Hi kuda,
The LM96551 ultrasound HV pulser supply sequence should be started with the VSUB being the most negative potential.
The recommended sequence of power-up & power voltages is (This sequence is required to prevent the part getting blowed up):
The power voltages are based on the Absolute Max. Ratings in the datasheet.
1. VSUB = -65V (it can be as high as -55V with the condition being VPP not being more than +50V)
2. VNN = -50V
3. VLL = 3V
5. VDN & VDD (you can choose to power up VDN before VDD) = -10V & +10V (respectively)
6. Last VPP = +50V
Similarly the power down sequence should be the reverse order:
1. VPP
2. VDD & VDN
3. VLL
4. VNN
5. Last VSUB
The collateral for the reference designs will be made available when you buy the boards at http://www.ti.com/tool/tx-sdk-v1.
Thanks,
Rohit