Power circuit design logic -- suggestion

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Hi,

 

I am designing a board where my AVR Microcontroller is powered by LM7805 regulator & my load requires 3.8 voltage, is powered by following voltage regulator NCP57302.

http://www.onsemi.com/PowerSolutions/product.do?id=NCP57302

 

As per the specs of NCP57302 to produce 3.8V i have selected :--
R1 = 30.96K
R2 = 15K

Is it the right set of resistors values to be used for producing 3.8V ?

 

Also I need some external circuit to be put on enable PIN of NCP57302. So that microntroller & control the turning ON/OFF the power supply of load.

Can some suggest this external circuit for controlling the enable pin of NCP57302 regulator by my microcontroller ?

I need my load to be switched off when board is powered on & microcontroller turn it ON after some time, for that i need some external circuit to fix enable pin at ground level on startup.

 

Please suggest.

 

//Katoch

 

Schematic

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Last Edited: Tue. Jan 27, 2015 - 06:41 AM
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Katoch,

 

1. Your calculation is correct.

2. Tie the enable pin to ground via a 10K resistor. That will disable the device until you set your microcontroller to provide a "high" to the enable pin.

3. Why do you talk of 3.8volts, but label your circuit with 1.8volts?

4. I have deleted your second thread on the same topic.

 

Cheers,

 

Ross

Moderator

 

 

Ross McKenzie ValuSoft Melbourne Australia

Last Edited: Tue. Jan 27, 2015 - 06:28 AM
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1. Your calculation is correct.

Thanks for replying.

 

2. Tie the enable pin to ground via a 10K resistor. That will disable the device until you set your microcontroller to provide a "high" to the enable pin.

   I have one question here.

   Now if i connect enable pin to ground by 10K resistance & apply high  on my microcontroller o/p pin. Then what level will be input to the EN pin of the regulator.

   Will it be high on Enable pin of regulator ? Or high signal send by the microcontroller will be get the ground through 10K (R33) resistor & enable pin will be on ground again ?

 

   It should be the potential drop across 10K resistor right ?

 

   Please suggest.

 

3. Why do you talk of 3.8volts, but label your circuit with 1.8volts?

      Corrected & reposted the schematic.

4. I have deleted your second thread on the same topic.

By mistake i posted twice

 

 

Last Edited: Tue. Jan 27, 2015 - 06:53 AM
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2. When the microcontroller outputs a high (~5volts), the enable pin will see that voltage and a small current will flow through the 10K resistor to ground. When the microcontroller either outputs a low or is in its tri-state mode during bootup, the enable pin will be grounded.

 

Ross McKenzie ValuSoft Melbourne Australia

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Thanks for helping me in this.

 

Regards,

katoch