LT3651-4.1/LT3651-4.2
15
365142ff
For more information www.linear.com/LT3651-4.1
Note the LT3651 internally integrates the input limit
signals. This should normally provide sufficient filtering
and reduce the sensitivity to current spikes. For the best
accuracy take care in provide good Kelvin connections
from R
CL
to CLP, CLN.
Further flexibility is possible by dynamically altering the
I
LIM
pin. Different resistor values could be switched in
to create unique input limit conditions. The I
LIM
pin can
also be tied to a servo amplifier for other options. See the
information in the following section concerning I
RNG/SS
programming for examples.
RNG/SS: Dynamic Current Adjust
The RNG/SS pin gives the user the capability to adjust
maximum charge current dynamically. The part sources
50µA from the pin, so connecting a resistor to ground
develops a voltage. The voltage on the RNG/SS pin cor
-
responds to approximately ten times the maximum volt-
age across the charge current sense resistor, R
SENSE
. The
defining equations for charge current are:
I
MAX(RNG/SS)
=
RNG/SS
10.8 • R
=
RNG/SS
10.8 • R
I
MAX(RNG/SS)
is the maximum charge current.
The programming range for RNG/SS is 0V to 1V. Voltages
higher than 1V have no effect on the maximum charge
current. The default maximum sense voltage is 95mV
and is obtained if R
RNG/SS
is greater than 20k or if the
pin is left open.
For example, say you want to reduce the maximum charge
current to 50% of the maximum value. Set RNG/SS to 0.5V
(50% of 1V), imposing a 48mV maximum sense voltage.
Per the above equation, 0.5V on RNG/SS requires a 10k
resistor. If the charge current needs to be dynamically
adjustable then Figure 5 shows one method.
Active servos can also be used to impose voltages on the
RNG/SS pin, provided they can only sink current. Active
circuits that source current cannot be used to drive the
RNG/SS pin. An example is shown in Figure 6.
RNG/SS: Soft-Start
Soft-start functionality is also supported by the RNG/SS
pin. The 50µA sourced from the RNG/SS pin can linearly
charge a capacitor, C
RNG/SS
, connected from the RNG/SS
pin to ground (see Figure 7). The maximum charge current
follows this voltage. Thus, the charge current increases
from zero to the fully programmed value as the capacitor
charges from 0V to 1V. The value of C
RNG/SS
is calculated
based on the desired time to full current (t
SS
) following
the relation:
C
RNG/SS
= 50µA • t
SS
The RNG/SS pin is pulled to ground internally when charg-
ing is terminated so each new charging cycle begins with
a soft-start
cycle. RNG/SS is also pulled to ground during
bad battery and NTC fault conditions, so a graceful recovery
from these faults is possible.
APPLICATIONS INFORMATION
LT3651
RNG/SS
10k
365142 F05
LOGIC HIGH = HALF CURRENT
Figure 5. Using the RNG/SS Pin for
Digital Control of Maximum Charge Current
LT3651
RNG/SS
SERVO
REFERENCE
365142 F06
+
–
Figure 6. Driving the RNG/SS Pin
with a Current-Sink Active Servo Amplifier
LT3651
RNG/SS
C
RNG/SS
365142 F07
Figure 7. Using the RNG/SS Pin for Soft-Start