LTC2918 with Vcc between 5V and 7V

The LTC2918 supports operation between 1.5 V and 5.5 V directly, but also incorporates a shunt regulator to allow operation at higher voltages. The "Shunt Regulator" section on Page 12 of the datasheet describes how to choose a series resistor to current limit the part for operation about 7V. By my reading, the lower bound of the resistor value is set to guarantee that the part is able to receive 5 mA of current, and the upper bound of the resistor value is set to guarantee that there's always at least 250 uA of current available to keep the shunt regulator in regulation.

I am hoping to use this part in an application where Vcc ranges from 4 V to 7 V. Limiting current to 5 mA is straightforward (R >= 260 Ohm). However, because the voltage range goes below the maximum regulation voltage of the shunt resistor, there's no way to guarantee at least 250 uA across all conditions.

What is the guidance on how to use the LTC2918 with input voltages that span the 5.7 V - 7 V range? I suspect that a 270 ohm resistor is unlikely to actually cause problems in this application, but by a strict reading of the datasheet this would violate the suggestion of always having 250 uA across the shunt. Suggestions?

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  • Your final assessment is correct. Since the part nominally only uses 30uA. The drop across a 270 Ohm resistor is somewhat negligible. Only 8mV. As long as there is a reasonable decoupling cap to absorb transient currents the part should be fine.

    The lower limit on the resistor is to ensure you never exceed 5mA into Vcc.