BQ24765_15 Datasheet, PDF (21/43 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery Charger With Integrated Power MOSFETs

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bq24765
SLUS999A – NOVEMBER 2009 – REVISED NOVEMBER 2015
8.3.22 Remote Sense
The bq24765 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 AGND.
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.
8.4 Device Functional Modes
8.4.1 Continuous Conduction Mode and Discontinuous Conduction Mode
In Continuous Conduction Mode (CCM), the inductor current always flows to charge battery, and the charger
always operates in synchronized mode. At the beginning of each clock cycle, high-side n-channel power
MOSFET turns on, and the turn-on time is set by the voltage on the EAO pin. After the high-side power MOSFET
turns off, the low-side n-channel power MOSFET turns on. During CCM, the low-side n-channel power MOSFET
stays on until the end of the clock cycle. The internal gate drive logic ensures there is break-before-make
switching to prevent shoot-through currents. During the 25 ns dead time where both FETs are off, the back-diode
of the low-side power MOSFET conducts the inductor current. Having the low-side FET turn-on keeps the power
dissipation low, and allows safely charging at high currents. With type III compensation, the loop has a fixed 2-
pole system.
As the ripple valley current gets close to zero, charger operation goes to non-synchronized mode. During non-
synchronous operation, after the high-side n-channel power MOSFET turns off, and after the break-before-make
dead-time, the low-side n-channel power MOSFET will turn-on 40 ns. After the 40 ns blank out time is over, if
V(CSOP-CSON) voltage falls below UCP threshold (typical 10 mV), the low-side power MOSFET will turn-off and
stay off until the beginning of the next cycle, where the high-side power MOSFET is turned on again. After the
low-side MOSFET turns off, the inductor current flows through back-gate diode until it reaches zero. The
negative inductor current is blocked by the diode, and the inductor current will become discontinuous. This mode
is called Discontinuous Conduction Mode (DCM).
During the DCM mode the loop response automatically changes and has a single pole system at which the pole
is proportional to the load current, because the converter does not sink current, and only the load provides a
current sink. This means at very low currents the loop response is slower, as there is less sinking current
available to discharge the output voltage. At very low currents during non-synchronous operation, there may be a
small amount of negative inductor current during the 40 ns recharge pulse. The charge should be low enough to
be absorbed by the input capacitance.
Whenever the converter goes into zero percent duty-cycle, the high-side MOSFET does not turn on, and the low-
side MOSFET does not turn on (no 40 ns recharge pulse) either, and there is no discharge from the battery;
unless the BOOT to PHASE voltage discharges below 4 V. In that case, it pulses once to recharge the boot-strap
capacitor.
8.5 Programming
8.5.1 Battery-Charger Commands
The bq24765 supports five battery-charger commands that use either Write-Word or Read-Word protocols, as
summarized in Table 2. ManufacturerID() and DeviceID() can be used to identify the bq24765. On the bq24765,
the ManufacturerID() command always returns 0x0040 and the DeviceID() command always returns 0x0006.
REGISTER
ADDRESS
0x14
0x15
0x3F
0xFE
0xFF
REGISTER NAME
ChargeCurrent()
ChargeVoltage()
InputCurrent()
ManufacturerID()
DeviceID()
Table 2. Battery Charger SMBus Registers
READ/WRITE
Read or Write
Read or Write
Read or Write
Read Only
Read Only
DESCRIPTION
6-Bit Charge Current Setting
11-Bit Charge Voltage Setting
6-Bit Input Current Setting
Manufacturer ID
Device ID
POR
STATE
0x0000
0x0000
0x0080
0x0040
0x0007
POR
VOLTAGE/CURRENT
0mA
0mV
256mA (10mΩ RAC)
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