\$ Sep 10, 2016 at 5:16

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If you connect a normal LED directly to 3.3 volts, it will try to draw a near-infinite amount of current, due to the LEDs I-V curve. This is from a completely random green LED I found:

enter image description here

Follow the horizontal axis to 3.3 volt, and you can see that the current, if possible, would be hundreds of amperes, or more.

This is a pointless exercise, because the LED would burn up in flames before that. Instead, what likely happens here, is that your regulator has a few protections:

The TPS788xx features internal current limiting and thermal protection. During normal operation, the TPS78833 limits output current to approximately 350 mA. When current limiting engages, the output voltage scales back linearly until the overcurrent condition ends. While current limiting is designed to prevent gross device failure, care should be taken not to exceed the power dissipation ratings of the package. If the temperature of the device exceeds approximately 165 °C, thermal-protection circuitry shuts it down. Once the device has cooled down to below approximately 140 °C, regulator operation resumes.

I don't know if it is the current or temperature protection that kicks in here, it depends on the frequency of your "blinking".

If you want to make sure that your regulator is still working, the easiest way is to connect a 22 Ω resistor from the output to ground. Make sure it can handle 0.5 watts. Then measure the voltage.

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  • \$\begingroup\$ So the regulator is just blinking to avoid this overcurrent the LED is trying to draw ? \$\endgroup\$ Sep 10, 2016 at 5:21
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    \$\begingroup\$ @AbdelrahmanElshafiey Yes, I believe that is what you are seeing. It's just a theory, you may need an oscilloscope to investigate further. \$\endgroup\$
    – pipe
    Sep 10, 2016 at 5:22
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Please be aware that led's idealy should be current driven. For that reason a serial resistor is mostly used to limit the current within a save range. Therefore if you have an led with a nominal voltage of 1,5V and a current of 20 mA and you wish to use a 3V supply then you need a series resistor of (3,3 - 1,8)/ 20 = 0,075 kOhm or 75 Ohm. Testing you circuit following this principle will show you directly if the regulator is working properly

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The TSP78918 is working because the voltage is less than the forward voltage of the LED and therefore not drawing excessive current.

The TPS78833 outputs enough voltage to turn on the LED with the regulator being the only current limiter.

The value of the current limiting resistor is dependent on how bright you want it to be and how much current is required to make it that bright.

Use the desired current according to the LED's datasheet and divide 3.3 by the desired current to get the ohms.

If the forward voltage of the LED is 3.0V, then the voltage across the resistor is 0.3. The wattage will be in the milli-watt range, So a very small wattage resistor will suffice.

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