AD7293 EVAL BOARD

Hi,

I have a few queries.

1. I have been working with the on board FETs(on daughter board) for the closed loop and everything was working fine.

Then, I connected an external FET CMPA0060002F1,  which has VDD=28V and IDQ=100mA . I changed the RSENSE3 resistor to 10ohms(to get a max of 20mA drain current) and it was also working fine in the closed loop. 

Then I tried the external power supply of 5V, 200mA and 10V, 100mA through vpp3 of the eval board (since I was working with closed loop3) and it worked well, until I changed the supply to 20V, 50mA. The gate voltage rose to something in mA range and now closed loop3 doesn't work  even for the onboard FETs. I checked that only upto 10mA drain current.

Fortunately, the FET is not dead and I tried closedloop2 which works fine with 5V and 10V external through VPP2.  I was wondering if there anything I should take into consideration before checking the board with 20V or above. Is there something on  the path which get destroyed by 20V or larger voltage? What is the max voltage that can be applied through VPP0-VPP3? What should I be careful with if I work with closed loop0 or closed loop1 with PA_ON and external power supply greater than 20 V? Are the currents from the supply high enough to destroy the current sensors?

2. Also, I think the board have AVSS issue. AVSS is only -4.0V.  Say I set VCLAMP1 =  1.77V with PU1 push button. So the power up gate voltage is supposed to be -3VCLAMP = -5V, but what I get is -4V, which is AVSS. The max negative voltage GATE can go is AVSS which makes sense. But why is  AVSS -4V only?  How can I get that to be ~-5V.?

This is my 2nd eval board and my first board always has the gate voltage of -2.9V on power up.  AVSS voltage was around -3.3V and what I got as power up gate voltage was -2.9V.  The IC ADP3605ARZ was also not damaged and I checked the voltage across the resistors R82 and R18, and it was -3.3V. I tried to apply external -5V, but something was drawing more current and AVSS was still -3.3V. I removed U5 and  U6 and tried again, but it didnt solve the issue. What could have gone wrong with AVSS ?