LTC4110_15 Datasheet, PDF (40/52 Pages) Linear Technology – Battery Backup System Manager

LTC4110
APPLICATIONS INFORMATION
where
V_CUTOFF = adjusted cutoff threshold voltage
V_CAL/V_DIS = voltage on V_CAL or V_DIS pin
V_BGR =1.220V
The resistor divider connected to V_REF pin will affect timer.
See the Programming Charge Time with TIMER and V_REF
Pins section for more details.
PROGRAMMING CHARGE TIME WITH TIMER AND
V_REF PINS
Charge time limits for Li-Ion batteries can be programmed
by selection of capacitance on the TIMER pin, but is
dependent upon resistance on the V_REF pin. Typical pro-
grammed bulk charge times range from 2 to 12 hours
and is set as follows:
C _{T IM E R} (F)
=
(944
T(Hrs)
• R_VREF (Ω))
As an example if R_VREF = 113k and the desired bulk charge
time limit is five hours then C_TIMER = 47nF. See F_TMR
which directly affects the 944 constant in the Electrical
Characteristics Table for the tolerance.
Avoid capacitors with high leakage currents. The V_REF
pin load resistor range is 49k to 125k or 10μA to 25μA of
load current. At 125k the maximum capacitance on V_REF
is limited to a maximum of 50pF to maintain sufficient AC
stability for the internal amplifier. At 49k the maximum is
125pF. The maximum capacitance is inversely proportional
to the resistance.
The voltage (V_REF) on the V_REF pin can be used as a
precision voltage for other uses with some limitations.
The total V_REF pin current must not exceed 25μA and the
capacitance must be limited as discussed above. Load
current fluctuations will modulate the programmed charge
time. In shutdown mode V_REF will drop to 0V.
In some applications a divided down V_REF voltage is needed
to program the SELA, SELC, TYPE, V_CHG, V_CAL and V_DIS
pins. This is easily implemented by use of a resistor divider
connected from V_REF to GND that sets the V_REF pin current
instead of a single resistor.
40
If the TYPE pin is set for SLA/LEAD ACID or any nickel
based smart battery, the TIMER pin is not used. You can
ground the TIMER pin. Furthermore, if there is no need
of any timer function and there is no need of any voltage
divider from V_REF to ground, you must still keep a load on
the V_REF pin between 10μA and 25μA. It is recommended
you place a 49.9k load resistor from V_REF to ground.
CHARGING BATTERIES OVER 12 HOURS
In situations where required bulk charge time cycle will
exceed the 12 hour time limit imposed by the charge TIMER
pin, you have two options. You can have an SMBus host
clear the CHG_FLT bit and force start another charge cycle
or you can switch to a smart version of the same battery.
If you chose the former, reduce the TIMER pin time to
about 2/3 of the actual time required. This will result in
faster termination in the second cycle and with autorestart
cycles when V_AR is tripped. If you choose the smart bat-
tery option, the smart battery itself safely controls charge
termination. Bulk charge can last as long as necessary
to charge the battery to 100%. No host is required to do
anything, as the battery will maintain its full charge state
using its SMBus charge commands.
PROGRAMMING AC PRESENT INDICATION DELAY
TIME WITH ACPDLY AND V_REF PINS
When the main supply, DCIN, returns after a power failure
the ACPb pin is driven low to indicate presence of main
power. This transition can be delayed to allow time for the
system to stabilize before actions are taken by the system
based on this pin status. The high to low transition only
delay on the ACPb pin can be programmed by selection
of capacitance on the ACPDLY pin, but is dependent upon
resistance on the V_REF pin. Typical programmed delay times
range from 10ms to 200ms and is set as follows:
C_ACPDLY (F)
=
2
•
T(s)
R_VREF (Ω)
As an example if R_VREF = 113k and the desired delay time
is 105ms then C_TIMER = 470nF. See t_AC in the Electrical
Characteristics Table for the tolerance.
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