AN2149 Datasheet, PDF (2/7 Pages) STMicroelectronics – INTEGRATED BUSHOLD CIRCUITRY

AN2149 - APPLICATION NOTE
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.
Eliminating pull-up resistors significantly reduces part count and assembly costs. Thus, the bus hold
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
t(s) ^{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
c _{avoided with a bus-hold input. Also, since the input will avoid unnecessary transitions through the input}
u _{threshold, the device power dissipation will be reduced (DIcc).}
rod _{3. CIRCUIT OPERATION}
P _{The Integrated Bus-hold circuits act like dynamic pull-up/pull-down resistors as follows (Refer to figure 2):}
te _{- When the input is “low”, the output of the inverter is "high" so that the lower NMOS is ON and acts like a}
le pull-down resistor.
o _{- When the input is "high", the output of the inverter is "low" so that the upper PMOS is activated and acts}
s _{like a pull-up resistor.}
Obsolete ^{Product(s) - Ob Figure 2. Bus Hold Schematic Representation}
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