TPS62912: Synchronize the TPS6291x by a source which is powered by the TPS6291x's output voltage.

Dear Sir, 

You can refer to datasheet 7.3.2 and 7.3.3 for the details about how to configure EN/SYNC.

The device is enabled by pulling the EN/SYNC pin high, and has an accurate rising threshold voltage of typically 1.01 V. Once the device is enabled, the operation mode is set by the configuration of the S-CONF pin. This occurs during the device start-up delay time tdelay. Once tdelay expires, the internal soft-start circuitry ramps up the output voltage over the soft-start time set by the CNR/SS capacitor. The start-up delay time tdelay varies depending on the selected S-CONF value. It is shortest with smaller S-CONF resistors. The EN/SYNC pin has an active pulldown resistor RPD. This prevents an uncontrolled start-up of the device, in case the EN/SYNC pin cannot be driven to a low level. The pulldown resistor is disconnected after start-up. With EN set to a low level, the device enters shutdown and the pulldown resistor is activated again.

The EN/SYNC pin is also used for device synchronization. Once a clock signal is applied to this pin, the device is enabled and reads the configuration of the S-CONF pin. The external clock frequency must be within the clock synchronization frequency range set by the S-CONF pin. When the clock signal changes from a clock to a static high, then the device switches from external clock to internal clock. To shutdown the device when using an external clock, EN/SYNC must go low for at least 10 µs.

BRs

Lucia

Dear Lucia

Thank you very much for the quick answer.

Exact these chapters in the data sheet led me to the question.

Applying 12V needs the EN/SYNC pin to be high to start the TPS62912. But because the OCXO ist not supplied with +5V its output signal is 0. So, I need to enable the TPS62912 somehow with the 12V and later, when the clock signal from the OCXO is available, I have to switch over to the clock signal.

Sure, there are cumbersome ways to accomplish that. But I hope you have a smart solution to fulfill this desired behavior. Because if the EN and SYNC functionality were separated the design would be easy.