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I2c I/O expander(PCA9555) and mosfet driving

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fuzzzy

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HI

I'm intended to use an I2C I/O expander to drive some N-Mosfet by an High side mosfet driver.
I already used this PCA955 to drive some relays through ULN2003. But when i power up my siystem PCA9555( at startup is set as Input) set at ON-State, for a moment, all relays.Untill MCU sets PCA9555 pins as Output.

My question is: do you think Mosfets will be on for a moment during startup? If it happens how can i prevent this problem?

My system is: I2C Master MCU-------> PCA9555---------> 0-5V logic mosfet driver--------> Mosfet.

Thanks a lot for helping me
 

Hi Fuzzzy,
I thihg that the Mosfets can be ON during startup. In the schematic picture you can see the reason. When an I/O is configured as an input, FETs Q1 and Q2 are off, creating a high impedance input with a weak pull-up 100k to VDD. One solution of this problem is connect resistor betwen Gate Mosfet and VSS.


JURS
 

Thanks for your suggestion.

I was thinking a similar solution but i would have a certainty. something under 10Kohm would fit. what do you think??


by the way should I use also a resistor in series between PCA9555 and mosfet driver input? where do you suggest me to put it? before or after pull-down resistor?
 

Resistor 10KOhm or lower will be good, it is only inportant don't cross Vgs Gate Threshold Voltage. . My opinion is that you needn't use the resistor in series between PCA and mosfet.

JURS
 

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