BQ24745_14 Datasheet, PDF (26/39 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery

bq24745
SLUS761D – DECEMBER 2007 – REVISED OCTOBER 2011
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HIGH-ACCURACY VICM USING CURRENT-SENSE AMPLIFIER (CSA)
An industry standard, high-accuracy current-sense amplifier (CSA) is used to monitor the input current by the
host or some discrete logic through the analog voltage output of the VICM pin. The CSA amplifies the input
sensed voltage of CSSP-CSSN by 20× through the VICM pin. The VICM output is a voltage source 20 times the
input differential voltage. Once DCIN is above 4.5 V and ACIN is above 0.6 V, VICM no longer stays at ground,
but becomes active. A user wanting to lower the voltage could use a resistor divider from VICM to GND and still
achieve accuracy over temperature.
A 100-pF capacitor connected on the output is recommended for decoupling high-frequency noise.
VDDSMB INPUT SUPPLY
The VDDSMB input provides bias power to the SMBus interface logic. Connect VDDSMB to an external 3.3-V or
5-V supply rail. SMBus communication can start between host and charger when the VDDSMB voltage is above
2.5 V and the VREF voltage is at 3.3 V. Bypass VDDSMB to GND with a 0.1-µF or greater ceramic capacitor.
INPUT UNDERVOLTAGE LOCKOUT (UVLO)
The system must have a minimum 4.5-V DCIN voltage to allow proper operation. When the DCIN voltage is
below 4 V, VREF LDO stays inactive, even with ACIN above 0.6 V. VREF turns on when DCIN > 4.5 V and ACIN
> 0.6 V. To enable VDDP requires DCIN > 4.5 V, ACIN > 2.4 V, and CE = HIGH.
VDDP GATE DRIVE REGULATOR
An integrated low-dropout (LDO) linear regulator provides a 6-V supply derived from DCIN for high efficiency,
and delivers over 90 mA of load current. The LDO powers the gate drivers of the n-channel switching MOSFETs.
Bypass VDDP to PGND with a 1-µF or greater ceramic capacitor. During thermal shutdown, the VDDP LDO is
disabled.
INPUT CURRENT COMPARATOR TRIP DETECTION
In order to optimize the system performance, the host monitors the adapter current. Once the adapter current is
above a threshold set via ICREF, the ICOUT pin sends a signal to the HOST. The signal alarms the host that
input power has exceeded the programmed limit, allowing the host to throttle back system power by reducing
clock frequency, lowering rail voltages, or disabling certain parts of the system. The ICOUT pin is an open-drain
output. Connect a pullup resistor to ICOUT. The output is logic HI when the VICM output voltage (VICM = 20 ×
V_CSSP-CSSN) is lower than the ICREF input voltage. The ICREF threshold is set by an external resistor divider
using VREF. The hysteresis can be programmed by a positive feedback resistor from the ICOUT pin to the
ICREF pin.
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