ZHCSQB8B July 2021 – October 2022 TPS38700-Q1
The TPS38700-Q1 is designed to give an accurate CLK32K output and it is used internally for setting the RTC time and alarms. TPS38700-Q1 is configured to be used with a 32.768 kHz crystal oscillator. To achieve a well-defined frequency of oscillations, all crystals oscillators are tuned at specific capacitive load such as 6.5 pF, 12 pF, or 20 pF (during manufacturing stage), which becomes a part of the crystal specification. The task of a designer is to design within the crystal's specifications to achieve the correct specified frequency.
For these crystal oscillators, the need for loading capacitors are required because the capacitive load is effectively split between the output and input capacitance in a typical Pierce Oscillator scheme. These capacitors are essentially connected in series with the crystal oscillator. Therefore, if a chosen crystal oscillator has a capacitive load that is specified for 12.5 pF load, then the need for two 12.5 pF capacitors are required for proper frequency output from the crystal oscillator.
The TPS38700-Q1 is configured to be used without the need of loading capacitors, as long as the 6.5 pF version of the external crystal oscillator is selected. External crystal oscillators will typically specify their internal capacitance such as 6.5 pF, 9 pF, 12.5 pF etc. If the external crystal oscillator has load capacitance specification requirement not equal to 6.5 pF, please contact the TI factory for an OTP (one time programming) configuration for the correct external capacitor loading.
The CLK32K signal can start as late as 50 ms from when the input voltage VDD exits out of UVLO. The accuracy of CLK32K is within ±100 ppm after one second of initial operation. If the frequency of CLK32K deviates for more than ±10%, a fault interrupt is asserted. The accuracy of the CLK32K will also depend on the choice of external crystal oscillator and its temperature rating.