TCA4307: Low output voltage of PCA9544A compatibility with TCA4307

Mohammad Medhat

Part Number: TCA4307
Other Parts Discussed in Thread: PCA9544A, ADS1115

Dear experts,

I will connect the TCA4307 out I2C interface to the I2C of PCA9544A. The maximum low input voltage of TCA4307 is 0.4V but I can't find the low output voltage of PCA9544A in the datasheet. Only, in page 5 it is mentioned the low output current as 6-10 mA when Vol = 0.6V and 3-7 mA when Vol = 0.4V.

My question is: As I need to guarantee that low output voltage of PCA9544A to be <= 0.4V then please confirm that if I limit the low output current to 3 mA at VCC=3.3V then the low output voltage will never exceed 0.4V.

Regards,

Mohammad.

7 months ago

0 Clemens Ladisch 7 months ago

Guru 271610 points

The PCA9544A is a passive switch; the voltages are just passed through. There is a voltage drop over the switch, but R_ON is quite small.

0 Mohammad Medhat 7 months ago in reply to Clemens Ladisch

Prodigy 50 points

Hi Clemens,

Thank you for your fast response.

Clemens Ladisch said:
The PCA9544A is a passive switch; the voltages are just passed through. There is a voltage drop over the switch, but R_ON is quite small.

So, the value mentioned in the datasheet is calculated according to the current passed and the Ron which is maximum 45 ohm.

ADS1115 will be connected behind the switch and according to its datasheet:

V(ol) can reach up to 0.4V at I(ol) = 3mA.

We will limit the current to less than 1 mA which will give maximum V(ol) after the switch about 0.42V but can we expect much less V(ol) from ADS1115 at 1mA so there will be good margin under the 0.4V?

Regards,

Mohammad.

0 Mohammad Medhat 7 months ago in reply to Mohammad Medhat

Prodigy 50 points

I want to ask about the values range in the datasheet. It depends on what? The tolerance in manufacturing process, working temperature, load current, VCC, or what?

For example, when it said the V(ol) ranging from 0.15-0.4V so when we must expect the 0.15V and when we expect the 0.4V? Can we do something to force the value we want?

+1 Clemens Ladisch 7 months ago in reply to Mohammad Medhat

Guru 271610 points

CMOS outputs behave like resistors, i.e., with 1 mA, the ADS1115's maximum V_OL is about 0.13 V. You can afford to go to about 2.2 mA. (See [FAQ] What is the output voltage (VOH or VOL) when the output current is X or the supply voltage is Y?)

The typical value of 0.15 V is measured at 25 °C and with a typical device. 0.4 V is the worst case at the most extreme temperature and taking into account manufacturing variations. In practice, the only reliable way to reduce V_OL is to reduce the current.

0 Mohammad Medhat 7 months ago in reply to Clemens Ladisch

Prodigy 50 points

So, if the 0.4V is the worst case at 3 mA then we can expect also the 0.133V is the worst case at 1 mA.

Excellent. I understand now.

Thanks for the explanation and the good FAQ.

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TCA4307: Low output voltage of PCA9544A compatibility with TCA4307