BQ2954_13 Datasheet, PDF (9/23 Pages) Texas Instruments – Lithium Ion Charge Management IC

bq2954
minimum current threshold, I_MIN. The I_FULL and I_MIN
thresholds are programmed using the I_TERM input pin
(See Table 4.)
Figures 4 and 5 show the bq2954 configured for display
mode 2 and I_FULL = I_MAX/5 while I_MIN = I_MAX/10.
Voltage and Current Monitoring
Battery insertion is detected within 500ms. Transition
to the fast-charge phase, however, will not occur for time
t_HO (approximately one second), even if voltage qualifi-
cation VMIN is reached. This delay prevents a voltage
spike at the BAT input from causing premature entry
into the fast-charge phase. It also creates a delay in
detection of battery removal if the battery is removed
during this hold-off period.
In low-side current sensing, the bq2954 monitors the
battery pack voltage as a differential voltage between
BAT and pins. In high-side current sensing, the bq2954
monitors the battery pack voltage as a differential volt-
age between BAT and V_SS pins. This voltage is derived
by scaling the battery voltage with a voltage divider.
(See Figures 6 and 7.) The resistance of the voltage di-
vider must be high enough to minimize battery drain
but low enough to minimize noise susceptibility. RB1 +
RB2 is typically between 150kΩ and 1MΩ. The volt-
age-divider resistors are calculated from the following:
RB1 ₌ N ∗ V_{CELL − 1}
(1)
RB2
VREG
where
VCELL = Manufacturer-specified charging cell voltage
N = Number of cells in series
VREG = 2.05V
The current sense resistor, R_SNS (see Figures 6 and 7),
determines the fast-charge current. The value of R_SNS
is given by the following:
R _SNS
=
0.25V
I _MAX
(2)
where IMAX is the current during the constant-current
phase of the charge cycle. (See Table 1.)
Battery Insertion and Removal
V_BAT is interpreted by the bq2954 to detect the presence
or absence of a battery. The bq2954 determines that a
battery is present when V_BAT is between the
High-Voltage Cutoff (V_HCO = V_REG + 0.25V) and
the Low-Voltage Cutoff (V_LCO = 0.8V). When V_BAT is
outside this range, the bq2954 determines that no battery
is present and transitions to the battery test state, testing
for valid battery voltage. The bq2954 detects battery re-
moval when V_BAT falls below V_LCO. The BTST pin is
driven high during battery test and can activate an exter-
nal battery contact pull-up. This pull-up may be used to
activate an over-discharged Li-Ion battery pack. The V_HCO
limit implicitly serves as an over-voltage charge fault. The
CHG output can be used to disconnect capacitors from the
regulation circuitry in order to quickly detect a battery-re-
moved condition.
Temperature Monitoring
Temperature is measured as a differential voltage be-
tween TS and BAT-. This voltage is typically generated
by a NTC (negative temperature coefficient) thermistor
and thermistor linearization network. The bq2954 com-
pares this voltage to its internal threshold voltages to
determine if charging is allowed. These thresholds are
the following:
I High-Temperature Cutoff Voltage: VTCO = 0.4 ∗ VCC
This voltage corresponds to the maximum temperature
(TCO) at which charging is allowed.
I High-Temperature Fault Voltage: VHTF = 0.44 ∗ VCC
This voltage corresponds to the temperature (HTF) at
which charging resumes after exceeding TCO.
I Low-Temperature Fault Voltage: VLTF = 0.6 ∗ VCC
This voltage corresponds to the minimum temperature
(LTF) at which charging is allowed.
Charging is inhibited if the temperature is outside the
LTF—TCO window. Once the temperature exceeds
TCO, it must drop below HTF before charging resumes.
RT1 and RT2 for the thermistor linearization network
are determined as follows:
0.6
∗
V_CC
=
1
+
RT1
∗
V
(RT2
+
R _LTF )
(3)
(RT2 ∗ R _LTF )
0.44 =
1
(4)
1 + ^{RT1 ∗ (RT2 + R HTF )}
(RT2 ∗ R _HTF )
where
RLTF = thermistor resistance at LTF
RHTF = thermistor resistance at HTF
V = VCC - 0.250 in low-side current sensing
V = VCC in high-side current sensing
TCO is determined by the values of RT1 and RT2. 1%
resistors are recommended.
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