How to Bias Hi-Z and Low-Z I/O Pins
Integrated circuits rely on control signals to perform important functions, and the way the signal interacts with the component is based on the impedance of I/O pins. I/O pins are typically characterized in two ways: high impedance and low impedance, or hi-Z and low-Z respectively. The input impedance of an I/O pin defines whether the pin is current controlled or voltage controlled at DC.
To ensure bias settings are correct, you would use sets of resistors to adjust the voltage and/or current going into these pins, and the resistor values are chosen based on the pins’ input impedance values. We will look at some examples of how to set these resistor values so that pins can be properly biased and thus perform their intended functions.
Biasing Hi-Z Pins
All hi-Z Pins have high input impedance. In other words, if you were to apply a voltage to a hi-Z pin, you would expect very little current to flow into that pin. However, if you were to apply a voltage onto that pin, then you would expect all of the voltage to be dropped across the internal high impedance circuit inside the IC.
Hi-Z I/O pins are very easily biased by simply applying a voltage, sometimes with a voltage divider. These are simple circuits that are built from a source voltage and a group of resistors. An example for feedback in a voltage regulator is shown below.
The FB pin in this circuit is a hi-Z input.
These pins have high input impedance, but they draw very little current at their rated bias voltage. If you look in a datasheet for a component with a hi-Z configuration pin, you will typically see a small bias current value, typically less than 1 mA. Low power devices can have much lower bias current values so that they do not consume excessive energy during operation.
In the above example, your job as a designer is to calculate the voltage across the second resistor in the voltage divider such that the parallel voltage meets the specification on the I/O pin. Typical resistor values are in the 1000’s of Ohms as this will limit excess current drawn from the voltage source to sub-mA levels.
Low-Z I/O Pins
Low-Z I/Os are controlled by the amount of current flowing into the pin. Low-Z I/Os have low input impedance, so they drop very little voltage across the pin. In some ICs, there may be a minimum voltage specification for the particular pin, which will require an adjustment of the supply voltage and any biasing resistors. As long as the input current is below some absolute maximum value, then the circuit will be biased correctly.
To limit the current into a low-Z pin, a series resistor is used. Consider the circuit below with an IO pin that is connected to a circuit with a diode. In order for the IC to function, the diode must have minimum forward voltage and forward current. Your job is to select the mix of input voltage and series resistor that limits the current into the pin.
If an 18V source is used, the required resistor value needed to bring the internal voltage above minimum would be found by solving:
(18 - 0.65)/0.5 = 34.7 Ohms
This is based on the current flowing into the pin having a value of 500 mA, which is the minimum bias current. A larger source voltage or smaller resistor value will increase the bias current and voltage on the internal circuit.
Where You Will See These Pins
Digital Components
Most of the pins on digital components are hi-Z pins and are designed to draw low current. Instead of drawing current through an I/O, digital ICs draw their current through a supply pin (VCC, VDD, etc.). This is because I/O pins on a digital component are almost always part of a logic circuit. Instead of supplying power into/out of a pin, the signal reaching a pin is representing a logic state, and it can toggle a logic circuit inside the receiving component.
Analog Components
The use of low-Z pins is much more common in analog ASICs, such as sensors or PMICs. If you're designing circuits with discretes, you will also need to perform this exercise. Note that the above example uses a diode, but a similar thought process supplies when calculating current limiting resistors for circuits that do not include a diode. Remember, you may need to adjust your supply voltage and current limiting resistor values in order to satisfy a minimum current and minimum voltage specification when both are present.
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