VOLTAGE REFERENCE CIRCUIT

Abstract: An improved voltage reference circuit uses a feedback loop in the lower portion of a bias network to clamp the output voltage at a lower-limit value.

Description: Description Voltage Reference Circuit Technical Field The field of the invention is that of voltage reference circuits used in integrated circuits. Background Art The prior art has long used a voltage divider circuit to provide a reference voltage that depends upon the supply voltage. A chronic problem in the art has been to provide a reference voltage that is insensitive to variations in the supply voltage. It is known to use a clamping circuit to limit the maximum value of the reference voltage in cases when the supply voltage exceeds its nominal value. It is also known to use a clamping circuit on the minimum voltage, but one prior art approach fails to work if the supply voltage is less than 2VT, twice the threshold voltage of the transistors used in the circuit. Disclosure of Invention The invention relates to an improved voltage reference circuit in which one-half of a bias network forms a feedback loop to adjust the impedance of the bias network to clamp the output voltage at a lowerlimit value. A feature of the invention is the extension of the permissible lower limit range to VT, a single threshold voltage. Brief Description of Drawings Figure 1 illustrates a prior art circuit. Figures 2A and 2B illustrate a prior art circuit. Figures 3A and 3B illustrate a circuit constructed according to the invention and its voltage dependance. Best Mode of Carrying Out the Invention Voltage reference circuits are used throughout integrated circuits. One particular application is the use of a reference circuit in a dynamic RAM to compare the voltage levels on the input terminals to the reference in order to detect whether a binary 1 or 0 is present. The stability of the voltage reference is important because it is directly related to the voltage margins that must be allowed to accommodate the electrical noise that will inevitably be present. It is a commercial requirement that integrated circuits work for supply voltages within some tolerance range. The wider the range, the more usable the circuit. Clamping the reference voltage-to a predetermined value to resist the effect of an increased supply voltage is straightforward. Proper reference voltages for a low value of the supply voltage have proven to be more difficult to obtain. The improvement afforded by this invention is a considerable increase in the lower limit of supply voltage that may be tolerated, from a value of twice the threshold voltage of the circuit to a value equal to the threshold voltage. For many dynamic RAMs, the reference voltage that is determined is approximately 0.29xVcc. The reasons for this choice are not relevant to the present invention, which is concerned with achieving that value of reference voltage. In a straightforward 2 - resistor voltage divider, the upper resistor between VCC and the reference voltage node has a value of 2.45 times the value of the lower resistor between the reference node and ground. Figure 1 illustrates a prior art circuit in which resistors 10 and 12 form a voltage divider having a standard ratio so that the voltage on node 15 is .29 Vcc. Clamp circuit 100, comprising two transistors 102 and 104 is a standard voltage clamp which limits the maximum value of the output voltage on node 15 to a value of the sum of the threshold voltages of transistors 102 and 104 (when transistors 102 and 104 have low impedance compared to the impedance of the divider chain). Figure 2A illustrates a prior art circuit in which the same resistive divider chain 10, 12 with node 15, is corrected for low values of Vcc. In this case, transistor 22 supplies current to node 15 to maintain the output voltage at a lower-limit value. The gate of transistor 22 is controlled by node 25, which is intermediate between resistor 24 and clamp circuit 100. Clamp circuit 100 controls node 25 to a voltage equal to two thresholds so the output voltage on node 15 is clamped by transistor 22 to one threshold; i.e. its source voltage (the output voltage) is one threshold below the gate voltage of node 25. When the supply voltage is decreased below two thresholds, the clamping function no longer works and the output voltage declines along with the supply voltage. The voltage dependance for low voltages is illustrated in Figure 2B, showing the clamping (VMIN) provided by transistor 22 which is effective until VCC reaches a value of 2VT, below which the output voltage declines. The circuit of Figure 2A can be combined with the clamping circuit 100 of Figure 1 to provide clamping in both the higher and lower ranges, of course. Referring now to Figure 3A, a voltage divider comprising the familiar resistors 10 and 12 with output node 15 is modified by the addition of transistor 20 between intermediate node 17 and ground. The gate of transistor 20 is connected to node 15, so that resistor 12 and transistor 20 together form a feedback loop to control the impedance of the lower half of the voltage divider in response to the output voltage on node 15. Circuit 100 is an optional voltage clamp for VMAx. The voltage dependance of VOUT on VCC is illustrated in Figure 3B, in which the clamp VMAX is shown as in the prior art. The difference between this invention and the prior art invention of Figure 2A is indicated by the solid and dotted lines. It can readily be seen that r;ze subject invention extends the operating range of VCC from a value equal to 2VT to a value equal to VT. The operation of the invention may be calculated by noting that the gate to source voltage of transistor 20 must be greater than threshold i.e. the voltage on node 15 must be greater than VT. Since the voltage on node 15 is a fraction of VCC (when a low impedance transistor 20 is turned on hard) then there is an inequality given by: VCC (Rl0/(Rl0 + R12)) > VT Rearranging this inequality, there is a lower limit of VCC: VCC lower > VT ([RIO + Rl2]/R12]. If VCC falls below this value, transistor 20 begins to turn off, so the current in the lower half of the voltage divider is reduced. The reduction in current allows the voltage on node 15 to rise until a stable value of the impedance of transistor 20 is reached. For best operation of the circuit, transistor 20 should be quite large so that the gain of the feedback loop is also large. An alternative embodiment of the invention can be seen in Figure 3A, if circuit 100 is removed and the ratio R10/R12 is changed. In that case, when the value of R10 is much greater than the value of R12, the operating range of the negative feedback loop will be much enlarged. This will yield a feedback regulated reference voltage which is equal to VT. Those skilled in the art will readily appreciate the uses of such a reference voltage in operational amplifiers and many other linear circuits.

Claims: Claims 1. A voltage reference circuit connected between a supply voltage and ground, for producing a reference output voltage greater than a minimum reference output voltage comprising: a resistive divider chain having a first portion comprising a resistive element connected between said supply voltage and an output node and a second portion connected between said output node and ground and comprising a second resistive element, said first and second resistive elements being related such that the voltage on said output node is at a predetermined value when said supply voltage is at a standard value; and a low-voltage correction means for maintaining said voltage on said output node at a constant predetermined low-limit voltage when said supply voltage is within a low-voltage range; characterized in that: said second portion further comprises a low-limit transistor connected between said second resistive element and ground and having a gate connected to said output node, whereby said low-limit transistor and said second resistive element form a feedback loop for modifying the impedance of said second portion to maintain said output voltage at said low-limit voltage within said low-voltage range of said supply voltage. 2. A circuit according to claim 1, in which at least one of said first and second resistive elements is a depletion transistor. 3. A circuit according to claim 1, in which at least one of said first and second resistive elements is a resistor. 4. A circuit according to claim 1, in which said first resistive element has a first magnitude that is large Compared o a second magnitude of said second resistive element, whereby said circuit has a wide operating range above said low-voltage range.

Inventors: JOHNSON, MARK GRIFFIN, TAYLOR, RONALD THOMAS
Applicants: THOMSON COMPONENTS-MOSTEK CORPORATION
Application year: 1984
Main classification: G05F3
IPC codes: G05F1, G05F3