how does this circuit works?

[Danesh_SA]

Hi All,
Im on a isolated digital output circuit,i found this circuit.could you expain me how it works? what does BAV70 do here?

[erikka]

See equivalent circuit of each driver
The diodes are to clamp the output between GND and Vcc

[Danesh_SA]

I design 4 channel digital output for industrial system ,I want to use ULN2803 instead of TBD62783 in my design.
but don't understand the usage of BAV70 here,this is DIGITAL OUTPUT ,why in this design use CLAMP diodes?

[awallin]

if you have inductive loads (relays) they might give enough 'kick' to damage the driver, without protecting diodes?

[PKTKS]

(..)
but don't understand the usage of BAV70 here,this is DIGITAL OUTPUT ,why in this design use CLAMP diodes?

ESD or worst spikes

[rstofer]

To prevent the signal lines from going above Vcc by more than a diode drop.  This protects the chip inputs (or outputs, in this case).  It doesn't matter WHY the signal line goes above VCC (ESD, whatever), it is necessary to protect the chip.

Given that each output is also fused, we can assume it is intended to protect the device from miswiring.  Inductive kickback is handled inside the driver chip for the case where the output goes below ground.  These external diodes clamp when the output is somehow pulled above Vcc.

[Danesh_SA]

These external diodes clamp when the output is somehow pulled above Vcc.

Thank you for detail answer.now i find out how it works,but still can't understand how BAV70 works in output mode!
I think it prevent damege to chip when we use it as INPUT ,which is wrong and these diodes to prevent from failing.
I have this ideas but I don't know do I think in right way?!