LTC3350_15 Datasheet, PDF (19/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Operation
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
capacitance 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 cur-
rent. 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 measure-
ments 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.
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.
Details of the mon_status and msk_mon_status registers
can be found in the Register Descriptions section of this
data sheet.
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