LTC3350
10
3350fc
For more information
www.linear.com/LTC3350
pin FuncTions
SCL (Pin 1):
Clock Pin for the I
2
C/SMBus Serial Port.
SDA (Pin 2):
Bidirectional Data Pin for the I
2
C/SMBus
Serial Port.
SMBALERT
(Pin 3):
Interrupt Output. This open-drain
output is pulled low when an alarm threshold is exceeded,
and will remain low until the acknowledgement of the part’s
response to an SMBus ARA.
CAPGD (Pin 4):
Capacitor Power Good. This open-drain
output is pulled low when CAPFB is below 92% of its
regulation point.
VC (PIN 5):
Control Voltage Pin. This is the compensation
node for the charge current, input current, supercapacitor
stack voltage and output voltage control loops. An RC
network is connected between VC and SGND. Nominal
voltage range for this pin is 1V to 3V.
CAPFB (Pin 6):
Capacitor Stack Feedback Pin. This pin
closes the feedback loop for constant voltage regulation.
An external resistor divider between VCAP and SGND with
the center tap connected to CAPFB programs the final
supercapacitor stack voltage. This pin is nominally equal
to the output of the V
CAP
DAC when the synchronous
controller is in constant voltage mode while charging.
OUTFB (Pin 7):
Step-Up Mode Feedback Pin. This pin
closes the feedback loop for voltage regulation of V
OUT
during input power failure using the synchronous controller
in step-up mode. An external resistor divider between
V
OUT
and SGND with the center tap connected to OUTFB
programs the minimum backup supply rail voltage when
input power is unavailable. This pin is nominally 1.2V when
in backup and the synchronous controller is not in current
limit. To disable step-up mode tie OUTFB to INTV
CC
.
SGND (Pin 8):
Signal Ground. All small-signal and com-
pensation components should be connected to this pin,
which in turn connects to PGND at one point. This pin
should also Kelvin to the bottom plate of the capacitor stack.
RT (Pin 9):
Timing Resistor. The switching frequency of
the synchronous controller is set by placing a resistor, R
T
,
from this pin to SGND. This resistor is always required.
If not present the synchronous controller will not start.
GPI (Pin 10):
General Purpose Input. The voltage on this
pin is digitized directly by the ADC. For high impedance
inputs an internal buffer can be selected and used to drive
the ADC. The GPI pin can be connected to a negative
temperature coefficient (NTC) thermistor to monitor the
temperature of the supercapacitor stack. A low drift bias
resistor is required from INTV
CC
to GPI and a thermistor
is required from GPI to ground. Connect GPI to SGND if
not used. The digitized voltage on this pin can be read in
the meas_gpi register.
ITST (Pin 11):
Programming Pin for Capacitance Test Cur-
rent. This current is used to partially discharge the capaci-
tor stack at a precise rate for capacitance measurement.
This pin servos to 1.2V during a capacitor measurement.
A resistor, R
TST
, from this pin to SGND programs the test
current. R
TST
must be at least 121Ω.
CAPRTN (Pin 12):
Capacitor Stack Shunt Return Pin. This
pin is connected to the grounded bottom plate of the first
super capacitor in the stack through a shunt resistor.
CAP1 (Pin 13):
First Supercapacitor Pin. The top plate of
the first supercapacitor and the bottom plate of the second
supercapacitor are connected to this pin through a shunt
resistor. CAP1 and CAPRTN are used to measure the voltage
across the first super capacitor and to shunt current around
the capacitor to provide balancing and prevent overvoltage.
The voltage between this pin and CAPRTN is digitized and
can be read in the meas_vcap1 register.
CAP2 (Pin 14):
Second Supercapacitor Pin. The top plate
of the second supercapacitor and the bottom plate of the
third supercapacitor are connected to this pin through a
shunt resistor. CAP2 and CAP1 are used to measure the
voltage across the second supercapacitor and to shunt
