lt3651-4.2 Linear Technology Corporation, lt3651-4.2 Datasheet - Page 14

• Manufacturer Part Number: lt3651-4.2
• Description: Monolithic 4a High Voltage Li-ion Battery Charger Features
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
LT3651-4.2
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
Further flexibility is possible by dynamically altering the
I
to create unique input limit conditions. The I
also be tied to a servo amplifier for other options. See the
information in the following section concerning I
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 ten times the maximum voltage across the
charge current sense resistor, R
tions for charge current are:
I
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
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 46mV 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.

LIM
MAX(RNG/SS)
I
MAX RNG SS
pin. Different resistor values could be switched in
CL
(
to CLP , CLN.
/
is the maximum charge current.
)
=
10 8
RNG/SS
. •
V
RNG SS
R
/
SENSE
is greater than 20k or if the
SENSE
=
50
. The defining equa-
10 8 8 • R
µA R
.
•
RNG SS
LIM
SENSE
pin can
/
RNG/SS
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
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
based on the desired time to full current (t
the relation:
C
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.
RNG/SS
Figure 7. Using the RNG/SS Pin for Soft-Start
Figure 5. Using the RNG/SS Pin for
Digital Control of Maximum Charge Current
LT3651-4.2
Figure 6. Driving the RNG/SS Pin
with a Current-Sink Active Servo Amplifier
= 50µA • t
RNG/SS
LT3651-4.2
RNG/SS
LT3651-4.2
RNG/SS
RNG/SS
SS
10k
365142 F05
365142 F06
LOGIC HIGH = HALF CURRENT
, connected from the RNG/SS
365142 F07
+
–
C
RNG/SS
SERVO
REFERENCE
RNG/SS
SS
is calculated
) following
365142f