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Start-up Method for a Sub-bandgap Reference Circuit

IP.com Disclosure Number: IPCOM000245270D
Publication Date: 2016-Feb-24
Document File: 6 page(s) / 377K

Publishing Venue

The IP.com Prior Art Database

Abstract

A sub-bandgap reference circuit is capable of providing reference voltages less than the bandgap reference value. Hence it is very useful in low voltage designs. However, sub-bandgap reference circuits have inherent startup problems especially with supply ramp up. In this paper we present a circuit that resolves the startup issue and provides a robust startup to a sub-bandgap circuit.

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Page 01 of 6

Start-up Method for a Sub-bandgap Reference Circuit

A sub-bandgap reference circuit is capable of providing reference voltages less than the bandgap reference value. Hence it is very useful in low voltage designs. However, sub-bandgap reference circuits have inherent startup problems especially with supply ramp up. In this paper we present a circuit that resolves the startup issue and provides a robust startup to a sub-bandgap circuit.

Introduction

The sub-bandgap reference circuit generates a voltage reference with a zero temperature coefficient (ZTAT). This is generated by summing a positive temperature coefficient (PTAT) current and a negative temperature coefficient (CTAT) current. The resultant current has zero dependence on temperature, and is converted to a bandgap voltage using a resistor ladder (Rztat).

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Abstract

Figure 1: Sub-bandgap reference circuit (without startup)


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    The sub bandgap circuit shown in Figure 1 includes ZTAT current generation branches having MOS transistors M1, M2 and current to voltage conversion branch having MOS transistor M3. The current generation arms includes BJTs Q1 and Q2 carrying PTAT current (Proportional to Absolute Temperature) and the resistors in parallel carry CTAT (Complementary to Absolute Temperature). These two currents add up to form a zero temperature coefficient (ZTAT) current that flows through transistors M1, M2 and which is mirrored by M3 in the voltage generating branch.

    The sub-bandgap circuit has multiple stable operating points where the circuit can stabilize in steady-state. These stable solutions to the bandgap circuit can be identified by sweeping equal currents through MOS transistors M1 and M2 and observing the node voltages Vleft and Vright. At all of the branch currents for which Vleft is equal to Vright we get a possible solution. A plot (Figure 1a) of node voltages Vleft and Vright is shown in red and green curves respectively. Observing the plots, the circuit has stable solutions for all values of branch current less than a certain threshold value of current (Imin_start). These are the undesired operating points not having the required ZTAT voltage profile. The intersection point after the Imin_start

current is the desired operating point, which has the required characteristics of ZTAT voltage.

Undesired

Desired operating

Solutions

point solution

Voltage

Vleft Vright

Imin_start

Branch Current

Figure 1a: Identification of possible operation point

solution

    These undesired operating points occur because for all these values of branch currents the BJTs Q1 & Q2 are in cutoff and all the current flows through the CTAT resistors making the node voltage Vleft and Vright equal.

    So there is a need of a startup circuit that ensures that the circuit stabilizes at the desired operating point which can be ensured only if the startup circuit guarantees that BJTs are turned on.

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Conventional circuit

    In Figure 2, the highlighted part (in red) is the con...