ltc1760 Linear Technology Corporation, ltc1760 Datasheet - Page 33

• Manufacturer Part Number: ltc1760
• Description: Dual Smart Battery System Manager
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Automatic Current Sharing
In a dual parallel charge configuration, the LTC1760 does
not actually control the current flowing into each individual
battery. The capacity, or Amp-Hour rating, of each battery
determines how the charger current is shared. This auto-
matic steering of current is what allows both batteries to
reach their full capacity points at the same time. In other
words, given all other things equal, charge termination will
happen simultaneously.
A battery can be modeled as a huge capacitor and hence
governed by the same laws.
I = C • (dV/dt) where:
The equivalent model of a set or parallel batteries is a set
of parallel capacitors. Since they are in parallel, the change
in voltage over change in time is the same for both batteries
one and two.
From here we can simplify.
At this point you can see that the current divides as the ratio
of the two batteries capacity ratings. The sum of the
current into both batteries is the same as the current being
supply by the charger. This is independent of the mode of
the charger (CC or CV).
From here we solve for the actual current for each battery.
Please note that the actual observed current sharing will
vary from manufacturer’s claimed capacity ratings since it
is actual physical capacity rating at the time of charge.
I = The current flowing through the capacitor
C = Capacity rating of battery (using amp-hour values
instead of capacitance)
dV = Change in voltage
dt = Change in time
dV/dt
I
I
I
I
I
BAT1
BAT2
CHRG
BAT2
BAT1
/C
BAT1
= I
= I
= I
= I
BAT1
BAT1
CHRG
CHRG
BAT1
= dV/dt
= dV/dt = I
C
C
C
+ I
BAT2
BAT2
BAT1
BAT2
U
BAT2
/C
/(C
/(C
BAT1
BAT1
BAT1
BAT2
U
/C
+ C
+ C
BAT2
BAT2
BAT2
W
)
)
U
Capacity rating will change with age and use and hence the
current sharing ratios can change over time.
Adapter Limiting
An important feature of the LTC1760 is the ability to
automatically adjust charging current to a level which
avoids overloading the wall adapter. This allows the prod-
uct to operate at the same time that batteries are being
charged without complex load management algorithms.
Additionally, batteries will automatically be charged at the
maximum possible rate of which the adapter is capable.
This feature is created by sensing total adapter output
current and adjusting charging current downward if a
preset adapter current limit is exceeded. True analog
control is used, with closed loop feedback ensuring that
adapter load current remains within limits. Amplifier CL1
in Figure 9 senses the voltage across R
between the CLP and DCIN pins. When this voltage ex-
ceeds 100mV, the amplifier will override programmed
charging current to limit adapter current to 100mV/R
lowpass filter formed by 5kΩ and 0.1µF is required to
eliminate switching noise. If the current limit is not used,
CLP should be connected to DCIN.
Setting Input Current Limit
To set the input current limit, you need to know the
minimum wall adapter current rating. Subtract 5% for the
input current limit tolerance and use that current to deter-
mine the resistor value.
*R
R
I
LIM
CL
CL
=
= 100mV/I
ADAPTER CURRENT LIMIT
= Adapter Min Current
– (Adapter Min Current • 5%)
CL1
100mV
+
–
100mV
LIM
+
Figure 9.
CLP
DCIN
+
0.1µF
C
IN
R
LTC1760
CL
*
5kΩ
CL
, connected
AC ADAPTER
33
INPUT
CL
1760 F09
V
1760f
. A
IN