Hello,
I am using an UCD9244 to generate power for a TMS320C6678 DSP. For the core voltage a UCD74120 is used, for 1V, 1.5V and 1.8V UCD74106s are used. The input voltage is 5V for all the converters, the 3.3V for UCD9244 is generated by a separate power module (and also used elsewhere on the board).
I set the switching frequency to 500kHz, but when I measure any switch node I never get more than 450kHz and this frequency is depending on the load currents of power stages, under heavy load conditions it drops down to 380kHz. I measured 3.3V, it remains stable, but the internal 1.8V measured on BPCAP pin of UCD9244 is not (1.7V when the system is unloaded, and drops down to 1.6V when loaded). I also noticed that the temperature readings of the controller are depending on the load currents, even the internal temperature of the controller goes up and down with the load currents (of course the current flowing through the power stages will heat them up, but not in a fraction of a second). It seems like there is a crosstalk between the input channels of the auxiliary ADC.
Voltages at the outputs are not even close to stable, loading the 1.5V rail with ~3A causes about 200mV drop and few mV increase at the other outputs, but UCD9244 displays far more than that, see attached figures.
What is going on here? Please help!
Thank you,
Gergely
Relcom Ltd.
~8A load step on CVDD rail (DSP disconnected):
~4A load step on 1.5V rail (note the internal temperature!):