how to use the adum3471 in dual supply mode

The Eval pcb datasheet advises the use of a 1:5 ratio transformer when designing dual output (+15/-15V) outputs.

With a 5V input, The transformer generates aprx 25V VPk in Open loop mode with a 5V input.

When i follwed the instructions in the eval unit. The output was +11 and -25V .

with no load.

At this point i noticed the LDO which powers the regulator ccts was no longer functioning. The chip was operating in Oloop mode.

I fitted a 1:3 ratio transformer and a new adum3471 and the cct worked

This is not good enough to provide more the 30mA of load apx 50 % of pmax.

My question is this

I understand that the 1:5 ratio is the correct value. The Oloop output is 25V or so and is to high for the LDO input. Will the supply wake up and acheive regulation before the LDO input Voltage Max is exceeded. Should i be able to follow the example in the datasheet without sacrificing more of these devices?. should this supply be loaded to stop the vreg input from exceeding its maximum value?  Any advice is very much apreciated

  • 0
    •  Analog Employees 
    on Sep 4, 2012 6:25 PM


    You are correct, the ADuM347x eval board manual advises the use of 1:5 ratio transformer for the +/-15V outputs.  This configuration should not be set with open loop, but by the feedback resistors to regulate the +15V output. It is important to have some load on the +15V output in order to regulate, since the +15v is also connected to the internal regulator input, and you are correct that 25V would be too high for the VREG pin which has an absolute maximum of +20V. The internal regulator provides the current for the side 2 digital isolator channels and the side 2 dc to dc  controller, so it is important to have at least a 10mA load on +15V to regulate its voltage.

    Regards, Brian Kennedy