I am working on a design using the TPS24750 to control the 12V rail for a PCI3 slot in a system. The design component values used are shown in the attached spreadsheet. During system testing, the in-rush current and voltage during start-up of a particular PCIe adapter card was observed and the constant power engine does not appear to be working as expected.
The attached scope picture shows the Imon voltage in green and the Vout voltage in yellow. The Imon voltage peak of 0.6V shows that the current thru the TPS24750 to peaks at 9.04A (0.6V * 15.061 A/V).
During the time the current is ramping up, the voltage on the output is also rising. When I calculate the power dissipated by the internal FET by looking at the voltage across the FET and the current, it appears that the constant power engine is not limiting the power level as expected. The design spreadsheet shows an expected power limit of 11.876 W. At the peak of the current, the output voltage is at 3V and with a 12V input, that would be 9 volts across the FET. The associated power level is 9V *9A = 81W.
Also, the current ramp does not match the expected exponential type curve as shown in the TPS24750 datasheet in Figures 31 and 32.
Can you help me understand what is happening here and why the power limiting does not seem to be working?
