How to read datasheets
Which correctly describes the change when the value of the pull-up resistor on an open-drain output is reduced?
- AThe rise becomes slower, but the current flowing while the output is low decreases, reducing power consumption
- BThe rise becomes faster, but the current flowing while the output is low increases, and VOL also tends to rise
- CThe high-level voltage rises above the pull-up supply voltage, making it easier to exceed the receiver's input rating
Show answer and explanation
Answer: B. The rise becomes faster, but the current flowing while the output is low increases, and VOL also tends to rise
The high level of an open-drain output is produced by the pull-up resistor pulling the signal line up while the output transistor is off. The rise speed is set by the time constant of the resistor and the line capacitance—for example, 1 µs for 10 kΩ and 100 pF. A smaller resistor makes it faster, but when the output is low, current flows from the supply through the resistor: 3.3 mA for 3.3 V and 1 kΩ. If this current exceeds the IOL condition in the datasheet, VOL will not stay within its specified value, so choose a value that satisfies both.