BQ25895_16 Datasheet, PDF (21/67 Pages) Texas Instruments – Adjustable Voltage 3.1-A Boost Operation

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bq25895
SLUSC88A – MARCH 2015 – REVISED MAY 2016
When the charge current is reduced to zero, but the input source is still overloaded, the system voltage starts to
drop. Once the system voltage falls below the battery voltage, the device automatically enters the Supplement
Mode where the BATFET turns on and battery starts discharging so that the system is supported from both the
input source and battery.
During DPM mode, the status register bits VDPM_STAT (VINDPM) and/or IDPM_STAT (IINDPM) is/are set high.
Figure 11 shows the DPM response with 9V/1.2A adapter, 3.2-V battery, 2.8-A charge current and 3.4-V
minimum system voltage setting.
Voltage
VBUS
SYS
3.6V
3.4V
3.2V
BAT
3.18V
Current
4A
3.2A
2.8A
ISYS
ICHG
1.2A
IIN
1.0A
0.5A
-0.6A
DPM
DPM
Supplement
Figure 11. DPM Response
8.2.6.3 Supplement Mode
When the system voltage falls below the battery voltage, the BATFET turns on and the BATFET gate is
regulated the gate drive of BATFET so that the minimum BATFET VDS stays at 30 mV when the current is low.
This prevents oscillation from entering and exiting the Supplement Mode. As the discharge current increases, the
BATFET gate is regulated with a higher voltage to reduce R_DS(ON) until the BATFET is in full conduction. At this
point onwards, the BATFET VDS linearly increases with discharge current. Figure 12 shows the V-I curve of the
BATFET gate regulation operation. BATFET turns off to exit Supplement Mode when the battery is below battery
depletion threshold.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
5 10 15 20 25 30 35 40 45 50 55
V(BAT_SYS) (mV)
D010
Figure 12. BATFET V-I Curve
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