Patent ID: 7495426

Claim:
A temperature setpoint circuit, comprising: first and second supply voltage terminals; a first bipolar transistor Q 1 having an emitter area of x; a second bipolar transistor Q 2 having the same polarity as Q 1 and an emitter area of N*x (N: an integer); first and second current sources connected to provide currents I 1 and I 2 to the collectors of Q 1 and Q 2 , respectively, said circuit arranged such that I 1 and I 2 are proportional-to-absolute-temperature (PTAT) currents which are maintained in a constant ratio to each other; a third transistor Q 3 arranged to drive the base of Q 1 such that Q 1 conducts I 1 ; a resistance R 1 connected between the bases of Q 1 and Q 2 such that the difference (ΔV be ) in the base-emitter voltages of Q 1 and Q 2 appears across R 1 ; and a third current source connected to provide a current I 3 , said circuit arranged such that I 3 is provided to the collector of Q 2 while Q 2 is off and is not provided to the collector of Q 2 while Q 2 is on, said circuit arranged such that I 3 is a PTAT current which is maintained in a constant ratio to I 1 and I 2 when provided to the collector of Q 2 ; said circuit arranged such that Q 2 is turned on and conducts a current equal to Ia when: Δ V be =( kT/q ) ln ( NI 1 /Ia ), where Ia=I 2 +I 3, the temperature T at which ΔV be =(kT/q)ln(NI 1 /Ia) being said circuit's setpoint temperature, such that said switching of current I 3 provides hysteresis for said setpoint temperature which is approximately constant over temperature.