I am developing an application with the LTC3785 that will use a single Lithium Cell as the power source (3V to 4.1V).
The LTC3785 has an onboard LDO regulator (4.35V) that provides power for the FET Gate Drive signals. The Block Digram in the Datasheet shows the input to the LDO is the VIN pin, so in my case, this will be the Lithium Cell and so the voltage available for the gate drive will be well below 4.35V, which could reduce efficiency.
Page 9 of the datasheet says that if desired, a Schotkky diode can be used to connect the Vout, which in my case will be 5V.
My question is; How should this diode be connected? Do I remove the VIN (pin 24) connection to the Cell and replace it with the cathode of the diode and then connect the anode of the diode to VOUT (pin 10)?
As VIN also powers the internal circuitry, wouldn't this prevent the controller from starting up? Perhaps a pair of diodes is required, with both cathodes attached to Vin and one anode to the cell and one anode to Vout?
connect vin to vin. connect diode anode to vout and cathode to vcc. the ldo will power-up from vin, and when vout is up it will be diode-ored in.
That makes sense - thank you. Most of the time Vin>Vout, so the diode will be reverse biased, but towards the end of discharge, when Vin<Vout, the diode will start to conduct and keep the LDO supplied with 5V-Vf which should be enough margin to maintain a regulated 4.35V output.