7.4.3.1 Buck Converter Enabled (VSTOR > VBAT_UV, EN = LOW and VOUT_EN = HIGH )

The bq25570 buck converter is hysteretic, peak current, discontinuous mode buck converter as summarized in Step Down (Buck) Converter. It has two startup responses: 1) from the ship-mode state (EN transitions from high to low with VOUT_EN already high), and 2) from the standby state (VOUT_EN transitions from low to high). The startup response out of ship-mode has the longest time duration due to the internal circuitry being disabled. This response is shown in Figure 35. The startup time takes approximately 100 ms due to the internal Nano-Power management circuitry needing to first complete the 64 ms sample and hold cycle. 

Startup from the standby state is shown in Figure 37. This response is much faster due to the internal circuitry being pre-enabled.  The startup time from this state is entirely dependent on the size of the output capacitor. The larger the capacitor, the longer it will take to charge during startup. With COUT = 22 µF, the startup time is approximately 400 µs. The buck converter can startup into a pre-biased output voltage.

The buck converter is disabled when the voltage on VSTOR drops below the VBAT_UV condition. The buck converter continues to operate in pass (100% duty cycle) mode, passing the input voltage to the output, as long as VSTOR is greater than VBAT_UV and less than VOUT.