Datasheet

BAT-
PowerSupplyground
bq2000groundandBAT-
SNS
R
f
C
f
1
2
bq2000
V
SS
R
SNS
MAX
SNS
0.05
I =
R
C
HYS
Hysteresis (V) = V
CC
(C + C )
HYS f
´
bq2000
SLUS138D –JANUARY 2008–REVISED DECEMBER 2009 www.ti.com
Table 2. RC Pin Status (continued)
bq2000 CHARGE STATE TS PIN STATE RC PIN BEHAVIOR
V
TS
> V
LTF
(in trickle maintenance charge 1-2V DC level (timer is paused and will
state) resume 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 1-2V 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 bq2000 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 bq2000 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
bq2000 modulates the MOD pin to regulate the current and voltage of the pack. The bq2000 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 9 for a typical current-sensing circuit.
Figure 9. Current-Sensing Circuit
R
SNS
is sized to provide the desired fast-charge current (I
MAX
).
(4)
If the voltage at the SNS pin is greater than V
SNSLO
or less than V
SNSHI
, the bq2000 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
bq2000 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 and 25 mV. The amount of hysteresis can be calculated by
examining the capacitive divider formed by C
HYS
and C
f
.
(5)
Being a hysteretic controller, the switching frequency of the bq2000 is determined by the values of several of the
external circuit components. The components that affect the switching frequency are: input voltage, R
SNS
value,
inductor value, hysteresis capacitor value (C
HYS
), and the value of the filter on the current sense signal (R
f
and C
f
values). R
f
and C
f
have the most impact on the switching frequency and are also the components that are
easiest to change to adjust the frequency, as they do not affect anything else in the circuit (besides, of course,
the cleanliness and quality of the current sense signal being fed to the bq2000). In general, increasing the input
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