Choosing an Appropriate Pull-up/Pull-down Resistor for Open Drain Outputshttp://www.ti.com/lit/an/sl

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Choosing an Appropriate Pull-up/Pull-down Resistor for Open Drain Outputs

Ben Hopf ....................................................................................... PMP-DC/DC Low-Power Converters

ABSTRACT

Many ICs contain digital output pins to indicate certain statuses to the rest of the system. These outputsfall into two categories: open drain (open collector for bipolar outputs) or push-pull (also known as totempole). Open drain outputs are commonly utilized because they offer several advantages when comparedto push-pull outputs. Unlike push-pull outputs, several open drain outputs from different devices can beconnected directly together to create an OR function. Also, open drain outputs provide more flexibility to adesigner as they can be pulled-up to any voltage found in the system, which can be useful when theyserve as inputs to a processor which might require a lower voltage level than the push-pull output wouldgive. Examples of open drain outputs commonly found on ICs include Power Good (PG) and Low Battery(LBO) on switching regulators, reset and Power Fail (PFO) on supply voltage supervisors (SVS), and LowBattery, Power Fail, and reset on power management units. All open drain outputs require the use of anexternal pull-up or pull-down resistor to keep the digital output in a defined logic state. This applicationreport discusses when to use a pull-up or pull-down resistor, the factors that should be considered whenselecting a pull-up or pull-down resistor, and how to calculate a valid range for the value of the resistor.