SLCA006 June   2025 LM339 , LM393 , LMH7322 , TL331 , TLV1812 , TLV1871 , TLV1872 , TLV3601 , TLV3604

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Output Types
    1. 2.1 Open Collector or Open Drain
      1. 2.1.1 Selectable Open Collector or Open Emitter Output
    2. 2.2 Push-Pull
      1. 2.2.1 Push-Pull with Separate Supplies
    3. 2.3 Differential
      1. 2.3.1 Differential Push Pull
      2. 2.3.2 ECL, RSECL, PECL, RSPECL, and LVPECL
      3. 2.3.3 Low Voltage Differential Signaling
      4. 2.3.4 Current Mode Logic
  6. 3Summary
  7. 4References

2.1 Open Collector or Open Drain

The Open Collector or Open Drain (used interchangeably) output consists of a single transistor that can only sink current. This can be thought as a mechanical (SPST) switch from the output to ground.

A pull-up resistor is required to create the output high voltage. The termination voltage applied to the top of the termination resistor sets the output high (VOH) voltage. When the output is logic High, the transistor is off (not conducting), and the pull-up resistor pulls the output node up to the termination voltage.

Open Drain Output Figure 2-1 Open Drain Output

NOTE: The output can be pulled up to any voltage less than the comparator supply voltage. In most cases, the comparator allows the output to be pulled above the comparator supply voltage up to a maximum voltage. However, some devices have an upper ESD clamp diode on the output that limits the maximum pull-up voltage to the supply voltage. Pulling up to a different voltage is known as Level-Shifting.

The data sheet Features or Electrical Specification table notes can mention if the output can be pulled above the supply. If not, then the Output Voltage line of the Absolute Maximum Ratings or Recommended Operating Conditions can give a clue.

  • If the output voltage rating is an absolute number, such as 5V or 36V, then this is a good sign that the output can be pulled above the comparator supply.
  • If the rating references the supply, such as VCC + 0.3 or (V+) + 0.3V, then the maximum output pull-up voltage is limited to the comparator supply voltage.

An advantage of the open collector output is that multiple outputs can be tied together to form a OR'ed output bus, where any output can pull the output bus low. This is common for system error buses and window comparators.

A list of Open Collector output devices is shown in Table 3-1.

Table 2-1 Advantages and Disadvantages of Open Drain or Open Collector Outputs
AdvantagesDisadvantages
  • Allows different Output High (VOH) voltage (Level shifting)
  • Allows OR'ing of multiple outputs
  • Allows charging or discharging capacitors in timing applications
  • Pullup voltage can be higher than supply voltage (in most cases)
  • Cannot source (output) current
  • Requires external Pull-Up resistor
  • Rise time dependent on capacitive loading
  • Asymmetrical output rise and fall times at high speeds
  • Dissipates current when output is low