Description

OUTPUT LEVEL VOLTAGE REGULATION

TECHNICAL FIELD

The invention relates to electrical interface circuits and, more particularly, to a circuit for matching an output voltage to a desired voltage.

BACKGROUND

Computer systems are comprised of a number of electronic components that must interoperate . To minimize the costs of various components, different components are made which operate at different voltages. If the output level is not compatible with an input level, the device may be damaged and interoperation thwarted.

In prior circuit design, a level shifter circuit commonly would be used to interconnect devices, for example by interconnecting sections of a bus system. Each section would have a different supply voltage and different logic level. In a typical example, each section would have a supply voltage; pull up resistors and devices connected to the supply voltage; and a serial data bus line and a serial clock bus line connected to the devices. An exemplary level shifter circuit would include a gate connected to the lower voltage supply, sources connected to the lower voltage bus lines, and drains connected to the higher voltage bus lines. One such level shifter circuit would be required for each different supply voltage.

Level shifter circuits are generally specific to specific voltage levels. Thus for each device or group of devices having a specific voltage operating

level, the system requires specific circuitry to allow use of the devices .

SUMMARY OF THE INVENTION A circuit and method of adapting an output voltage level toward a target voltage level includes applying an output voltage from a pin or pad to an output line and applying a reference voltage to a reference voltage line for comparison by a comparator having a logical output, high or low. A state machine receives a signal from the comparator. The state machine generates a trim bus signal in response to the signal from the comparator and in response to the registered prior logic output. If the two polarities match, further corrective voltage feedback is required. If the two polarities do not match, corrective voltage feedback of the opposite sign is indicated by flipping the output state of the comparator. A clocked trim bus signal is sent to a sense circuit and voltage regulator, which changes a switched resistor value to step the output voltage one step towards the reference voltage. Thus if the output voltage was below the reference voltage, the resistor value (resistance) of the sense circuit and voltage regulator is changed to increase the output voltage one step towards the reference voltage. The comparison between the output voltage and the reference voltage would then be repeated on the next clock cycle, as will the subsequent steps effected by the comparator, the state machine, and the sense/resistor. Sequential voltage steps would be completed until the output voltage crossed the reference voltage threshold in polarity. At this point the comparator signal would flip states, signaling the state machine to save the trim bus value into a register.

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BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a circuit overview of an output adapter.

Fig. 2 is a graph of signal voltage over time using a circuit such as the circuit of Fig. 1.

Fig. 3 is a circuit diagram of the resistor network shown as a block in Fig. 1.

Fig. 4 is a circuit diagram of the decoder circuit shown as a block in Figs. 1 and 3. Fig. 5 is a circuit diagram of the regulator shown as a block in Fig. 1.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Fig. 1, an output pad 12 produces an output voltage applied at output line 14. A reference supply, such as a tester 16, produces a stable and fixed reference voltage onto line 18. These are compared by comparator 20, an analog error amplifier that produces an output by sensing an error either above or below the reference threshold voltage. The resulting comparator output will be either a logic high or low, depending upon the polarity of the error.

Referring to Fig. 2, the initial output voltage level 40 is compared to the reference voltage level 60. Since initially the value of the voltage 40 at node 38 in

Fig. 1 is below the reference voltage level, in this example the state of the comparator output remains at low state 50.

Returning to Fig. 1, the comparator 20 sends a signal to the state machine 22, a logic device. The logic device has registered what it did on the previous cycle. If the error signal was of the same polarity on the previous clock cycle then the incremental output signal is set to be one step larger than the prior output signal. If the error signal has a reverse polarity, then

the incremental output signal is one step of opposite polarity. The state machine 22 receives a clock signal from the clock 24, allowing clocking of logic signals and register of the state machine. The state machine sends a three-bit trim bus value signal to decoder 27 for establishing a correction voltage in a resistor network 28. The signal is sent on three-bit line 26, such that a three-bit word may be transmitted. A three-bit word may encode eight states of the trim bus value, or four bits and a sign bit, presenting a number of corresponding voltage step heights that are possible. The sense circuit 21 and voltage regulator 30 are part of a voltage feedback loop that may be changed to alter output voltage at node 38. This is done in defined increments corresponding to the eight step heights described above. The output signal is stepped up or down a maximum of one step per clock cycle until the polarity of the error signal in the comparator reverses.

Again with reference to Fig. 2, output signal 42 is shown stepped up one step per clock cycle compared to output level 40. At this voltage output level, the output voltage 42 is still below the reference voltage 60. Thus, as explained in Fig. 1, the comparator output logic level remains at low state 50. The state machine, a clocked logic device, adds one to the registered state of the trim bus value in a signal which is again transmitted to the sense circuit and voltage regulator. The regulator again incrementally changes the resistance of the feedback loop, again increasing the output voltage in a stepped manner so long as the error signal maintains its polarity.

In Fig. 2, the result of this is the output voltage level 44 is maintained at a one step increase for one clock cycle with clock intervals indicated by vertical dashed lines. This process is repeated. An

increase, to output voltage 46, is compared in the comparator to reference voltage 60. At this point the output voltage is above the reference voltage 60 and the polarity of the error signal is reversed. The comparator output then flips states to high state 52. This signals the state machine to memorize the trim bus value into a memory.

In the example of Fig. 2, the output voltage is initially below the reference voltage. This voltage is incrementally stepped up at clocked intervals until the output voltages crosses the reference voltage threshold. The incremental stepping of the output voltage in the direction of the reference voltage is effected by modifying a switched resistor network described below with reference to Figs. 3 and 4. This is repeated in clocked cycles until the output voltage crosses the reference voltage threshold. It is also possible that the initial output voltage be above the reference voltage. In this case, the comparator logic level would start in the high state, indicating to the state machine that the output voltage level is above the reference voltage threshold. The trim bus logic signal value would then signal the switched resistor network to modify resistance for the output, incrementally stepping down the voltage. This would be repeated until the output voltage crossed the reference voltage threshold in polarity. At that time the comparator would flip state, in this instance from high to low. The trim bus logic signal value would again be saved into a register for comparison with polarity of the next logic signal value.

If the polarity is the same, the previous logic signal value is increased by one unit. If the polarity is the opposite, the previous logic signal value is decremented by one unit .

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With reference to Fig. 3, decoder 27 is seen to receive three-trim bits on line 26. The three-trim bits, also shown in Fig. 4, generate eight unique signal in decoder block 27. In Fig. 4 there is a logic arrangement wherein three parallel bits are translated to eight possible signals. The eight possible logic signals, appearing one at a time, are taken as logic output signals 32a-32h and their corresponding complements 34a- 34h. The complementary output signals, e.g., 32c and 34c, are delivered as simultaneous logical outputs.

Returning to Fig. 3, the simultaneous logical outputs are split to drive corresponding CMOS driver transistors pairs of opposite conductivity types in resistor network 76. For example, p-channel CMOS transistor 38a has a control gate operated by logic signal 34a, while the corresponding n-channel CMOS transistor 36a has a control gate operated by logic signal 32a. Each p-channel transistor 38a, 38b, etc. delivers regulated bias voltage on line 70 to a lower level in the series of resistors R2. Transistor 38a bypasses one resistor with bias voltage. Transistor 38b bypasses two resistors, and so on. On the other hand, each n-channel transistor 36a, 36b, etc. shorts resistors to ground 72 in the series of resistors Rl. The n-channel transistor 36a shorts one resistor to ground. Transistor 36b shorts two resistors, and so on. It is seen that the p-channel and n-channel transistors behave in a complementary manner to deliver an intermediate output voltage, V _MED/ on output line 74 that may be half way, or some calculated value based upon selected resistor values, between the regulated supply voltage and ground. Resistor values in resistor network 76 are selected to give V _MED the proper voltage for a given regulated supply voltage applied across selected resistors in Rl and R2 groups.

Resistor network 76 is seen in Fig. 5 to deliver the voltage output, V _MED to an operational amplifier 80 in regulator 30 which was discussed with reference to Fig. 1. The voltage level V _MED is combined with a bandgap reference level from device 82 to adjust a supply transistor 84 to which external supply 86 is connected. The regulated supply output voltage on line 88 is fed to resistor network 76, the node 38 associated with output pad 12 and an input line to comparator 20 in Fig. 1. The circuit of Fig. 1 is not intended to be limited by any specific type of regulator nor any other specific circuit well known in the art.