We have utilized the LTC6909 device in some of our radio products that have been in the field in excess of 12 years, in 24/7/365 operation. We have recently had field returns where the failures seen to stem from the LTC6909. The major symptom is that the output clock frequency has shifted down from the expected 600 KHz to a significantly lower frequency, as low as 220 KHz in at least one case. In addition, the spread-spectrum aspect of the clock output, which is enabled in our design, is no longer working. When we observe the resulting clock outputs on a spectrum analyzer we see a discrete line at one frequency, vs. when we compare with a good, working unit and the spread-spectrum aspect of the clock is clearly visible.
We utilize these clock outputs to synchronize some switching regulator power supplies (buck converter topology), so the shift in the operating frequency has impacted the stability and output ripple from those regulators, which supply DC power to an Altera FPGA on our main processing board. This can lead to all sorts of subtle problems.....but the root cause appears to be the LTC6909 device. We use the Industrial grade version of the part which is specified for operation from -40C to +85C.
It seems clear that some analog circuitry internal to the device has shifted. The power supply input to the device remains at the nominal voltage = V+A = V+D = +5.2V. The R-set value is unchanged at approximately 41.9K, and we have all three phase control pins pulled high (i.e. PH = 8) so we would have an expected clock frequency = 597 KHz. We have over 25,000 of these radios deployed in South Korea for over 12 years, and we now have at least 25 units returned with this problem. The amount of the frequency shift to lower output frequencies varies, but the loss of the spread-spectrum mode seems consistent.
Moved from Power Management to Clock and Timing on Friday, November 10, 2023 11:48:09 PM by MattGorder