Part Number: TCA9800
We are building ultra low power devices and would like to isolate one 1-Wire device from the microcontroller. If enable pin is low (set through GPIO), there is current flow from VCCA to the microcontroller. According to datasheet Page no 5, Pull up Resistor for enable pin is 250Kohm. current flowing into microcontroller = (3.3/250 x 1000) = 13.2uA which is very high in our case. This is solved by making GPIO pin of microcontroller as input.
Adding pull up resistor on SDAA and SCLA has increased the current consumption to 650uA while enable pin is low. What could be the reason for this behaviour?
That is more current than I would expect. When the enable line is low, the input current to each SDAA/SCLA pin should be at most 10 uA (see "input leakage current" specification in Table 7.5 of the datasheet). What else do you have connected to these signals? Have you checked the expected leakage current into each of these components? (Or, does the current drop if the TCA9800 device is removed from the PCB?)
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In reply to Max Robertson:
Thanks for the reply. Yes, we have isolated the issue to be TCA9800. As mentioned in the datasheet there is no pull resistor on the B side. VCCB is powered through a load switch and powered off until there is no communication with a slave device. Here is the observation.
Device current consumption: 2uA
1) device current consumption without TCA9800 = 2uA
2) device current consumption with TCA9800 and EN pin(GPIO of uC) as input and load switch is off (VCCB ) = 2.2uA
3) device current consumption with TCA9800 and EN pin(GPIO of uC) as output low and load switch is off (VCCB) = 15.6uA.
4) device current consumption with TCA9800 and EN pin(GPIO of uC) as output low and load switch is on (VCCB) = 650uA.
In reply to Ashok R41:
Sorry for the delay. Are the currents that you listed the total currents (ICCA + ICCB) or just ICCA? If they are total currents, then are any other components enabled when the VCCB rail is switched on? In this condition where you are measuring 650 uA, what are the voltages observed on the I2C lines on each side of the TCA9800?
The currents listed in the previous reply was total current (ICCA + ICCB + other circuity), 650uA was measured at power supply. No other components are enabled when VCCB is switched on. The voltage at I2C lines are as follows.
Power supply = 3.3V
VCCA = 3.3V, SCLA = SCDA = 3.285V
VCCB = 0V, SCLB = SDAB = 250mV
Note that these are voltages when enable was low.
It's a bit odd that you are seeing SCLB/SDAB at 250mV when VccB is 0V. The B side current source is disabled so we shouldn't be seeing a voltage generated on B side unless it's floating around. The way you describe the set up, pulling ENable low or high should not matter if VccB is not powered because the device is held in under voltage lock out.
Do you have any series resistors on the SDA/SCL lines? Maybe we can check to see which direction the current is coming from as it seems we are seeing some unknown leakage if you are seeing 250mV.
In reply to BOBBY:
SCLB and SDAB are left open. No there are no series resistors on the SDA and SCL lines.
Just for testing purposes, could you ground the SDA/SCL pins of side B during this event?
I'm wondering if B side floating, is causing some kind of shoot through current.
Also, is there anything else on the SDAB/SCLB bus that is powered?
Have you had a chance to look further into Bobby's suggestions above?
We tried connecting one wire slave to the B side of the TCA9800, there was no change in the current consumption. We are thinking to use dual analog buffer for the slave and power it through load switch.
I wanted to revisit this again.
Our TCA9800 device was intended to be used in low voltage, low power applications (designed for it) so the amount of current consumption you are seeing seems oddly high.
Can you share a schematic of your set up for us?
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