BQ2000_15 Datasheet, PDF (12/29 Pages) Texas Instruments – Programmable Multi-Chemistry Fast-Charge Management IC

bq2000
SLUS138D – JANUARY 2008 – REVISED DECEMBER 2009
www.ti.com
Table 2. RC Pin Status (continued)
bq2000 CHARGE STATE
Top-off charge
Trickle maintenance charge (after fast charge)
Charge complete
TS PIN STATE
RC PIN BEHAVIOR
V_TS > V_LTF (in trickle maintenance charge
state)
V_LTF > V_TS > V_HTF
V_HTF > V_TS > V_TCO
1-2V DC level (timer is paused and will
resume when V_TS < V_LTF)
Active
Active (timer is still counting, even though
charging is suspended)
N/A
1-2V DC level
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.
BAT-
R_f
R_SNS
1 _SNS
Cf
2 _VSS
Power Supply ground
bq2000 ground and BAT-
bq2000
Figure 9. Current-Sensing Circuit
R_SNS is sized to provide the desired fast-charge current (I_MAX).
0.05
I_MAX
=
R_SNS
(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.
Hysteresis (V) = VCC
´
CHYS
(CHYS + Cf )
(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
12
Submit Documentation Feedback
Product Folder Link(s): bq2000
Copyright © 2008–2009, Texas Instruments Incorporated