Input buffers are used to condition signals transmitted to an integrated circuit from an external source. A common input buffer includes a simple CMOS (complementary metal-oxide semiconductor) inverter circuit having a P-channel transistor coupled in series with an N-channel transistor. The gates of the transistors are coupled together to receive an input signal. A common problem with this type of circuit is that as the input signal is transitioning from one logic state to another there is a point when both the P-channel and N-channel transistors are both conducting at the same time, causing a current, sometimes referred to as a crowbar current, to flow through both devices for a short period. The circuit of FIG. 1, described below, has been used to reduce the crowbar current in an input buffer.
FIG. 1 illustrates a schematic diagram of a prior art input buffer circuit 10. To reduce the crowbar current, input buffer circuit 10 includes a resistor 13 between a P-channel transistor 12 and an N-channel transistor 14 that make up an inverter circuit. There are two outputs from the inverter, labeled “A” and “B”, separated from each other by resistor 13. Output A is coupled to drive the gate of P-channel transistor 16 and output B is coupled to drive the gate of N-channel transistor 18. The resistor functions to prevent P-channel transistor 16 from being conductive at the same time N-channel transistor 18 is conductive when an input signal IN transitions from, for example, a logic low voltage to a logic high voltage. This reduces the crowbar current and thus reduces power consumption.
Frequently, input buffers with hysteresis characteristics are necessary, such as in circuits that handle low frequency signals. A circuit with hysteresis has two switching voltage levels, an upper level and a lower level. As a signal transitions from a logic low to a logic high, the circuit will switch states as the signal passes the upper voltage level. As the signal transitions from a logic high to a logic low, the circuit will switch states as the signal passes the lower voltage level. This prevents the circuit from inadvertently switching states in response to a rapidly changing signal due to, for example, noise. A problem with some existing input buffers with hysteresis is that a relatively high amount of power is consumed to provide the hysteresis. Therefore, what is needed is an input buffer with hysteresis with lower power consumption.