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Notecarrier A Series Solar JST Input

The JST input powers the on-board Texas Instruments bq24210 Li-Ion Battery Solar Charger IC through an ON Semiconductor FSV1045V Schottky rectifier .

Section 7 of the charger datasheet states that the Absolute Maximum Ratings of the VBUS input are -0.3V and 20V. However, these are merely the limits of the device beyond which damage will occur and do not inform the Blues customer recommendations below.

Section 7.3 of the charger datasheet gives a VBUS Voltage Range of 3.5V - 18V, and a VBUS Operating Voltage Range of 3.5 to 7V. Accounting for the forward voltage drop across the rectifier, these ranges will be used to recommend the absolute maximums and operating range of the Notecarrier SOLAR input.

The ELECTRICAL CHARACTERISTICS table of the rectifier reports the Forward Voltage Drop under various conditions. Conservatively ignoring the conditions and just picking the minimum and maximum values gives a range of 0.18V - 0.44V.

Combining this information in the most conservative way gives the follow values for our SOLAR input:

  • VSOLAR Absolute Maximum Ratings
    • Minimum: 0V
    • Maximum: 18V + 0.18V = 18.18V
    • This is the allowable voltage range, although the battery is unlikely to charge outside of the Operating Range calculated below.
  • VSOLAR Charging Range
    • Minimum: 3.5V + 0.44V = 3.94V
    • Maximum: 7V + 0.18V = 7.18V
    • This is the voltage range which will keep the charger in the domain where it is capable of charging a battery.

But Will It Charge?

Section 10 of the charger datasheet (Power Supply Recommendations) reads in its entirety:

In order for the IC to charge a battery, the power source at VBUS must be larger than the undervoltage lockout threshold of 3.3 V typical and the battery voltage, V(BAT) plus 200 mV typical to prevent SLEEP mode but less than the OVP threshold of 7.5 V typical. The input power source can be a low impedance USB port or wall adapter or a high impedance solar panel if the VBUS_DPM feature if appropriately configured.

Our recommended Operating Range avoids under-voltage lockout since we specify at least 3.5V and the maximum UVLO level is 3.45V per Table 7.5 (Electrical Characteristics).

Our recommended Operating Range avoids the over-voltage protection threshold since we specify no more that 7V and the minimum OVP threshold is 7.3V per Table 7.5 (Electrical Characteristics).

Table 7.5 (Electrical Characteristics) states that the Input Power Good threshold is 250mV +/- 50mV above the present battery level. Accounting for the typical drop across the rectifier at 25°C, the charger is expected to enter Charge Mode whenever the solar panel generates a voltage that is typically 0.25V + 0.28V = 0.53V above VBAT. Again accounting for the rectifier, if the SOLAR input voltage rises above 7.3V + 0.28V = 7.58V (typical), the charger will go into over-voltage protection mode until the SOLAR input falls about 200mV below the level at which it entered OVP - typically around 7.38V.

What About Airnote?

For reference, our Airnote solar panel has a maximum open-circuit voltage of 6.22V. Thus, it will never put the charger into over-voltage protection and will always charge its LiPo battery whenever it exceeds VBAT by 0.53V (typical).