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SMB135 View Datasheet(PDF) - Summit Microelectronics

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SMB135 Datasheet PDF : 34 Pages
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SMB135
APPLICATIONS INFORMATION (CONTINUED)
EN (ENABLE)
EN is a logic input pin (active low) for enabling/disabling
the device and/or restarting a charge cycle. When EN is
held at a high logic level, IN goes into a high
impedance state, the internal VDD regulator is powered
down, no communication can occur over the I2C bus,
and no charge cycles may proceed. EN must be held
low in order to get any functionality out of the device. If
unused, this pin should be tied to GND.
Charge Enable
The initiation of a charge cycle is controlled via the
state of the volatile command register (Register 31, bit
4) and Register 0F bit 7. Bit 0F[7] controls the polarity
of the command bit 31[4]; if 0F[7] is low, then a 0 at
31[4] will cause a charge cycle to start. Since the
volatile register always powers-up to all 0’s, then 0F[7]
determines whether a charge cycle may begin on
power up, or whether an I2C command must be issued
to initiate charging.
USB500/100
USB500/100 is a logic input that allows the user to
select a maximum fast charge current of 100mA or
500mA. When a logic high signal is applied on this pin,
the charge current level may be as high as 500mA.
When a logic low signal is applied on this input, the
charge current level is limited to 100mA. In all cases, a
lower register value setting will impose an overriding
current limit. When the USB500/100 input is not
used, this pin should be tied to VDDCAP or GND (do
not leave floating). The USB500/100 functionality can
also be controlled over the I2C interface (ignoring the
state of the pin), allowing for full software control of
charge current levels. This function is accomplished
via register 31 bit 3, when Register 7 bit 0 is
programmed high.
Automatic Battery Recharge
The SMB135 allows the battery to be automatically
recharged (topped off) when the battery voltage falls by
a value of VRECH (115mV typical) below the
programmed float voltage. Provided that the input
power supply is still present, charging remains enabled
and all the pre-qualification parameters are still met, a
new charging cycle will be initiated. This ensures that
the battery capacity remains high, without the need to
manually re-start a charging cycle. The automatic
battery recharging can be disabled if not required by
the application (Register 03h).
Safety Timers
The integrated safety timers provide protection in case
of a defective battery pack. The pre-charge timer starts
after the pre-qualification check is completed and
resets when the transition to the constant current mode
happens. At that point, the fast charge timer is initiated.
The fast charge timer expires and charge cycle is
terminated if the termination current level is not reached
within the pre-determined duration. Each safety timer
has three programmable timeout periods, which
eliminates the need for external timing capacitors and
allows for maximum design flexibility. In addition, each
timer can be disabled by the appropriate bit selection in
Register 05h.
Thermal Monitor
A temperature sensing I/O (THERM) is provided to
prevent excessive battery temperatures during
charging. The battery temperature is measured by
sensing the voltage between the THERM pin and
ground. The voltage is created by injecting a current
into the parallel combination of Negative Temperature
Coefficient (NTC) thermistor and a resistor. This
voltage is then compared to two predetermined
voltages representing the maximum and minimum
temperature settings of the battery. The purpose of the
resistor in parallel to the NTC thermistor is to linearize
the resistance of the thermistor. The table below,
shows the 1% resistor that should be placed in parallel
with the corresponding thermistor.
If the temperature limits are exceeded, battery charging
will be suspended until the temperature level has fallen
within the safe operating range. The over-temperature
limit is programmable from 30°C to 65°C, and the
under-temperature limit is programmable from 20°C to
15°C, each in 5°C increments using Register 04h. In
addition, the user can easily select the required bias
current, based on the value of the negative temperature
coefficient (NTC) thermistor located in the battery pack:
10k, 25k, 100k (Register 04h). Disabling the thermal
monitor is also possible by selecting the appropriate
bits in Register 04h.
As the temperature changes, the resistance of the
thermistor changes creating a voltage proportional to
temperature. The temperature coefficient or Beta (Β) of
the thermistor must be as close to 4400 as possible to
achieve the maximum temperature accuracy.
NTC THERMISTOR
RESISTANCE
10K
24.9K
25K
61.9K
100K
249K
Table: NTC values and associated parallel
resistances.
Summit Microelectronics, Inc
2106 3.1 11/4/2008
14
 

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