BAT-

PowerSupplyground

bq2000groundandBAT-

SNS

Rf

Cf

1

2

bq2000

VSS

RSNS

MAX

SNS

0.05

I

=

R

CHYS

Hysteresis (V) = VCC

(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

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

state)

resume 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

1-2V 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 VMCV. During trickle maintenance charge,

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

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, RSNS, in series with the battery

pack. See Figure 9 for a typical current-sensing circuit.

Figure 9. Current-Sensing Circuit

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

(4)

If the voltage at the SNS pin is greater than VSNSLO or less than VSNSHI, the bq2000 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

bq2000 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 and 25 mV. The amount of hysteresis can be calculated by

examining the capacitive divider formed by CHYS and 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, RSNS value,

inductor value, hysteresis capacitor value (CHYS), and the value of the filter on the current sense signal (Rf and Cf

values). Rf and Cf 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

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