BQ25015 Datasheet, PDF (14/28 Pages) Texas Instruments – SINGLE-CHIP CHARGER AND DC/DC CONVERTER IC FOR PORTABLE APPLICATIONS

bq25015
bq25017
SLUS721A – DECEMBER 2006 – REVISED MARCH 2007
www.ti.com
Thermal Shutdown and Protection
The bq25015/7 monitors the junction temperature, T_J, of the die and suspends charging if T_J exceeds T_(SHTDWN).
Charging resumes when T_J falls below T_(SHTDWN) by approximately 15°C.
TImer Fault Recovery
As shown in Figure 14, bq25015/7 provides a recovery method to deal with timer fault conditions. The following
summarizes this method:
Condition 1: Charge voltage above recharge threshold (V_(RCH)) and timeout fault occurs.
Recovery method: bq25015/7 waits for the battery voltage to fall below the recharge threshold. This could
happen as a result of a load on the battery, self-discharge or battery removal. Once the battery falls below the
recharge threshold, the bq25015/7 clears the fault and starts a new charge cycle. A POR or CE toggle also
clears the fault.
Condition 2: Charge voltage below recharge threshold (V_(RCH)) and timeout fault occurs.
Recovery method: Under this scenario, the bq25015/7 applies the I_(FAULT) current. This small current is used to
detect a battery removal condition and remains on as long as the battery voltage stays below the recharge
threshold. If the battery voltage goes above the recharge threshold, then the bq25015/7 disables the I_(FAULT)
current and executes the recovery method described for Condition 1. Once the battery falls below the recharge
threshold, the bq25015/7 clears the fault and starts a new charge cycle. A POR or CE toggle also clears the
fault.
DC-DC CONVERTER
The bq25015/7 provides a low quiescent-current synchronous DC-DC converter. The internally compensated
converter is designed to operate over the entire voltage range of a single-cell Li-Ion or Li-Pol battery. Under
nominal load current, the device operates with a fixed PWM switching frequency of typically 1 MHz. At light load
currents, the device enters the power save mode of operation; the switching frequency is reduced and the
quiescent current drawn by the converter from the BAT/OUT pin is typically only 15 µA.
During PWM operation the converter uses a unique fast-response voltage mode controller scheme with input
voltage feedforward to achieve good line and load regulation allowing the use of small ceramic input and output
capacitors. At the beginning of each clock cycle initiated by the clock signal (S), the P-channnel MOSFET switch
is turned on and the inductor current ramps up until the comparator trips and the control logic turns off the
switch. The current limit comparator also turns off the switch in case the current limit of the P-channel switch is
exceeded. After the dead time preventing current shoot through the N-channnel MOSFET rectifier is turned on
and the inductor current ramps down. The next cycle is initiated by the clock signal again turning off the
N-channel rectifier and turning on the on the P-channel switch. The g_M amplifier as well as the input voltage
determines the rise time of the saw-tooth generator and therefore any change in input voltage or output voltage
directly controls the duty cycle of the converter giving a very good line and load transient regulation.
Power Save Mode Operation
As the load current decreases the converter enters the power save mode operation. During power save mode
the converter operates with reduced switching frequency in PFM mode and with a minimum quiescent current to
maintain high efficiency.
Two conditions allow the converter to enter the power save mode operation. One is the detection of
discontinuous conduction mode. The other is when the peak switch current in the P-channel switch goes below
the skip current limit. The typical skip current limit can be calculated as:
I_SKIP
+
66
mA
)
V_IN
160 W
(5)
During the power save mode the output voltage is monitored with the comparator by the thresholds comp low
and comp high. As the output voltage falls below the comp low threshold (set to typically 0.8% above VOUT
nominal) the P-channel switch turns on. The P-channel switch is turned off as the peak switch current is
reached. The typical peak switch current can be calculated as:
I_PEAK
+
66
mA
)
V_IN
80 W
(6)
14
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