ISOW7741: External Supply for VISO_IN?

Hi John,

Thanks for reaching out.

John Griffith said:

I have asked for more information on why they are trying to this, but my guess is that they want 5V on the isolated side but might also have 3.3V signals. So by separating these and using a 5.0V to 3.3V regulator they can get both rails with a simple LDO as opposed to a boost converter from 3.3V to 5.0V.

Yes, that possibly could be the case and ISOW7741 allows such usage where VISOOUT is 5V and VISOIN is 3.3V. Figure 10-1 in ISOW7741 includes an optional LDO drawn for this use-case.

John Griffith said:

If we leave pin 20 floating - can ISOW7741 be damaged?

The answer is possibly yes. Since the digital isolator part of device carries a feedback channel for the DC/DC converter, it needs to be powered on all the time VDD is powered so that the output VISOOUT is regulated and not exceeding device ratings.

John Griffith said:

We  supply 3.3V from external LDO to pin 20 - is there a power sequencing restriction between pin 12  (5V ) and pin 20 (3.3V)?

There is no specific power sequencing defined but the VISOIN needs to be supplied with valid voltage before VISOOUT reaches its final voltage that way the feedback channel powered, and takes controls of the duty cycle. If the external LDO that is powering VISOIN is powered by VISOOUT then there is no need for any additional sequencing. VISOIN will power-up much before (1.71V) the VISOOUT hits its final voltage (5V). Hence, this is not an issue. Thanks.

Regards,
Koteshwar Rao