BQ24747 Datasheet, PDF (27/36 Pages) Texas Instruments – SMBus-Controlled Level 2 Multi-Chemistry Battery Charger With Input Current Detect Comparator and Charge Enable Pin

bq24747
www.ti.com ............................................................................................................................................................................................. SLUS988 – OCTOBER 2009
REMOTE SENSE
The bq24747 has a dedicated remote sense pin, VFB, which allows the rejection of board resistance and
selector resistance. To fully utilize remote sensing, connect VFB directly to the battery interface through an
unshared battery-sense Kelvin trace, and place a 0.1-μF ceramic capacitor near the VFB pin to GND (see
Figure 1).
Remote Kelvin Sensing provides higher regulation accuracy, by eliminating parasitic voltage drops. Remote
sensing cancels the effect of impedance in series with the battery. This impedance normally causes the battery
charger to prematurely enter constant-voltage mode with reducing charge current.
INPUT CURRENT MEASUREMENT
Use VICM to monitor the system-input current sensed across CSSP and CSSN. The voltage at VICM is
proportional to the input current by the equation:
VICM = 20 × V_(CSSP-CSSN) = 20 × (I_in × R_sense)
where I_in is the input DC current supplied by the AC adapter, 20 is the gain, and R_sense is the input sense resistor.
VICM has a 0 to (VREF–100 mV) output voltage range. Leave VICM open if not used. Use a 100 pF (maximum)
ceramic capacitor.
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 75 mA of load current. The LDO powers the gate drivers of the n-channel MOSFETs. VDDP
has a minimum current limit of 90 mA. This allows the bq24747 to work with high gate charge (both high-side
and low-side) MOSFETs. Bypass VDDP to PGND with a 1-μF or greater ceramic capacitor.
AC ADAPTER DETECTION
The bq24747 includes a hysteretic comparator that detects the presence of an AC power adapter. When ACIN is
greater than 2.4 V, the open-drain ACOK output becomes high impedance. Connect a 10-kΩ pullup resistor
between the pull-up rail and ACOK. Use a resistive voltage-divider from the adapter’s output to the ACIN pin to
set the appropriate detection threshold. Select the resistive voltage-divider not to exceed the 7 V absolute
maximum rating of ACIN.
Copyright © 2009, Texas Instruments Incorporated
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