Power-on Detection Circuit

The present invention relates to power supply threshold detectors, and in particular power supply threshold detectors incorporated in integrated circuitry without Zener reference elements.

The detection of adequate power supply voltage upon equipment power up requires an accurate and consistent determination of the earliest point of adequate power supply voltages, and minimization of false signals and changes the threshold at which a signal is produced indicating the proper supply potential. Previous power-on detection circuits which produce under-voltage lock-out signals for inadequate power supply voltage have almost universally relied upon comparison of an input voltage to a Zener diode reference element. However, the actual potential of the Zener diode reference is notoriously process and temperature dependent. As a result, the change in Zener potential due to manufacturing and temperature variances significantly affect product yield. Alternately, construction of discrete under-voltage lock-out circuits having selected Zener diodes is both time consuming and expensive.

The power-on detection circuit according to the present invention comprises a comparator having hysteresis, with an integral threshold circuit wherein the internal threshold is provided without a Zener reference element. The input signal to be measured is received by first transistor which in turn provides multiple current signals therefrom from a split collector. Each collector current is received by separate current sinking elements including additional transistors and resistors which produce an under-voltage lock-out signal in response to changes in the input voltage. Changes in input voltage cause corresponding changes in current in each of the various collector current paths. The threshold at which the under-voltage lock-out signal is produced is determined by resistance ratios in the various collector current paths and semiconductor band-gap voltage (Vbg) of the semiconductor material.

The circuit can be easily implemented on a bipolar monolithic integrated circuit, consumes very little supply current, and consumes very little circuit space. The preferred embodiment comprises 5 transistors and 3 resistors, and is independent of resistor absolute value.

These and other features according to the present invention will be better understood by reading the following detailed description taken together with drawing wherein:

FIG. 1 is a schematic diagram of one embodiment of the present invention; and

FIG. 2 is a graph of the characteristics of the circuits according to FIG. 1.

As shown in FIG. 1, the power supply voltage to be monitored is received by the emitter of transistor Q.sub.1 through resistor R.sub.1 at input V+. The transistors Q.sub.2 and Q.sub.3 along with R.sub.2 and R.sub.3 form a temperature compensating current sink. Transistor Q.sub.4 acts an error amplifier, which amplifies slight differences between the currents from Q.sub.1 collector 54 and the temperature compensating current from the collector of Q.sub.3 and R.sub.3. Transistors Q.sub.5 is the output driver providing the current flow exemplified by FIG. 2.

For V+ less than Vbe, every transistor is off. For increasing monitored voltage above 2Vbe, current begins to flow through R1, the emitter-base junction of transistor Q.sub.1 and through the emitter-base junction transistor Q.sub.5, initially turning both Q.sub.1 and Q.sub.5 on. However, at the same time, a current equal to the base current of Q.sub.5 flows into the base and collector of Q.sub.2 and into the base of Q.sub.3. Also, a current equal to the base current of Q.sub.5 flows into R.sub.3, the base of Q.sub.4 and the collector of Q.sub.3. For V+ below the turn-on threshold, the current in the collector of Q.sub.3 plus the current in R.sub.3 equals the current in the corresponding collector 54 of Q.sub.1, providing no (or insufficient) base current to turn on transistor Q.sub.4. However, as the monitored voltage V+ increases to the turn-on threshold, Q.sub.4 will begin to turn on, causing an increasing current to flow through resistor R.sub.1 and thus increasing current flow to the collectors of transistor Q.sub.1. As Q.sub.4 turns on, it pulls additional current from the base of Q.sub.1, which provides more current from Q.sub.1 (and therefore collector 54), which turns on Q.sub.4 harder. Transistor Q.sub.4 turning on instantly turns off transistor Q.sub.5, which provides an output signal shown by graph 60 curve portions 62, 64, 66, depending on device temperature. Thus, the turn-on threshold of the present apparatus is determined in part by the current resulting from the selection of resistor R.sub.1 relative to the single base emitter voltage (Vbe) of transistor Q.sub.1. For any voltage above that thershold the circuit state remains the same and Q.sub.5 stays off.

As the monitored power supply voltage at V+ begins to decrease, such as in a power down sequence, transistors Q.sub.1 and Q.sub.4 initially stay on, but the current out of any Q.sub.1 collector (52, 54, 56) decreases). When the current out of collector (54) of Q.sub.1 drops below the current sinking through the collector of transistor Q.sub.3 plus the current Vbe/R.sub.3, transistor Q.sub.4 will lose sufficient base drive and turn off, turning Q.sub.5 on, shown by curve section 74 on FIG. 2.

The separate turn-off threshold which provides the hysteresis characteristic shown in FIG. 2 of the circuit of 1 is due to the fact that transistor Q.sub.4, no longer being saturated allows the Vbe of transistor Q.sub.5 to be added to (in series with) the Vbe of transistor Q.sub.1 thereby affecting the current produced from collectors 52, 54 and 56 of transistor Q.sub.1 (in response to the selected resistor R.sub.1 and the monitored voltage V+). An approximation of the turn-on and turn-off thresholds are enumerated below in equations 1 and 2 respectively, where Vbg is the band-gap potential of the semiconductor material, such as silicon in the present invention: ##EQU1##

Note that for a particular choice of components, the turn-on and turn-off thresholds can be made nearly independent of temperature. For instance, in the embodiment shown, R.sub.1 =30K ohms, R.sub.2 =3K ohms, and R.sub.3 =80k ohms to provide a voltage threshold of approximately 4.2 volts with .+-.200 millivolts (mv) of hysteresis, FIG. 2.

A graph showing the turn-on and turn-off points of the embodiment shown in FIG. 1 of the present invention shows the response of a circuit for 0.degree. C., 50.degree. C. and 100.degree. C., wherein it can be appreciated that the hysteresis characteristics is repeatable in the manufacturing process and maintained under a wide temperature variation and the threshold points are sufficiently consistent to eliminate the necessity of Zener reference elements in circuitry embodying the present invention. Thus the apparatus according to the present invention provides a repeatable accuracy significantly exceeding that achievable with standard process Zener diodes, and is applicable to various devices such as linear and switching voltage regulators, and inductive load drivers and in complex control systems.

Modifications and substitutions of the present invention by one skilled in the art is considered to be within the scope of the present application, which is not to be limited except by the claims which follows.