LT3651-4.1_15 Datasheet, PDF (14/22 Pages) Linear Technology – Monolithic 4A Wide Input 1 Cell Li-Ion Battery Charger

LT3651-4.1/LT3651-4.2
APPLICATIONS INFORMATION
System Input Current Limit
The LT3651 contains a PowerPath^TM 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 sources 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_CL
=
50µA • R_ILIM
11.5 • R_CL
INPUT
SUPPLY
CLP
LT3651
CLN
R_CL
SYSTEM LOAD
V_IN
I_LIM
R_ILIM
365142 F03
Figure 3. Input Current Limit Configuration
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 time 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.
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.
3
INPUT CURRENT
2
1
CHARGE
CURRENT
(V_IN REFERRED)
0
0
1
2
365142 F04
SYSTEM LOAD CURRENT (A)
Figure 4. Input Current Limit for 4A Maximum Charger
and 6A System Current Limit
14
For more information www.linear.com/LT3651-4.1
365142ff