Datasheet

BAT-
PowerSupplyground
bq2000TgroundandBAT-
SNS
R
f
C
f
1
2
bq2000T
V
SS
R
SNS
MAX
SNS
0.05
I =
R
C
HYS
Hysteresis (V) = V
CC
(C + C )
HYS f
´
bq2000T
www.ti.com SLUS149D –MAY 1999–REVISED JANUARY 2010
Table 2. RC Pin Status (continued)
bq2000T CHARGE STATE TS PIN STATE RC PIN BEHAVIOR
V
TS
< V
LTF
Active
Fast charge (current and voltage regulation)
V
TS
> V
LTF
(in battery conditioning state) 1-V to 2-V DC level (timer is paused and
resumes when V
TS
< V
LTF
)
V
TS
> V
LTF
(in trickle maintenance charge 1-V to 2-V DC level (timer is paused and
state) resumes when V
TS
< V
LTF
)
Top-off charge V
LTF
> V
TS
> V
HTF
Active
Active (timer is still counting, even though
V
HTF
> V
TS
> V
TCO
charging is suspended)
Trickle maintenance charge (after fast charge) N/A 1-V to 2-V DC level
Charge complete N/A Active
Both top-off and trickle maintenance charge are terminated and the pack never receives any more charge (until a
charge initialization occurs) if the voltage on the BAT pin reaches V
MCV
. During trickle maintenance charge,
charging is suspended if V
TS
< V
HTF
. It resumes when V
TS
> V
HTF
. The bq2000T is designed to remain in trickle
maintenance charge forever (excluding the two faults just mentioned) in order to keep a Nickel pack full.
Charge Current Control
The bq2000T implements a hysteretic control loop that regulates the current being delivered to the battery pack
to a user programmable value that is set by the value of the R
SNS
resistor. A second, outer control loop reduces
the average current delivered to the pack in order to clamp the voltage at the BAT pin to a maximum of V
MCV
.
The bq2000T controls the MOD pin to regulate the current and voltage of the pack. The bq2000T monitors
charge current at the SNS input by sensing the voltage drop across a sense-resistor, R
SNS
, in series with the
battery pack. See Figure 8 for a typical current-sensing circuit.
Figure 8. Current-Sensing Circuit
R
SNS
is sized to provide the desired fast-charge current (I
MAX
).
(3)
If the voltage at the SNS pin is greater than V
SNSLO
or less than V
SNSHI
, the bq2000T switches the MOD output
high to pass charge current to the battery. When the SNS voltage is less than V
SNSLO
or greater than V
SNSHI
, the
bq2000T switches the MOD output low to shut off charging current to the battery. A hysteresis capacitor (C
HYS
) is
required between the C
MOD
pin and the SNS pin to add a healthy amount of hysteresis to the current sense
signal. Typical hysteresis values are between 5 mV and 25 mV. The amount of hysteresis can be calculated by
examining the capacitive divider formed by C
HYS
and C
f
.
(4)
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