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The MH251 Hall Effect Switch has a CMOS output, which doesn't need a pull-up resistor. The typical application circuit on the datasheet is as shown: enter image description here enter image description here

Since the same design will also be used by other sensors that require pull-up resistors. I am wondering if it is okay to add a pull-up resistor to the output of a CMOS Output sensor. What would be the difference between with and without a pull-up resistor?

enter image description here

RoastDuck
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1 Answers1

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There is no problem with adding a pull-up resistor to the output as long as you don't exceed the current sinking capability of the CMOS driver.

The MH251 has a abs max output current of 1mA according to the datasheet, so your pull-up resistor must conduct less than this valu when Vcc is applied over it. For safety you want some margin, so lets say a max of 0.5mA. For a 3.3V supply, that gives \$R >> V/I=3.3V/0.5mA=6.6k\Omega\$. Something like a \$10k\Omega\$ pull-up should be fine.

Of course, even if you add the footprint for the pull-up resistor to the design, there is no requirement to populate it with a component.

When using the open-collector chip populate the footprint with the necessary resistor. Otherwise leave it unpopulated/DNI for the push-pull chip.

Tom Carpenter
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  • What happens when the push-pull chip output HIGH? What interaction will occur between the output signal and the pull-up resistor? – RoastDuck May 30 '20 at 10:42
  • @RoastDuck The resistor is connected to VCC, and the output pulls up to Vcc. There will be no voltage across the resistor, so no current. The resisor will do nothing in that situation. – Tom Carpenter May 30 '20 at 10:45
  • The sensor will be placed remotely from the MCU. The pull-up resistor will be placed on a PCB where the MCU is connected to. Will the voltage drop raise any concern? – RoastDuck May 30 '20 at 10:57
  • @RoastDuck it shouldn't have any impact. – Tom Carpenter May 30 '20 at 11:06