LTC3350
19
3350fc
For more information
www.linear.com/LTC3350
operaTion
stack. This measurement is performed with minimal impact
to the system, and can be done while the supercapacitor
backup system is online. This measurement discharges
the capacitor stack by a small amount (200mV). If input
power fails during this test, the part will go into backup
mode and the test will terminate.
The capacitance test is performed only once the
supercapacitors have finished charging. The test
temporarily disables the charger, then discharges the
supercapacitors by 200mV with a precision current.
The discharge time is measured and used to calculate
the capacitance with the result of this measurement
stored in the meas_cap register. The number reported is
proportional to the capacitance of the entire stack. Two
different scales can be set using the ctl_cap_scale bit in
the ctl_reg register. If ctl_cap_scale is set to 0 (for large
value capacitor stacks), use the following equation to
convert the meas_cap value to Farads:
C
STACK
=
R
T
R
TST
•
336µF
•
meas_cap
If ctl_cap_scale is set to 1 (for small value capactor stacks),
use the following equation to convert the meas_cap value
to Farads:
C
STACK
=
R
T
R
TST
•
3.36µF
•
meas_cap
In the two previous equations R
T
is the resistor on the RT
pin and R
TST
is the resistor on the ITST pin.
The ESR test is performed immediately following the
capacitance test. The switching controller is switched on
and off several times. The changes in charge current and
stack voltage are measured. These measurements are
used to calculate the ESR relative to the charge current
sense resistor. The result of this measurement is stored
in the meas_esr register. The value reported in meas_esr
can be converted to ohms using the following equation:
R
ESR
=
R
SNSC
64
•
meas_esr
where R
SNSC
is the charge current sense resistor in series
with the inductor.
The capacitance and capacitor ESR measurements do not
automatically run as the other measurements do. They
must be initiated by setting the ctl_strt_capesr bit in the
ctl_reg register. This bit will automatically clear once the
measurement begins. If the cap_esr_per register is set to
a non-zero value, the measurement will be repeated after
the time programmed in the cap_esr_per register. Each
LSB in the cap_esr_per register represents 10 seconds.
The capacitance and ESR measurements may fail to
complete for several reasons, in which case the respective
mon_cap_failed or mon_esr_failed bit will be set. The ca-
pacitance test may fail due to a power failure or if the 200mV
discharge trips the CAPGD comparator. The ESR test will also
fail if the capacitance test fails. The ESR test uses the charger
to supply a current and then measures the supercapacitor
stack voltage with and without that current. If the ESR is
greater than 1024 times R
SNSC
, the ESR measurement will
fail. The ESR measurement is adaptive; it uses knowledge of
the ESR from previous measurements to program the test
current. The capacitance and ESR tests should initially be
run several times when first powering up to get the most
accuracy out of the system. It is possible for the first few
measurements to give low quality results or fail to complete
and after running several times will complete with a quality
result. The leakage on supercapacitors is initially very high
after being charged. Many supercapacitor manufacturers
specify the leakage current after being charged for 72 hours.
It is expected that capacitor measurements conducted prior
to this time will read low.
Monitor Status Register
The LTC3350 has a monitor status register (mon_status)
which contains status bits indicating the state of the ca-
pacitance and ESR monitoring system. These bits are set
and cleared by the capacitor monitor upon certain events
during a capacitor and ESR measurement, as described
in the Capacitance and ESR Measurement section.
There is a corresponding msk_mon_status register. Writing
a one to any of these bits will cause the
SMBALERT
pin to
pull low when the corresponding bit in the msk_mon_sta-
tus register has a rising edge. This allows reduced polling
of the LTC3350 when waiting for a capacitance or ESR
measurement to complete.
