PCF8575: Pull-Down Resistor Values for PCF8575

Priya Lakshmi

Part Number: PCF8575
Other Parts Discussed in Thread: SN74AHC14, SN74LVC14A, SN74LV14A

Tool/software:

Hi,

The PCF8575 datasheet indicates that the I/Os are high at power-on. To prevent any backdriving of the previous circuitry connected to these pins, I plan to add pull-down resistors. What would be the appropriate value for these resistors?

Thanks,

Priya

6 months ago

0 Clemens Ladisch 6 months ago

Guru 317390 points

I_OH is at most 300 µA. If you want the voltage to be at most 0.1 V, the resistor must be less than 333 Ω.

I would recommend to use a different expander without internal pull-ups.

0 Priya Lakshmi 6 months ago in reply to Clemens Ladisch

Prodigy 60 points

Input to expander in my application is coming from inverter SN74AHC14 which can sink upto 8mA. If Ioh of PCF8575 is limited to 300uA, I think it should be ok?

0 Clemens Ladisch 6 months ago in reply to Priya Lakshmi

Guru 317390 points

When the inverter is powered off, it does not sink any current, and can be backdriven.

Just use inverters with I_off support, e.g., SN74LV14A/SN74LVC14A.

0 Priya Lakshmi 6 months ago in reply to Clemens Ladisch

Prodigy 60 points

I am planning to use a power supervisor to delay the power to the PCF8575. This is to meet the timing requirement I saw in the datasheet that off time should be >100ms and rise time <10ms. With this setup, my inverters are powered first, followed by the PCF8575. Therefore, I don't need to change my inverters, correct?

Regarding the necessity of the supervisor circuit, how critical is it to implement this? Have you encountered any issues when this was not implemented?

0 Clemens Ladisch 6 months ago in reply to Priya Lakshmi

Guru 317390 points

TOFF does not matter for the initial power up.

AHC/LV/LVC inputs are overvoltage tolerant; you can simply power up the inverters together with the PCF.

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PCF8575: Pull-Down Resistor Values for PCF8575