Hello,


I’m trying to build a circuit with the PCF8575 (I2C 16xGPIO port expander). The outputs should be able to activate/deactivate either 5V or 12V to turn on/off several components (IC’s, relays, buzzers, etc.). I considered to use a 2N2222 transistor for that switching. However, it seems the ports aren’t able to supply enough current to drive the transistor. It seems that the outputs of this PCF8575 are open drain. In the “zero” state they are capable is sinking up an average of ~20 mA and in “one” state only ~165uA.


According to the datasheet:

I/Os; P00 to P07 and P10 to P17

LOW-level output current: min=10mA; typ=25mA

HIGH-level output current: min=-30uA; max=-300uA


Note 3. From datasheet:

3. A single LOW-level output current (IOL) must not exceed 20 mA for an extended time. The sum of all IOLs at any point in time must not exceed 100 mA

So, as I’ve understood by reading around, I need some P-channel device. 2N2222 is a N-channel and it somehow it doesn’t work well with the “open-drainess” of PCF8575. Also, assuming a scenario where I want to have all GPIO’s at the same level (0), I have to have 100mA/16= 6,25mA for each port, which seems very little to me. (and even much less if I want to have it as (1) ).


Having this in mind, could anyone recommend a device (mosfet, transistor, whatever) that I could use to connect to the PCF8575 and could allow to:


A) Act as a switch for 12V load (ex: some relays, 12v buzzers)

B) Act as a switch for 5V load (ex: some IC’s, 5v buzzers)



Thanks in advance,

Jabss

 

2n7000 'Fetlington'
ULN2803
ULN2003
Max.

 

2n2906, 2n4403,

 

Hello,

Thanks for replying.
Regarding 2n2906 and 2n4403 I'm not familiar with, but I've tried before 2n7000, and the ULN's directly to the PCF8575 and it doesn't work.... Maybe I need a special circuit? Maybe pull-up resistors?
Isn't 2N7000 a N-Channel mosfet? In that case, shouldn't a P-channel be needed in this situation? And could the same mosfet model be used to switch 12V and 5V? I don't know much about mosfets, but isn't there a limitation about the Vgs?
Just asking.....

Thanks,
Jabss

 

If you add a pull up resistor to each output, then any NPN-based device )2222, 4401, ULN2003, etc.) will work as an inverting saturated switch. In this case the loads are tied to the power source and GND is applied by the switch devices. Without the pullup you can drive a PNP device (3906, 4403, etc.) as an inverting saturated switch. In this case the loads are tied to GND and power is applied by the switch devices.

ak

 

If you want non-inverting look at the UDN2981 series.
Max.