7462.SCHEMATIC1 _ POWER DC TO DC.pdfIn
attached schematics you can see how we had implemented two step down power supplies using TPS54260DGQ.
These supplies provide 4.2VDC and 5.0VDC.
The problem we have is as follows:
When the input voltage (NET 12V_PS_IN) goes below 8.1VDC, the power supplies stop working properly.
You can assume that pin#3 (EN) of each is above 1.5VDC
Please let me know if there is a solution, that will allow generation of the 5VCC supply and the 4V2 supply,
When the input (NET 12V_PS_IN) goes below 8.1VDC, as much as possible.
We need to operate this circuit with voltages as low as 6.0VDC
Does the operation look similar to what is shown in Figure 3 of the following app note? http://www.ti.com/lit/an/slva444/slva444.pdf
This shows a sawtooth shape waveform with an amplitude of ~2.1V on the output.
If so this is due to the BOOT UVLO being tripped at light loads. Let me know and I can share you some solutions in addition to the one presented in the app note I linked above.
DC/DC Power Applications Engineer
It could be a UVLO issue.Can you please send me your swolutions?
Assuming it is due to the boot UVLO following is a description of what is happening. In the non synchronous buck the bootstrap capacitor is charged only when the SW node transitions to ground. When there is a load on the output the inductor current turns on the diode when the high side switch turns on pulling SW to ground. If you have no load on the output the diode doesn't conduct, the SW does not get pulled to ground and the bootstrap capacitor is not charged. If there isn't enough load the diode does not conduct long enough, the bootstrap capacitor is not charged enough each cycle and will eventually trigger the BOOT UVLO.
The application note I linked previously presents a solution to this. Another would be to add a dummy load on the output to ensure the low side diode conducts long enough to charge the bootstrap capacitor. The size of the dummy load would need to be selected empirically. If you look at Figure 26 in the datasheet it gives somewhat of an idea the load needed.
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