SLUAB05 April 2025 BQ25856-Q1

5 Application Example - Electrolytic Capacitor Backup

In the following scenario, there is a car battery powering a 1.4A system load. This system needs to maintain the 1.4A system load for 600ms at 5V. The backup power allows the car systems to safely shut down. The requirements are summarized in Table 6-4.

Table 5-1 Design Requirements

Parameter	Value
Input voltage	12V
System load	1.4A
Reverse voltage needed	5V
Load hold time needed	600ms
Total energy needed	4.9J

Because the energy needed is 4.9J, an electrolytic capacitor can be used for the backup capacitor. If board space is constrained, then the high voltage electrolytic capacitors can hold more energy compared to increasing the capacitance. 58V can be used for the capacitor. Equation 7 is used to calculate the needed capacitance.

Equation 7.

\frac{2 I_{L o a d} V_{L o a d} Δ t_{L o a d}}{{V_{C a p}}^{2} - 6.25 V} = C

The capacitance needed is 2,502µF. This number is rounded up to 3000µF. The total capacitance is 3160µF once the EVM output capacitors are included.

Table 5-2 Hardware Setup

Parameter	Value
Output capacitor	3160µF (3x1000µF +160µF)
Output voltage	58V
ACUV setting	6.2V

Table 5-3 Register Settings

Register	Description
REG0x14[0]=0	Sets EN_PRECHG=0 and sets the charger to skip the trickle charge and precharge modes. This sets the BQ25856-Q1 to only be in fast charge mode or taper charge mode.
REG0x14[3]=0	Sets EN_TERM = 0 and makes sure the charger doesn’t stop charging while VIN is good. This makes sure that the capacitor always has the max possible charge.
REG0x02=0x0A0	Sets ICHG_REG = 2000mA.
REG0x06=0x01E0	Sets IAC_DPM=15000mA. The input supply has a max current output of 15A. This makes sure the input doesn't crash.
REG0x19[1]=1	Sets EN_AUTO_REV=1 and this enables auto reverse mode.
REG0x0C=0x03E8	Sets VAC_REV = 5000mV and sets the voltage that the BQ25856-Q1 regulates VAC too while in reverse mode.
REG0x1E[5]=1	Sets SYSREV_UV=1. By default, reverse mode turns OFF if VAC is below 80% of the reverse voltage target. SYSREV_UV=1 fixes the undervoltage setting to 3.3V. This makes sure reverse mode turns ON even if VAC has dropped below the reverse voltage target.

Figure 5-1 shows a full cycle of charging and discharging. The BQ25856-Q1 takes around 100ms to charge the output capacitors from 0V to 60V at a charge current of 2A. For reference, channel 1 is the input source and is colored blue. Channel 2 is the capacitor voltage and is colored teal. Channel 3 is the system current. Channel 4 is the current out of the capacitor. Current into the capacitor is negative. When the input voltage drops out, the BQ25856-Q1 goes into reverse mode and the system load current stays constant.

Figure 5-1 BQ25856-Q1 Full Charging And Discharge Cycle

Figure 5-2 zooms in on the output capacitor charge speed.

Figure 5-2 BQ25856-Q1 Electrolytic Capacitor Charging Speed

Figure 5-3 zooms in on the capacitor discharge. The BQ25856-Q1 provides backup power to a 1.4A system load for 610ms.

Figure 5-3 BQ25856-Q1 Provides Backup Power with a 1.4A Load

Figure 5-4 shows how fast reverse mode activates. As soon as the VAC voltage crosses the ACUV threshold, the BQ25856-Q1 takes an average of 100µs to activate reverse mode and begin sourcing power. The exact time depends on the load current and voltages. In Figure 5-4 from the same circuit as above, the switchover time is about 50µs to transition from charging to discharging. For reference, Channel 1 is VAC and is colored blue. Channel 3 is switch node 2 and is colored magenta. Channel 4 is the current into the capacitors and is colored green. Current into the capacitors is assigned a negative value. The capacitor current goes from negative to positive as the BQ25856-Q1 goes from charging to discharging.

Figure 5-4 BQ25856-Q1 Auto Reverse Mode Response Time