the programmable hysteresis circuit of the present invention can be used with a wide range of ics , including sensors , controllers , analog - to - digital converters and comparators , to adjust the amount of hysteresis in the ic . in this way a standard chip can be used for a variety of applications , such as temperature sensing , that require different amounts of hysteresis . the invention will be described in conjunction with a programmable temperature controller , but it is generally applicable to many other ics and electrical circuits . the temperature controller is provided with a hysteresis circuit for establishing a hysteresis , and an input pin for receiving a signal to adjust the magnitude of the hysteresis . the hysteresis is programmable between a number of preset and intermediate levels . as shown in fig1 a programmable temperature controller ic 10 produces a voltage proportional to absolute temperature , and compares it to four set point voltages . when the temperature voltage exceeds a set point , the controller produces an &# 34 ; on &# 34 ; binary output voltage . for example , the controller may be used to monitor and control a power supply &# 39 ; s temperature . when the first ( lowest ) set point is exceeded , the associated binary output voltage trips a fan that is used to cool the power supply . when the second set point is surpassed , the fan &# 39 ; s speed is increased and a compressor is turned on to provide additional cooling . if the temperature rises higher than the third set point a warning signal is produced , and if the fourth and highest set point is traversed the power supply is shut down . hysteresis is built into the system to prevent the outputs from jittering on and off when the temperature voltage hovers close to one of the set points . once the temperature voltage exceeds a set point it must fall back at least a hysteresis differential below the set point to switch the output &# 34 ; off &# 34 ;. it would be very inefficient to turn the fan , compressor or entire system on and off every few seconds . the temperature controller ic 10 includes a bandgap reference and temperature sensor 12 that is typically referred to as a &# 34 ; brokaw &# 34 ; cell , and is disclosed in u . s . pat . no . 5 , 225 , 811 , &# 34 ; temperature limit circuit with dual hysteresis .&# 34 ; separate sensing and reference voltage circuits could be used . the brokaw cell outputs a reference voltage v ref , suitably 2 . 5 volts , and a voltage proportional to absolute temperature v ptat . v ptat is amplified by amplifier 14 , typically having a gain of 2 . 5 , to give the controller a sensitivity of approximately 5 mv /° c . since the output of amplifier 14 is also proportional to absolute temperature , it is also designated v ptat . the 16 - pin ic package 15 includes a v s supply voltage pin 16 and a gnd pin 17 for biasing the ic &# 39 ; s internal circuitry . the supply voltage , suitably 5 volts , and ground are provided externally . for clarity the ic &# 39 ; s internal supply voltage and ground connections are not shown . the ic includes a v ref pin 18 and a v ptat pin 20 that are connected to the reference voltage v ref and amplified temperature voltage v ptat , respectively . to conserve power a shutdown feature is included for the temperature sensor . a shutdown pin 22 is connected internally to the temperature sensor and receives an externally applied signal . the set point voltages are established externally so that they can be adjusted to the needs of the particular application . a voltage divider 24 consisting of five series resistors r1 through r5 is connected between the v ref pin and ground . the four set point voltages are tapped off of the voltage divider between each successive pair of resistors , and are connected to input pins in1 ( 26 ) through in4 ( 32 ), respectively . the ratio of the resistance between each tap and ground to the total resistance of the divider determines the value of the set point voltage for that tap . the voltage divider is preferably added by the user to establish the desired set points . alternatively , programmable digital - to - analog converters ( dacs ) could be used to establish the set point voltages from digital inputs or the voltage divider could be internal to the ic . the ic includes four comparators c1 - c4 for comparing v ptat to the respective set point voltages . the comparators &# 39 ; inverting inputs 34a - 34d are connected to the input pins in1 - in4 , respectively . the amplified v ptat is connected through resistors r6 - r9 to the comparators &# 39 ; non - inverting inputs 36a - 36d , respectively . a hysteresis circuit 38 produces hysteresis currents i hc1 , i hc2 , i hc3 and i hc4 that can be applied through switches s1 - s4 to the comparators &# 39 ; non - inverting inputs , respectively , to shift the input signal , v ptat . a hysteresis pin h pin 40 is connected to the hysteresis circuit 38 and can be used to sink or source current to adjust the magnitude of the hysteresis currents . the preferred external connections for the hysteresis pin are shown in fig3 . because the input impedances to the comparators are very large , when the switches are open the voltages at the non - inverting comparator inputs substantially equal v ptat . when the switches are closed , the hysteresis i hc1 - i hc2 currents flow through the respective resistors r6 - r9 , thus increasing the voltages at the non - inverting comparator inputs by a hysteresis differential . the switches are independently controlled by the voltage levels at the output terminals t1 - t4 of the respective comparators . if v ptat rises above a particular set point voltage , the corresponding comparator &# 39 ; s output goes high and closes its associated switch , allowing the associated hysteresis current to flow . v ptat must then fall below the set point by at least the hysteresis differential before the comparator will switch low . in terms of the previous example , if the lowest set point is 373 ° k . and the hysteresis differential is 4 ° k ., the first comparator will transition high at 373 ° k ., turning on the fan , and will remain high until the temperature falls below 369 ° k . the same principle can be applied to sensing temperatures below set points by reversing the direction of the hysteresis current , switching the polarities of the comparators &# 39 ; inputs and closing the switch when the comparator output is low . when v ptat falls below the set point , the comparator output turns &# 34 ; on &# 34 ; and stays &# 34 ; on &# 34 ; until v ptat exceeds the set point by the hysteresis differential . the circuit produces a corresponding output signal when any of the comparators turns &# 34 ; on &# 34 ;. the output signals are preferably low voltages because it is typically more efficient to drive external circuitry from a low voltage . to invert the signals at the comparator output terminals t1 - t4 , the terminals are connected to the bases of npn transistors q1 - q4 , respectively . the transistors &# 39 ; collectors are connected to output pins out1 - out4 ( 42 , 44 , 46 and 48 ), respectively , and their emitters are grounded . therefore , if v ptat exceeds a set point the corresponding output pin will assume a low voltage level . fig2 is a plot of the voltage levels v 01 - v 04 at the respective output pins out1 - out4 versus v ptat . the voltages at the output pins remain high until v ptat surpasses their corresponding set points sp1 - sp4 . when this happens the output voltage switches low , and stays low until v ptat falls to at least a hysteresis differential 50 below the set point . the magnitudes of the hysteresis differentials are proportional to the respective hysteresis currents i hc1 , i hc2 , i hc3 and i hc4 , and can be adjusted by sinking or sourcing current through the h pin 40 . as shown in fig3 the hysteresis circuit 38 produces a hysteresis current i h that is mirrored to each comparator circuit . for purposes of explanation only the first comparator circuit c1 is shown , but similar circuitry is provided for the other comparators c2 - c4 . in the preferred embodiment a current source is is connected to the emitter of an npn transistor qs . the current source establishes the reference hysteresis current i h , for example 15 μa , flowing through the transistor &# 39 ; s collector . q5 &# 39 ; s emitter is also connected through a resistor r10 to the hysteresis pin h pin . connecting the h pin to different voltages increases or reduces the amount of emitter current , and hence changes the hysteresis current i h . the transistor &# 39 ; s base is biased so that the voltage at the emitter is insensitive to fluctuations in temperature , and lies between ground and v ref . for example , a suitable base voltage would be v b + v be volts , where v be is the base - emitter voltage drop and v b = 2 . 011 volts , so that the emitter voltage is also 2 . 011 volts . the hysteresis current i h flowing through q5 &# 39 ; s collector is reflected through a current mirror 51 to the comparator c1 to supply the comparator &# 39 ; s hysteresis current i hc1 . the current mirror comprises a pair of pnp transistors q6 and q7a having a common base connection , and emitter degeneration resistors r11 and r12a connected between their respective emitters and the supply voltage v s . the collector of q6 is connected to the collector of q5 to supply the hysteresis current i h , and the collector of q7a mirrors the hysteresis current to comparator c1 . the emitter and base of a transistor q8 are connected to q6 &# 39 ; s base and collector , respectively . the emitter degeneration resistors and transistor q8 reduce the error between collector currents on either side of the current mirror that would otherwise result from mismatches between q6 and q7a . alternatively , the current mirror could be a wilson , cascode or base current mirror . the hysteresis current i hc1 supplied by q7a will be equal to the current i h in q6 if the transistors have equal emitter areas and the degeneration resistors have equal values . in general the current applied to the comparator can be a multiple or fraction of i h . for example , if the emitter area of q6 is n times greater than the emitter area of q7a and the value of the degeneration resistor r12a is n times the value of r11 , i hc1 = i h /( n ). this property allows the hysteresis currents and hence the hysteresis differentials to be different for each set point . however , the magnitudes of the hysteresis differentials relative to each other are fixed for a given ic design , and cannot be altered via the hysteresis pin . typically the hysteresis differentials for the different set points are all the same . the switch s1 ( shown in fig1 ) is preferably implemented as a switch s1a and a diode d1 . the switch s1a may be implemented by one or more transistors , which could be bipolar , mosfets , or jfets . similarly , the diode d1 may be replaced by one or more transistors , or anything which implements the diodes functionality as a cutoff valve . when the v ptat voltage is lower than the voltage at in1 , c1 &# 39 ; s output is low . this closes switch s1a and shunts the hysteresis current i hc1 to ground . conversely , when the v ptat voltage is higher than the voltage at in1 , c1 &# 39 ; s output switches high . this opens switch s1a , thus allowing the hysteresis current i hc1 to forward bias and conduct current through diode d1 . as the current flows through r6 it shifts the input signal ( v ptat ) by the hysteresis differential . the reference hysteresis current i h , and hence i hc1 and the hysteresis differential , are adjustable by connecting the hysteresis pin h pin to different voltage levels . for example , i h can have three preset levels : low , medium and high . these levels correspond to connecting h pin to the v ref pin , leaving it unconnected and connecting it to the gnd pin , respectively . an important aspect of the invention is that the user can select one of the three preset levels without providing any additional external biasing circuitry . in general , the number of preset levels would be limited only by the availability of reference voltages . leaving h pin unconnected has no effect on q5 &# 39 ; s emitter current , and hence the medium level is determined by the reference hysteresis current provided by the current source is . tying h pin to v ref , which is greater than the voltage at q5 &# 39 ; s emitter , causes the pin to source current . hence the current source is draws less current through q5 , which reduces i h . conversely , tying h pin to ground causes it to sink current such that both is and h pin draw current from q5 . the exact values for the high and low levels are determined by the selected voltage levels , such as 2 . 5 v for v ref and 0 v for ground , and by the value of r10 . intermediate hysteresis levels can be selected by tying h pin through a resistor r13 to v ref or gnd . the h pin setting for a desired hysteresis differential is described in table 1 below . table 1______________________________________desired hysteresisdifferential (° c .) h . sub . pin setting______________________________________h = h . sub . low tie to v . sub . refh . sub . low & lt ; h & lt ; h . sub . med tie to v . sub . ref through ## str1 ## h = h . sub . med leave unconnectedh . sub . med & lt ; h & lt ; h . sub . high tie to ground through ## str2 ## h = h . sub . high tie to ground______________________________________ for example , under the following conditions : is = 15 μa , q5 &# 39 ; s emitter voltage = 2 . 011 v , r10 = 57 . 5 kω , r6 = 2ω , i hc1 / i h = 1 / 4 and v ref = 2 . 5 v , the preset hysteresis levels would be h low = 0 . 5 ° c ., h med = 1 . 5 ° c . and h high = 5 ° c . an intermediate hysteresis of 1 ° c . would be achieved by connecting a 40 . 3 kω resistor between the hysteresis pin and v ref , and a hysteresis of 3 ° c . would result from connecting a 76 . 6 kω resistor between the pin and ground . by providing a separate external hysteresis pin that is independent of the reference voltage pin , the hysteresis differential can be set without affecting the set point voltages and vice versa . the calculations for the voltage divider resistors for the desired set points and the calculations for the hysteresis resistor r13 are independent , and thus much simpler . furthermore , the reference voltage pin can be used to bias other circuits without requiring a high impedance buffer to avoid loading the pin . while several illustrative embodiments of the invention have been shown and described , numerous variations and alternate embodiment will occur to those skilled in the art . for example , while the invention has been described in terms of shifting the input signal to produce hysteresis , it may also be possible to produce hysteresis by shifting a set point , and making the set point shift programmable . such variations and alternate embodiments are contemplated , and can be made without departing from the spirit and scope of the invention as defined in the appended claims .