Level Shifter Documentation

Level Shifter
Rebecca Pullen 10/03/09
Version 1

A level shifter is a simple device that interfaces a lower voltage circuit with a higher voltage device. In a given system it may be that some digital logic devices use 3.3V signals, whilst others use 5V signals. Where it is not possible to drive a device with a 5V signal with 3.3V, a level shifter is used to produce a voltage that can be used by the device.

Figure 1 – Circuit diagram for level shifter circuit

Figure 1 is a diagrammatic representation of the level shifter circuit that will be used in this project. The values of R1 and R2 are 10kΩ and 100kΩ respectively. The FET used must be a small signal p-channel FET with a threshold voltage of less than 4V and a drain/gate-source voltage between 10-20V.

The table below shows the voltage output for each combination of input voltages.

 Input 1 Input 2 Output 1 0V 5.137mV 1 3.3V 5V 1 5V 5V

The table below shows the current drain for a range of loads. It can be seen that the current drain doesn’t vary at all between 10kΩ and 1MΩ.

 Input 1 Input 2 (V) Load Current 1 3.3 10kΩ 5.163A 1 0 10kΩ 51.429nA 1 3.3 100kΩ 5.163A 1 0 100kΩ 51.429nA 1 3.3 200kΩ 5.163A 1 0 200kΩ 51.429nA 1 3.3 300kΩ 5.163A 1 0 300kΩ 51.429nA 1 3.3 400kΩ 5.163A 1 0 400kΩ 51.429nA 1 3.3 500kΩ 5.163A 1 0 500kΩ 51.429nA 1 3.3 604kΩ 5.163A 1 0 604kΩ 51.429nA 1 3.3 704kΩ 5.163A 1 0 704kΩ 51.429nA 1 3.3 806kΩ 5.163A 1 0 806kΩ 51.429nA 1 3.3 909kΩ 5.163A 1 0 909kΩ 51.429nA 1 3.3 1MΩ 5.163A 1 0 1MΩ 51.429nA

Parts list:
1 x 10k-ohm resistor
2 x 100k-ohm resistor
1 x Si9953DY MOSFET from Fairchild Semiconductor
1 x 2N3904 Small Signal Transistor (NPN) from Vishay Semiconductors

page revision: 5, last edited: 08 Apr 2009 13:54