BQ2002C Datasheet, PDF (5/12 Pages) Texas Instruments – NiCd/NiMH Fast-Charge Management IC

bq2002C
PVD and -∆V Termination
There are two modes for voltage termination, depend-
ing on the state of TM. For -∆V (TM = high), if VBAT is
lower than any previously measured value by 12mV
±3mV, fast charge is terminated. For PVD (TM = low or
mid), a decrease of 2.5mV ±2.5mV terminates fast
charge. The PVD and -∆V tests are valid in the range
1V < VBAT < 2V.
Synchronized Voltage Sampling
Voltage sampling at the BAT pin for PVD and -∆V termi-
nation may be synchronized to an external stimulus
using the INH input. Low-high-low input pulses
between 100ns and 3.5ms in width must be applied at
the INH pin with a frequency greater than the “maxi-
mum synchronized sampling period” set by the state of
the TM pin as shown in Table 1. Voltage is sampled on
the falling edge of such pulses. If the time between
pulses is greater than the synchronizing period, voltage
sampling “free-runs” at once every 17 seconds. A sample
is taken by averaging together voltage measurements
taken 57µs apart. The IC takes 32 measurements in
PVD mode and 16 measurements in -∆V mode. The
resulting sample periods (9.17 and 18.18ms, respec-
tively) filter out harmonics centered around 55 and
109Hz. This technique minimizes the effect of any AC
line ripple that may feed through the power supply from
either 50 or 60Hz AC sources. If the INH input remains
high for more than 12ms, the voltage sample history
kept by the IC and used for PVD and -∆V termination
decisions is erased and a new history is started. Such a
reset is required when transitioning from free-running
to synchronized voltage sampling. The response of the
IC to pulses less than 100ns in width or between 3.5ms
and 12ms is indeterminate. The tolerance on all timing
is ± 12%.
Voltage Termination Hold-off
A hold-off period occurs at the start of fast charging.
During the hold-off time, the PVD and -∆V terminations
are disabled. This avoids premature termination on the
voltage spikes sometimes produced by older batteries
when fast-charge current is first applied. Maximum
voltage and temperature terminations are not affected
by the hold-off period.
Maximum Voltage, Temperature, and Time
Any time the voltage on the BAT pin exceeds the maxi-
mum cell voltage, VMCV, fast charge is terminated.
Maximum temperature termination occurs anytime the
voltage on the TS pin falls below the temperature cut-off
threshold VTCO, where
VTCO = 0.5 ∗ VCC ± 5%.
Maximum charge time is configured using the TM pin.
Time settings are available for corresponding charge
rates of C/2, 1C, and 2C. Maximum time-out termina-
tion is enforced on the fast-charge phase, then reset, and
enforced again on the top-off phase, if selected. There is
no time limit on the trickle-charge phase.
Pulse-Trickle Charge
Pulse-trickle is used to compensate for self-discharge
while the battery is idle in the charger. The battery is
pulse-trickle charged by driving the CC pin active once
per second for the period specified in Table 1. This
results in a trickle rate of C/32.
TM Pin
The TM pin is a three-level pin used to select the
charge timer, top-off, voltage termination mode, trickle
rate, and voltage hold-off period options. Table 1
describes the states selected by the TM pin. The
mid-level selection input is developed by a resistor
divider between VCC and ground that fixes the volt-
age on TM at VCC/2 ± 0.5V. See Figure 4.
Charge Status Indication
A fast charge in progress is uniquely indicated when the
LED pin goes low. In the charge pending state, the LED
pin is driven low for 500ms, then to high-Z for 500ms.
The LED pin is driven to the high-Z state for all other
conditions. Figure 2 outlines the state of the LED pin
during charge.
Charge Inhibit
Fast charge and top-off may be inhibited by using the
INH pin. When high, INH suspends all fast charge and
top-off activity and the internal charge timer. INH
freezes the current state of LED until inhibit is
removed. Temperature monitoring is not affected by the
INH pin. During charge inhibit, the bq2002C continues
to pulse-trickle charge the battery per the TM selection.
When INH returns low, charge control and the charge
timer resume from the point where INH became active.
Low-Power Mode
The IC enters a low-power state when VBAT is driven
above the power-down threshold (VPD) where
VPD = VCC - (1V ±0.5V)
Both the CC pin and the LED pin are driven to the
high-Z state. The operating current is reduced to less
than 1µA in this mode. When VBAT returns to a value
below VPD, the IC pulse-trickle charges until the next
new charge cycle begins.
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