Analog Design

Kevin Aylward B.Sc.

Low Area, Low Power Startup Circuit

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Overview

The circuit design described here was developed to solve some specific issues in a design of a Band Gap voltage reference in a BiCmos process. However, the technique described here can be applied to many alternative situations.

The principle issues involved were:

1 Die Area - The product was to be packaged in a pico package.

2 Low power - The total supply current was to be at the 1ua level.

The fundamental consequence of these specifications is the difficulty in achieving such low current without using high, mega ohm resistors, or long channel mosfets, which take up significant die area.

Standard Approach

Standard PTAT (proportional to absolute temperature) bias circuits automatically solve one aspect of generating a low current with low value resister by applying a small delta Vbe of the order of 50mv across the resistor. However, the classic problem to these circuits is that they need a startup current. This is because they can start in a stable mode where all currents are zero. To avoid this, a current must be applied to ensure startup, but is then switched of once startup has occurred. The issue is that this startup current usually has to run all the time, it is only diverted from the bias circuit. If this startup current is to be small, very large resistors or alternatively, long depletion mode mosfets must be used. Fig. 1 shows such an approach.

Fig.1

Basic Operation of Fig.1

Fig. 1 is a basic band gap reference, with main currents being set by delta Vbe/R2. In this case delta Vbe is a standard 52mv from 25mv.Ln(8), giving a current of 3ua. If QN6 and QP8 are non conducting, indicating that the main circuit is off, MN1 will pull current through QP1 forcing QN11 and QN12 to turn on, which will force Q6 to turn on. Once Q6 is on, it will cause the startup current of MN1 to be diverted away from QP1.

New Circuit

The modified circuit also uses a depletion mosfet, but in this case, the mosfet is minimum sized, rather than a long channel device, and the startup current is disabled completely once started.

Fig.2

If VBG is lower than the pinch off of MN1, MN1 will feed current into that node. This will startup up the circuit via QN2 and QN1. Once the circuit starts up, VBG will set itself to the bandgap voltage say 1.2V. This voltage will causeMN1 to turn off, since it results in a pinch off voltage being applied to MN's gate and source.

Fig.3 shows simulation results from Fig.2, using SuperSpice, with temperature stepped from -50 to 150 in increments of 50 degrees., without MN1 in circuit.

Fig.3

Fig.4 shows Fig.2 simulation results using SuperSpice, with temperature stepped from -50 to 150 in increments of 50 deg., with MN1 in circuit.

Fig. 4

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