LTC1960 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1960' PDF : 28 Pages View PDF
LTC1960
APPLICATIONS INFORMATION
Automatic Current Sharing
In a dual parallel charge configuration, the LTC1960 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 value
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
dV
=
dtBAT1 dtBAT2
From here we can simplify.
IBAT1/CBAT1 = dV/dt = IBAT2/CBAT2
IBAT2 = IBAT1 CBAT2/CBAT1
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).
ICHRG = IBAT1 + IBAT2
From here we solve for the actual current for each battery.
IBAT2 = ICHRG CBAT2/(CBAT1 + CBAT2)
IBAT1 = ICHRG CBAT1/(CBAT1 + CBAT2)
Please note that the actual observed current sharing will
vary from manufactures 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.
In dual charge mode, the charger uses feedback from the
BAT2 input to determine charger output voltage. When
charging batteries with significantly different initial states of
charge (i.e., one almost full, the other almost depleted), the
full battery will get a much lower current. This will cause a
voltage difference across the charge MUX switches, which
may cause the BAT1 voltage to exceed the programmed
voltage. Using MOSFETs in the charge MUX with lower
RDS(ON) will alleviate this problem.
Adapter Limiting
An important feature of the LTC1960 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
8 senses the voltage across RCL, 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/RCL. 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.
100m+V
CLP
CL1
0.1µF
5kΩ
DCIN
+
*RCL
=
100mV
ADAPTER CURRENT
LIMIT
RCL*
CIN
AC ADAPTER
INPUT
VIN
11960 F08
Figure 8. Adapter Current Limiting
1960fb
19
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