BQ24725A_17 Datasheet, PDF (17/47 Pages) Texas Instruments – 1-4 Cell Li+ Battery SMBus Charge Controller with N-Channel Power MOSFET Selector and Advanced Circuit Protection

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bq24725A
SLUSAL0A – SEPTEMBER 2011 – REVISED AUGUST 2014
Device Functional Modes (continued)
8.4.2 Adapter Over Voltage (ACOVP)
When the ACDET pin voltage is higher than 3.15V, it is considered as adapter over voltage. ACOK will be pulled
low, and charge will be disabled. ACFET will be turned off to disconnect the high voltage adapter to system
during ACOVP. BATFET will be turned on if turns on conditions are valid. See the System Power Selection
section for details.
When ACDET pin voltage falls below 3.15V and above 2.4V, it is considered as adapter voltage returns back to
normal voltage. ACOK will be pulled high by external pull up resistor. BATFET will be turned off and ACFET and
RBFET will be turned on to power the system from adapter. The charge can be resumed if enable charge
conditions are valid. See the Enable and Disable Charging section for details.
8.4.3 System Power Selection
The bq24725A automatically switches adapter or battery power to system. The battery is connected to system at
POR if battery exists. The battery is disconnected from system and the adapter is connected to system after
default 150ms delay (first time, the next time default is 1.3s and can be changed to 150ms) if ACOK goes HIGH.
An automatic break-before-make logic prevents shoot-through currents when the selectors switch.
The ACDRV drives a pair of common-source (CMSRC) n-channel power MOSFETs (ACFET and RBFET)
between adapter and ACP (see Figure 18 for details). The ACFET separates adapter from battery or system, and
provides a limited di/dt when plugging in adapter by controlling the ACFET turn-on time. Meanwhile it protects
adapter when system or battery is shorted. The RBFET provides negative input voltage protection and battery
discharge protection when adapter is shorted to ground, and minimizes system power dissipation with its low
R_DS(on) compared to a Schottky diode.
When the adapter is not present, ACDRV is pulled to CMSRC to keep ACFET and RBFET off, disconnecting
adapter from system. BATDRV stays at V_SRN + 6V to connect battery to system if all the following conditions are
valid:
• V_VCC > UVLO;
• V_SRN > UVLO;
• V_ACN < 200mV above V_SRN (ACN_SRN comparator);
Approximately 150ms (first time; the next time default is 1.3s and can be changed to 150ms) after the adapter is
detected (ACDET pin voltage between 2.4V and 3.15V), the system power source begins to switch from battery
to adapter if all the following conditions are valid:
• Not in LEARN mode or in LEARN mode and V_SRN is lower than battery depletion threshold;
• ACOK high
The gate drive voltage on ACFET and RBFET is V_CMSRC + 6V. If the ACFET/RBFET have been turned on for
20ms, and the voltage across gate and source is still less than 5.9V, ACFET and RBFET will be turned off. After
1.3s delay, it resumes turning on ACFET and RBFET. If such a failure is detected seven times within 90
seconds, ACFET/RBFET will be latched off and an adapter removal and system shut down is required to force
ACDET < 0.6V to reset the IC. After IC reset from latch off, ACFET/RBFET can be turned on again. After 90
seconds, the failure counter will be reset to zero to prevent latch off. With ACFET/RBFET off, charge is disabled.
To turn off ACFET/RBFET, one of the following conditions must be valid:
• In LEARN mode and V_SRN is above battery depletion threshold;
• ACOK low
To limit the in-rush current on ACDRV pin, CMSRC pin and BATDRV pin, a 4kΩ resistor is recommended on
each of the three pins.
To limit the adapter inrush current when ACFET is turned on to power system from adapter, the Cgs and Cgd
external capacitor of ACFET must be carefully selected. The larger the Cgs and Cgd capacitance, the slower turn
on of ACFET will be and less inrush current of adapter. However, if Cgs or Cgd is too large, the ACDRV-CMSRC
voltage may still go low after the 20ms turn on time window is expired. To make sure ACFET will not be turned
on when adapter is hot plugged in, the Cgs value should be 20 times or higher than Cgd. The most cost effective
way to reduce adapter in-rush current is to minimize system total capacitance.
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