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

Please suggest.

enter image description here


You're resistors are correct.

The Enable pin is CMOS and TTL compatible. So you should be able to drive it from your microcontroller directly.

  • \$\begingroup\$ Ok actually i was having a doubt that 30.96K resistor will be available easily or not, so i can select a combination of resistor easily available. 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. \$\endgroup\$ – user6363 Jan 27 '15 at 5:52
  • \$\begingroup\$ use a pull down resistor on the enable pin. When the micro powers up (the I/O are generally configured as input - from what i've seen). Then your micro will go through its initialization and then drive the pin as needed. \$\endgroup\$ – efox29 Jan 27 '15 at 5:59
  • \$\begingroup\$ Additionally, if you can't find the resistance you need or can't find a suitable sum of resistances, recalculate your R1 and R2 values with a new set of values. \$\endgroup\$ – efox29 Jan 27 '15 at 6:03
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    \$\begingroup\$ You won't find a 30.96k resistor without parting with more money than the rest if the circuit combined! I'd recommend a fixed 27k resistor and a trimmer/potentiometer at 4.7k or 5k. Either wire it up as a simple variable resistor or place it as a potentiometer between your 27k and 15k resistors, with the wiper connected to the ADJ pin. Both will allow you to overcome tolerance errors in the resistor values and get the exact output voltage you need. When the trimmer is set to the correct position, put a SMALL dab of nail varnish or similar on it to prevent it 'self adjusting' in the future. \$\endgroup\$ – CharlieHanson Jan 27 '15 at 10:40

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