BAT-

PowerSupplyground

bq2000TgroundandBAT-

SNS

Rf

Cf

1

2

bq2000T

VSS

RSNS

MAX

SNS

0.05

I

=

R

CHYS

Hysteresis (V) = VCC

(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

VTS < VLTF

Active

Fast charge (current and voltage regulation)

VTS > VLTF (in battery conditioning state)

1-V to 2-V DC level (timer is paused and

resumes when VTS < VLTF)

VTS > VLTF (in trickle maintenance charge 1-V to 2-V DC level (timer is paused and

state)

resumes when VTS < VLTF)

Top-off charge

VLTF > VTS > VHTF

Active

Active (timer is still counting, even though

VHTF > VTS > VTCO

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 VMCV. During trickle maintenance charge,

charging is suspended if VTS < VHTF. It resumes when VTS > VHTF. 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 RSNS 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 VMCV.

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, RSNS, in series with the

battery pack. See Figure 8 for a typical current-sensing circuit.

Figure 8. Current-Sensing Circuit

RSNS is sized to provide the desired fast-charge current (IMAX).

(3)

If the voltage at the SNS pin is greater than VSNSLO or less than VSNSHI, the bq2000T switches the MOD output

high to pass charge current to the battery. When the SNS voltage is less than VSNSLO or greater than VSNSHI, the

bq2000T switches the MOD output low to shut off charging current to the battery. A hysteresis capacitor (CHYS) is

required between the CMOD 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 CHYS and Cf.

(4)

Copyright © 1999–2010, Texas Instruments Incorporated

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