LM317 regulator : 3.3V Power supply for Micro-Controller

Introduction:

A power supply for standalone micro-controller or development board generally have to be either a 3.3V or 5V. This voltage is supplied as one/more Vcc inputs to the micro-controller. Based upon the make/manufacturer of the micro-controller this can vary.

Popular and most commonly available adaptors that are available for AC to DC conversions are 12V or 9V or 5V. To bring these DC voltage outputs to the level of Vcc inputs of micro-controller the LM317 circuits can be used.

Salient Features of LM317:

• Adjustable output can be generated in the range of 1.25V~37V
• Output current generated is greater than 1.5A
• Internal short circuit limitation
• Thermal overload protection

Operating Conditions:

Symbol	Description	Min	Max	Unit
Vo	Output Voltage	1.25	37	Volt
Vi – Vo	Input-to-output differential voltage	3	40	Volt
Io	Output Current	0.01	1.5	Amp
Tj	Operating virtual junction temperature	0	125	°C

Hence for generating 37V output minimum 40V should be given as input.

Design:

• To generate an adjustable output voltage, R1 and R2 are required.
• An input bypass filter capacitor (Ci) can be added in case power supply filter capacitor is not present.
• An output capacitor (Co) can be added to improve transient response.
• Protection diode (D1) should be added in case output capacitor (Co) is used, diode provides a low-impedance discharge path to prevent the capacitor from discharging into the output of the regulator. 

Calculations:

Where

V_REF        = 1.25V

I_ADJ          = 50uA

Since I_ADJ is negligible, hence (I_ADJ × R₂) factor can be neglected. Hence the above equation becomes

For example

Voltage output requirement is 3.3V.

Voltage Input is 12V

If R₁ = 330Ω

Then R₂ = 560Ω

Schematic Diagram:

The following schematic diagram is generated in Proteus ISIS for generating 3.3V voltage output.

The Bill of Materials considered is as follows

********************************************************************************************************

Bill Of Materials For 3_3VSupply.DSN
Design Title : 3_3VSupply.DSN Author : Aditya Raj Nayak Revision : 1.0 Design Created : 31 October 2016 Design Last Modified : 31 October 2016 Total Parts In Design : 6
2 Resistors ******************************************************************************************************
	Quantity:		References		Value		Order Code
	1		R1		330R		W330R
	1		R2		560R		W330R
2 Capacitors
******************************************************************************************************
	Quantity:		References		Value		Order Code
	1		C1		220u		Digikey 399-4962-2-ND
	1		C2		10u		Digikey PCC2233TR-ND
1 Integrated Circuits ******************************************************************************************************
	Quantity:		References		Value		Order Code
	1		U1		LM317K
1 Diodes ******************************************************************************************************
	Quantity:		References		Value		Order Code
	1		D1		1N4002

R1 and R2 Matrix for variable voltage supply:

Below is a matrix created to show the various values of Vo generated for different combinations of R1 and R2.

The voltage value of 38.75 is not possible as the maximum output can be 37V.

Further additions:

Currently the hardware version of this circuit is under development, it will be posted on this site once completed with actual component list and their costs.