Hi,
I am using MPQ8623 with 1.1 MHz CCM. What is the dropout voltage in this configuration? When Vout is set for 5V, and I supply Vin=5V (with EN at 4.5V), the buck starts and then immediately shuts down. Same with Vin at 6 and 7V. It starts up correctly only around 7.6V. When gradually reducing Vin (starting from 8V), it shuts down around 6.8V.
Worst-case UVP threshold specified in the datasheet (p.5) is 83% of Vref, which I take to mean that the shutdown will happen if Vout goes down to 4.15V (when the target is 5V). Given my observations, that would correspond to a very large dropout. Is it really the case or did I miss something?
Thanks,
Milek
To add to the above, I did some quick measurements and the dropout voltage on MP2225 appears to be around 0.3V, which is more in line with my expectations.
Poking at it a bit more, with 1.1 MHz CCM the highest duty cycle I was able to measure is about 0.80. That aligns well with the info from the datasheet, which says that the minimum mosfet OFF time is 180 ns. At 1.1 MHz single cycle is 909 ns. (909 - 180) / 909 = 0.80. So even if efficiency of everything is at 100%, the best you can get from 5V is 4V. And given that PGOOD is not going up unless 92.5% of target voltage is reached (4.62 V), well, it’s clear why it does not work.
Switching to 600 kHz CCM improves the duty cycle to 0.89, which is still not great, and still not sufficient to start the chip correctly.
Looks like MPQ8623 is a really bad fit for low-dropout applications.