Datasheet

LT3650-8.2/LT3650-8.4
14
36508284fd
RNG/SS: Dynamic Charge Current Adjust
The LT3650 gives the user the capability to adjust maximum
charge current dynamically through the RNG/SS pin. The
voltage on the RNG/SS pin corresponds to ten times the
maximum voltage across the sense resistor (R
SENSE
). The
default maximum sense voltage is 100mV, so maximum
charge current can be expressed as:
I
MAX(RNG/SS)
= I
MAX
• V
RNG/SS
where I
MAX(RNG/SS)
is the maximum charge current if
V
RNG/SS
is within 0V to 1V. Voltages higher than 1V have
no effect on the maximum charge current.
The LT3650 sources 50µA from the RNG/SS pin, such that
a current control voltage can be set by simply connecting
an appropriately valued resistor to ground, following the
relation:
R
RNG/SS
=
V
RNG/SS
50µA
For example, to reduce the maximum charge current to 50%
of the original value, which corresponds to a maximum
sense voltage of 50mV, RNG/SS would be set to 0.5V.
R
RNG/SS
=
0.5V
50µA
= 10kΩ
This feature could be used, for example, to switch in a
reduced charge current level. 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. Resistive pull-
ups can be used, but extreme care must be taken not to
exceed the 2.5V absolute maximum voltage on the pin.
RNG/SS: Soft-Start
Soft-start functionality is also supported by the RNG/SS
pin. 50µA is sourced from the RNG/SS pin, so connecting
a capacitor from the RNG/SS pin to ground (C
RNG/SS
) cre-
ates a linear voltage ramp. 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
Figure 6. Using the RNG/SS Pin for Digital
Control of Maximum Charge Current
Figure 7. Driving the RNG/SS Pin with a
Current-Sink Active Servo Amplifier
Figure 8. Using the RNG/SS Pin for Soft-Start
365082 F06
RNG/SS
10k
LOGIC HIGH = HALF CURRENT
LT3650
365082 F08
RNG/SS
C
PROG
LT3650
365082 F07
RNG/SS
SERVO REFERENCE
LT3650
+
–