The bq25504 from Texas Instruments is a good candidate to become a milestone on the road to micro-power management and energy harvesting. A prominent feature of this IC is its ability to start up at a supply voltage as low as 330 mV typically, and 450 mV guaranteed. With an SMD inductor and a few capacitors and resistors, it forms a dc-dc converter with a high power efficiency that is unprecedented, especially in the ultralow-power region.
A possible explanation for this breakthrough in achieving an extremely low value of start-up voltage could be the use of an internal oscillator based on submicron-wide-channel FET transistors. It is known that the narrower the FET’s channel is, the lower its threshold value of gate-source voltage will be—down to a few hundred millivolts. You could assume that FETs in the internal oscillator of the bq25504 have a threshold voltage on the order of 200 mV.
The circuit in Figure 1 differs from those shown in the bq25504 data sheet in that it exploits a feature of the bq25504: The reference voltage at VBAT_OV (overvoltage) is internally multiplied by a factor of 3/2, as compared with the reference voltage at VBAT_UV (undervoltage). If limiting the battery voltage within an interval of VBAT_UV and (3/2)×VBAT_UV suits your application, you can connect the VBAT_UV and VBAT_OV inputs as shown in the figure and use a single resistor divider network for both, instead of two separate divider networks.
Thus, not only do you save two resistors, but you also can lower the value of the sum of resistances as RUV1+RUV2=5M without sacrificing the overall power efficiency of the circuit. As a by-product, you get a higher insensitivity to EMI at the VBAT_UV and VBAT_OV inputs.
Read more: DC-DC converter starts up and operates from a single photocell
