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Constant Current Generator

IP.com Disclosure Number: IPCOM000029133D
Publication Date: 2004-Jun-16
Document File: 4 page(s) / 37K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method that uses poly resistors and a constant current generator to generate the constant current which is independent of VCC, temperature, and process variation. Benefits include improving efficiency and reducing process variation.

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Constant Current Generator

Disclosed is a method that uses poly resistors and aconstant current generator to generate the constant current which is independent of VCC, temperature, and process variation. Benefits include improving efficiency and reducing process variation.

Concept

Bandgap voltage does not change with temperature, process, or power supply. A conventional bandgap reference circuit consist of an opamp, resistors, and bipolar diodes. Figure 1 shows the basic block diagram for the bandgap circuit. The temperature coefficient of resistor is positive and the junction voltage of the bipolar diode has a negative temperature coefficient. In order to  cancel the effect with temperature, a proper weighted sum of resistor voltage is added to the bipolar junction voltage. This effect is cancelled at one temperature and minimized at another, depending on the proper combination of the bipolar diodes at the two legs of the opamp. The bandgap voltage variation across the temperature range is shown in Figure 2. The circuit diagram  for the bandgap is shown in Figure 3. Instead of PMOS devices, two equal resistors are used for bandgap voltage generation.

Circuit Description

The complete circuit diagram for the constant current generator is shown in Figure 4. A simple cascode bias circuit is used to bias the transistors M3, M4, and M6.  The purpose of these transistors is to decouple the noise between the power supply (vcc) and the opamp inputs. The kicker circuit is a digital circuit with a group of delay elements; this circuit ensures that the opamp is initialized to zero when the circuit is powered up. The transistor M14 and M15 are driven...