The BQ2562x family of chargers use the Narrow VDC (NVDC) architecture where system and battery are separated by BATFET. The way VSYS is regulated depends on two key conditions: whether charging is enabled or disabled and whether VBAT is greater than or less than VSYSMIN, the minimum system voltage register setting.
The typical system voltage regulation spec, VSYS_REG_ACC, is provided in the datasheet for two sets of conditions. Both sets of conditions include the case where charging is disabled and there is no system current. They are differentiated by the level of VBAT in relation to VSYSMIN, and the parameter for which VSYS_REG_ACC is offset from. This is summarized below:
Charging Enabled
Initially, VBAT < VSYSMIN. Then, VBAT rises above VSYSMIN, fully turning on BATFET.
The difference between VBAT and VSYS is the VDS(on) of BATFET. Therefore, VSYS is regulated at VBAT + VDS(on).
VBAT is less than VSYSMIN.
VSYS is regulated around 230mV (typical) above VSYSMIN, and BATFET operates in linear mode (LDO mode).
Charging Disabled
VSYS is regulated 50mV (typical) above VBAT when VBAT > VSYSMIN
VSYS is regulated 230mV (typical) above VSYSMIN when VBAT < VSYSMIN
It is important to emphasize that this is only true when charging is disabled. When charging is enabled, BATFET becomes a factor and VSYS is regulated differently; the above conditions no longer apply. Thus, there are different conditions which determine how VSYS is regulated. Similarly to when charging is disabled, they still relate to the minimum system voltage.
No Adapter/Input Connected
The battery directly powers system through BATFET. Depending on VBAT, VSYS may not necessarily be maintained above VSYSMIN.
