I have the following TPS5450 circuit designed into a new product:
Design is for Vout of 5.6V at up to 5A over a Vin range of 8V to 30V, and SwitcherPro says it'll work fine. I've checked the datasheet equations too and have found nothing that would indicate that I might have a problem. But we just built up the first prototypes last week and I'm seeing some *very* strange behavior. The regulator works fine when Vin is between 8V & 10V, and when Vin is between 15V & 20V. But outside of those ranges I see two very distinct failures....
1. If Vin ever exceeds ~20V or so, the TPS5450 fails permanently with a hard short between its Vin and GND pins (measures just over 1ohm between Vin & GND after failure). This is irreversible, and only by replacing the TPS5450 chip can we get the regulator circuit to work again.
2. Whenever Vin is between 10V & 15V, the output jumps to 6.3V, and the waveform at the PH pin is like nothing I've seen. Here are scope shots of the regulator waveforms in the three different Vin operating ranges:
In all three scopeshots: The bottom (yellow) trace is the PH pin. The middle (purple) trace is Vout. The top (green) trace is Vin.
The top & bottom scopeshots shows "as expected" behavior with Vin at around 10V & 17V respectively. The middle scopeshot shows erroneous behavior with Vin at around 12V. I'm dumbfounded by this behavior. The TPS5450 regulator should have a fixed frequency variable duty-cycle behavior across its entire operating range. Why is it behaving so strangely at voltages between 10V & 15V here???? Note that these plots are taken with an 8ohm load connected between Vout and GND, but I've tried other loads and I always see this odd behavior whenever Vin is between 10V & 15V.
So.... WTF is going on here?? I've always had good experiences with other TI switchers I've used, and I've never seen anything at all like this behavior. We seem to have two completely independent failure modes here, and need a solution to both. We're already behind schedule for this product development, and need a solution FAST. Would very much appreciate any assistance.
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