I think that TPS65251 want to use external synchronization.
TPS65251Datasheet ROSC mention is that If external synchronization is used resistor should be fitted and set to ～70% of external clock frequency.
Internal clock frequency is less than 70% of external clock frequency?
Will it be to say that 70% are good at 0% as follows?
I theoretically think that consistency does not match.
Regard.
Bob Lee
Bob,
What external frequency are you trying to sync to?
The TPS65251 will only sync to an external clock when two conditions are met:
1) The PGOOD signal is asserted.
2) The internal oscillator frequency is less than that of the external clock.
During startup the TPS65251 ignores the external clock and will startup to the internal oscillator frequency set by Rosc.
In the datasheet equation 1 needs frequency (f) in MHz
For example if your external clock is 800kHz, your Rosc should be 330kOhms so the device will start up with 560kHz then sync to the external clock of 800kHz once PGOOD is asserted. Refer to the electrical characteristics table in the datasheet for max and min switching frequency (Fsw_bk) and resistor size (Rosc or Rfsw).
I am looking to see what the minimum external clock can be. Based on the metric that the internal oscillator should be set at ~70% of the external sync clock it reads as though the minimum external clock can be 470kHz with a max Rosc of 600kOhms.
