BQ2004 datasheet(6/16 Pages) TI

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Top-off Charge

An optional top-off charge phase may be selected to

follow fast charge termination for the C/2 through 4C

rates. This phase may be necessary on NiMH or other

battery chemistries that have a tendency to terminate

charge prior to reaching full capacity. With top-off en-

abled, charging continues at a reduced rate after

fast-charge termination for a period of time equal to

the fast-charge safety time (See Table 1.) During top-

off, the MOD pin is enabled at a duty cycle of 260

µsac-

tive for every 1820

µs inactive. This modulation results

in an average rate 1/8th that of the fast charge rate.

Maximum voltage, time, and temperature are the only

termination methods enabled during top-off.

Pulse-Trickle Charge

Pulse-trickle charging follows the fast charge and op-

tional top-off charge phases to compensate for self-

discharge of the battery while it is idle in the charger.

The configured pulse-trickle rate is also applied in the

charge pending state to raise the voltage of an over-

discharged battery up to the minimum required before

fast charge can begin.

In the pulse-trickle mode, MOD is active for 260

µsofa

period specified by the settings of TM1 and TM2. See

Table 1. The resulting trickle-charge rate is C/64 when

top-off is enabled and C/32 when top-off is disabled.

Both pulse trickle and top-off may be disabled by tying

TM1 and TM2 to VSS.

Charge Status Indication

Charge status is indicated by the LED1 and LED2 out-

puts. The state of these outputs in the various charge cy-

cle phases is given in Table 2 and illustrated in Figure 2.

In all cases, if VCELL exceeds the voltage at the MCV

pin, both LED1 and LED2 outputs are held low regard-

less of other conditions. Both can be used to directly

drive an LED.

Charge Current Control

The bq2004 controls charge current through the MOD

output pin. The current control circuitry is designed to

support implementation of a constant-current switching

regulator or to gate an externally regulated current

source.

When used in switch mode configuration, the nominal

regulated current is:

IREG = 0.225V/RSNS

Charge current is monitored at the SNS input by the

voltage drop across a sense resistor, RSNS, between the

low side of the battery pack and ground. RSNS is sized to

provide the desired fast charge current.

If the voltage at the SNS pin is less than VSNSLO, the

MOD output is switched high to pass charge current to

the battery.

Corresponding

Fast-Charge

Rate

TM1

TM2

Typical

Fast-Charge Safety

Time (minutes)

Typical

PVD, -

∆V Hold-Off

Time (seconds)

Top-Off

Rate

Pulse-

Trickle

Rate

Pulse-

Trickle

Period (Hz)

C/4

Low

360

137

Disabled

C/2

Float

Low

180

820

Disabled

C/32

240

High

Low

410

Disabled

C/32

120

Low

Float

200

Disabled

C/32

Float

100

Disabled

C/32

C/2

High

Float

180

820

C/16

C/64

120

Low

High

410

C/8

C/64

Float

High

200

C/4

C/64

High

100

C/2

C/64

Note:

Typical conditions = 25°C, VCC = 5.0V.

Table 1. Fast-Charge Safety Time/Hold-Off/Top-Off Table

bq2004

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