LT3651-8.2/LT3651-8.4
14
36518284fa
For more information www.linear.com/LT3651-8.2
Acceptable power inductors are available from several
manufacturers such a Würth Elektronik, Vishay, Coilcraft
and TDK.
System Input Current Limit
The LT3651-8.2/LT3651-8.4 contain a PowerPath
control
feature to help manage supply load currents. The charger
adjusts charger output current in response to a system
load so as to maintain a constant input supply load. If
overall input supply current exceeds the programmed
maximum value the charge current is diminished in an
attempt to keep supply current constant. One application
where this is helpful is if you have a current limited input
supply. Setting the maximum input current limit below
the supply limit prevents supply collapse.
A resistor, R
CL
, is placed between the input supply and the
system and charger loads as shown in Figure 3.
The LT3651-8.2/LT3651-8.4 source 50µA from the I
LIM
pin,
so a voltage is developed by simply connecting a resistor
to ground. The voltage on the I
LIM
pin corresponds to
11.5 times the maximum voltage across the input sense
resistor (R
CL
). Input current limit is defined by:
I
INPUT(MAX)
=
V
ILIM
11.5 •R
=
50µA •R
ILIM
11.5 •R
The programming range for I
LIM
is 0V to 1V. Voltages higher
than 1V have no effect on the maximum input current. The
default maximum sense voltage is 95mV and is obtained
if R
ILIM
is greater than 20k or if the pin is left open.
For example, say you want a maximum input current of
2A and the charger is designed for 4A maximum average
charge current, which is 1A V
IN
referred (4A times duty
cycle). Using the full I
LIM
range, the maximum voltage
across R
CL
is 95mV. So R
CL
is set at 95mV/2A = 48mΩ.
When the system load exceeds 1A (= 2A – 1A) charge
current is reduced such that the total input current stays
at 2A. When the system load is 2A the charge current is 0.
This feature only controls charge current so if the system
load exceeds the maximum limit and no other limitation
is designed, the input current exceeds the maximum
desired, though the charge current reduces to 0A. When
the input limiter reduces charge current it does not impact
the internal system timer if used. See Figure 4.
APPLICATIONS INFORMATION
Figure 3. Input Current Limit Configuration
Figure 4. Input Current Limit for 4A Maximum Charger
and 6A System Current Limit
CLP
CLN
SUPPLY
V
IN
R
LIM
R
CL
LT3651-8.2
LT3651-8.4
I
LIM
INPUT CURRENT
CHARGE
CURRENT
(V
IN
REFERRED)
2
2
365142 F04
1
10
0
3
If reduced voltage overhead or better efficiency is required
then reduce the maximum voltage across R
CL
. So for
instance, a 10k R
ILIM
sets the maximum R
CL
voltage to
43mV. This reduction comes at the expense of slightly
increased limit variation.
Note the LT3651-8.2/LT3651-8.4 internally integrate the
input limit signals. This should normally provide sufficient
filtering and reduce the sensitivity to current spikes. For
the best accuracy take care to provide good Kelvin con
-
nections from R
CL
to CLP, CLN.
