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LM25148: External Sync

Part Number: LM25148


A customer would like to use this device as it has an external sync feature.

They customer would like to externally sync a clock to 500kHz.

In the datasheet(page 24- 8.4.2) it is written that the LM25148 can be synchronized to ±20% of the programmed frequency up to a maximum of 2.5 MHz.

Could you please explain the meaning of this? By him using an external 500kHz clock, would there be a possible error of ±20% ?

In page 7 (PULSE FREQUENCY MODULATION Table), the test condition of the frequency range, an  RRT of 9.53 kΩ is used. Is there a reason for having this resistor ?

What is the reason for it being labeled FSYNCIN3 ?

Thank you, 


  • Hi VL,

    the 20% is referencing the programmed RT resistance, if your RT configures 500kHz then you can sync in ~400-600kHz. Sync outside of the 20% range may not synchronize as intended and could lead to instability.

    The 20% is not frequency error.

    We used 9.53kΩ test RT as 2.1-2.2MHz is a common switching frequency.

    There were other test frequencies but this was the one included in the datasheet.

    Hope this helps,