Datasheet

LTC1760
36
1760fa
APPLICATIONS INFORMATION
Figure 9.
100mV
–
+
5kΩ
CLP
DCIN
1760 F09
0.1μF
+
R
CL
*
C
IN
V
IN
CL1
AC ADAPTER
INPUT
*R
CL
=
100mV
ADAPTER CURRENT LIMIT
+
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:
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
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 1 and 2.
dV/dt
BAT1
= dV/dt
BAT2
From here we can simplify.
I
BAT1
/C
BAT1
= dV/dt = I
BAT2
/C
BAT2
I
BAT2
= I
BAT1
C
BAT2
/C
BAT1
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).
I
CHRG
= I
BAT1
+ I
BAT2
From here we solve for the actual current for each battery.
I
BAT2
= I
CHRG
C
BAT2
/(C
BAT1
+ C
BAT2
)
I
BAT1
= I
CHRG
C
BAT1
/(C
BAT1
+ C
BAT2
)
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.
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 auto-
matically adjust charging current to a level which avoids
overloading the wall adapter. This allows the product to
operate at the same time that batteries are being charged
without complex load management algorithms. Addition-
ally, 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 cur-
rent 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
CL
, connected between the
CLP and DCIN pins. When this voltage exceeds 100mV,
the amplifier will override programmed charging current
to limit adapter current to 100mV/R
CL
. A lowpass filter
formed by 5kΩ and 0.1μF is required to eliminate switch-
ing 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 mini-
mum wall adapter current rating. Subtract 5% for the input
current limit tolerance and use that current to determine
the resistor value.
R
CL
= 100mV/I
LIM
I
LIM
= Adapter Min Current
– (Adapter Min Current • 5%)