Other Parts Discussed in Thread: BQ25756,
Tool/software:
Hi,
We are developing a battery backup system, required to provide 24V at 10A into a load for 1hr. An input supply of 24V 14A is used to power the load in normal operation with a pickoff to supply current to charge the battery. If the input supply fails the battery needs to automatically take over supplying the load voltage / current. The system does not use a microcontroller.
We were designing in a BQ25756 chip to use it in Standalone mode, but its operation in Reverse Mode (taking battery power and feeding back through the converter to drive out to the load) is not well defined.
We now plan to use a BQ25750 device, using the device Reverse mode buck-boost converter path to ensure a stable output voltage (24V). This is catered for in the device datasheet section 8.1 last half of the last sentence: '..or via reverse power flow, discharging the battery through the buck-boost converter to generate a programmable, regulated voltage on system which is above or below the battery voltage'.
During board programming we are planning on setting the following bits:
REG0x19:
REG_RST = 0
EN_REV = 1
EN_AUTO_REV = 1
REG0x0C:
VSYS_REV == 24000mV.
Figure 8.6 of the datasheet shows the operation of Auto reverse mode, but only for a VBAT greater than the required system output voltage SYS (value programmed by the VSYS_REV register), i.e. buck operation of the converter. The datasheet does not follow this up with an illustration of the operation of the converter if the battery is partally discharged and VBAT is less than the VSYS_REV value, i.e. boost mode.
The REG0x19 bit 1 EN_AUTO_REV description states: 'Auto Reverse Mode to regulate SYS when VBAT < VSYS_REV register:' This implies the device can act as a boost converter to maintain SYS at the required voltage.
Q1. Is my interpretation of the operation of the system correct? Can the device regulate the battery stack voltage (18V to 26V), both up and down as necessary to support a defined VSYS voltage?
Q2. Is the operation from charge to reverse mode, with no micro attached (device will be programmed during build - no I2C available during operation) completely automatic and seamless?
Q3. Will the device auto-recover and be able to continue its buck-boost reverse mode operation after a fault condition has cleared? If not, the device is, like the BQ25756, not actually standalone.
Regards
Andy
