We are using ADuM2400 in device and recently we noticed a strange and undesired behavior at power on.
When the system starts, the V DD2=5V (output side supply of ADuM2400) is the first power supply which goes up.
About 100÷200ms later, the V DD1=3.3V goes up.
All inputs are kept low from system start to about 2 seconds (booting time of a microprocessor).
According to the Table 11 of the datasheet, if V E2 (Ouput enable) is left unconnected or High, the outputs go High till the V DD1=is unpowered and the V DD2 is powered, even if the inputs are Low. This is and undesired condition.
In order to have the outputs low when the inputs are low, the V E2 (Ouput enable) is kept low till the VDD1 goes up and a 10k pull-down is connected to each output of ADuM2400. To ensure that outputs are not wrongly set high at the start-up, a simple delay circuit keeps the V E2 low for about 6ms after Vdd1 goes up.
We noticed that, as soon as the VE2 goes high, the outputs go high too, even if the inputs are low. This is in violation of the true table in Table 11. I looks that the outputs start following the input level only after a High-to-low transition of the inputs.
Is there a required a power-up sequence to have true table of Table 11 respected?
hi Andrea C:
When we say VDD1 unpowered, it means VDD1 does not reach the UVLO point which is around 2V. please check the timing between VE2 goes high and VDD1 reaches UVLO. if VE2 goes high happens before VDD1 beyond UVLO. A H output could be observed.