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AN2149 - APPLICATION NOTE

2/7

Bus hold circuitry holds the input of the devices at the last known state. This feature eliminates the need

for using pull-up resistors to prevent the device inputs from floating and causing potential system errors.

option is particularly important in applications where space and cost are at a premium and the elimination

of pull-up resistors is a possibility.

In floating inputs, there is a possibility that the external bias appearing on the input buffer makes the buffer

operate around its threshold point. In such cases, the floating inputs might start oscillating.Since a floating

input has very high impedance, noise injected from other components on the board can cause the input

to move around the threshold point of the input buffer. Each time the input voltage crosses the threshold

point, the input buffer changes state, adding noise to the system which can potentially cause the device

to oscillate. These oscillations can also increase the power consumption of the device. The heat

dissipation thereof might also damage the device.

Adding a pull-up resistor adds bus loading every time the bus toggles low. With bus-hold, the devices are

optimally sized to reduce loading on the bus. Once the transition has been made through the toggle point,

the bus-hold input will be assisting the driver rather than fighting it and causing a current drain. This way,

it consumes much less current compared to a circuit with a pull-up resistor when the device switches low.

In other words, it is easier to overdrive a bus-hold input than to drive a pull-up resistor of equivalent value

LOW.

When using pull-up resistors, the bus will be pulled high when released, causing unnecessary transitions

where the logic was initially LOW. Bus-hold components always retain the last known state, thus avoiding

transitions. Thus, there is no toggling when the bus is released.

In all power sensitive applications, the current leakage of the pull-up resistor (when the bus is low) can be

avoided with a bus-hold input. Also, since the input will avoid unnecessary transitions through the input

threshold, the device power dissipation will be reduced (

DIcc).

3. CIRCUIT OPERATION

The Integrated Bus-hold circuits act like dynamic pull-up/pull-down resistors as follows (Refer to figure 2):

- When the input is “low”, the output of the inverter is "high" so that the lower NMOS is ON and acts like a

pull-down resistor.

- When the input is "high", the output of the inverter is "low" so that the upper PMOS is activated and acts

like a pull-up resistor.

Figure 2. Bus Hold Schematic Representation