Datasheet

Not Recommended for New Designs
bq24725
SLUS702A –JULY 2010–REVISED NOVEMBER 2010 www.ti.com
Continuous Conduction Mode (CCM)
With sufficient charge current the bq24725’s inductor current never crosses zero, which is defined as continuous
conduction mode. The controller starts a new cycle with ramp coming up from 200mV. As long as EAO voltage is
above the ramp voltage, the high-side MOSFET (HSFET) stays on. When the ramp voltage exceeds EAO
voltage, HSFET turns off and low-side MOSFET (LSFET) turns on. At the end of the cycle, ramp gets reset and
LSFET turns off, ready for the next cycle. There is always break-before-make logic during transition to prevent
cross-conduction and shoot-through. During the dead time when both MOSFETs are off, the body-diode of the
low-side power MOSFET conducts the inductor current.
During CCM mode, the inductor current is always flowing and creates a fixed two-pole system. Having the
LSFET turn-on keeps the power dissipation low, and allows safely charging at high currents.
Discontinuous Conduction Mode (DCM)
During the HSFET off time when LSFET is on, the inductor current decreases. If the current goes to zero, the
converter enters Discontinuous Conduction Mode. Every cycle, when the voltage across SRP and SRN falls
below 5mV (0.5A on 10mΩ), the under current-protection comparator (UCP) turns off LSFET to avoid negative
inductor current, which may boost the system via the body diode of HSFET.
During the DCM mode the loop response automatically changes. It changes to a single pole system and the pole
is proportional to the load current.
Both CCM and DCM are synchronous operation with LSFET turn-on every clock cycle. If the average charge
current goes below 125mA on 10mΩ current sensing resistor or the battery voltage falls below 2.5V, the LSFET
keeps turn-off. The battery charger operates in non-synchronous mode and the current flows through the LSFET
body diode. During non-synchronous operation, the LSFET turns on only for a refreshing pulse to charge the
BTST capacitor. If the average charge current goes above 250mA on 10mΩ current sensing resistor, the LSFET
exits non-synchronous mode and enters synchronous mode to reduce LSFET power loss.
Input Over Current Protection (ACOC)
The bq24725 cannot maintain the input current level if the charge current has been already reduced to zero.
After the system current continues increasing to the 1.66X of input current DAC set point (with 2.5ms blank out
time), ACFET/RBFET is turned off and charge is disabled for 1.3s and will soft start again for charge if the ACOC
condition goes away. If such a failure is detected seven times in 90 seconds, the ACFET and RBFET latch off
and an adapter removal and system shutdown is required to force ACDET < 0.6V to reset 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.
The ACOC function can be disabled or the threshold can be set to 1.33X, 1.66X or 2.22X of input DPM current
via SMBus command (ChargeOption() bit [2:1]).
Charge Over Current Protection (CHGOCP)
The bq24725 has a cycle-by-cycle peak over current protection. It monitors the voltage across SRP and SRN,
and prevents the current from exceeding of the threshold based on the DAC charge current set point. The high-
side gate drive turns off for the rest of the cycle when the over current is detected, and resumes when the next
cycle starts.
The charge OCP threshold is automatically set to 6A, 9A, and 12A on a 10mΩ current sensing resistor based on
charge current register value. This prevents the threshold to be too high which is not safe or too low which can
be triggered in normal operation. Proper inductance should be selected to prevent OCP triggered in normal
operation due to high inductor current ripple.
Battery Over Voltage Protection (BATOVP)
The bq24725 will not allow the high-side and low-side FET to turn-on when the battery voltage at SRN exceeds
104% of the regulation voltage set-point. If BATOVP last over 30ms, charger is completely disabled. This allows
quick response to an over voltage condition – such as occurs when the load is removed or the battery is
disconnected. A 4mA current sink from SRN to GND is on only during BATOVP and allows discharging the
stored output inductor energy that is transferred to the output capacitors.
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