LTC3350_15 Datasheet, PDF (20/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Operation
Charge Status Register
The LTC3350 charger status register (chrg_status) contains
data about the state of the charger, switcher, shunts, and
balancers. Details of this register may be found in the
Register Description sections of this data sheet.
Limit Checking and Alarms
The LTC3350 has a limit checking function that will check
each measured value against I²C/SMBus programmable
limits. This feature is optional, and all the limits are dis-
abled by default. The limit checking is designed to simplify
system monitoring, eliminating the need to continuously
poll the LTC3350 for measurement data.
If a measured parameter goes outside of the programmed
level of an enabled limit, the associated bit in the alarm_reg
register is set high and the SMBALERT pin is pulled low.
This informs the I²C/SMBus host a limit has been exceeded.
The alarms register may then be read to determine exactly
which programmed limits have been exceeded.
A single ADC is shared between the 11 channels with about
18ms between consecutive measurements of the same
channel. In a transient condition, it is possible for these
parameters to exceed their programmed levels in between
consecutive ADC measurements without setting the alarm.
Once the LTC3350 has responded to an SMBus ARA the
SMBALERT pin is released. The part will not pull the pin
low again until another limit is exceeded. To reset a limit
that has been exceeded, it must be cleared by writing a
one to the respective bit in the clr_alarms register.
A number of the LTC3350’s registers are used for limit
checking. Individual limits are enabled or disabled in the
msk_alarms registers. Once an enabled alarm’s measured
value exceeds the programmed level for that alarm the
alarm is set. That alarm may only be cleared by writing a
one to the appropriate bit of the clr_alarms register. All
alarms that have been set and have not yet been cleared
may be read in the alarm_reg register.
All of the individual measured voltages have a corresponding
undervoltage (uv) and overvoltage (ov) alarm level. All of
the individual capacitor voltages are compared to the same
alarm levels, set in the cap_ov_lvl and cap_uv_lvl registers.
The input current measurement has an overcurrent (oc)
alarm programmed in the iin_oc_lvl register. The charge
current has an undercurrent alarm programmed in the
ichg_uc_lvl register.
Die Temperature Sensor
The LTC3350 has an integrated die temperature sensor
monitored by the ADC and digitized to the meas_dtemp
register. An alarm may be set on die temperature by
setting the dtemp_cold_lvl and/or dtemp_hot_lvl registers
and enabling their respective alarms in the msk_alarms
register. To convert the code in the meas_dtemp register
to degrees Celsius use the following:
T_DIE (°C) = 0.028 • meas_dtemp – 251.4
General Purpose Input
The general purpose input (GPI) pin can be used to measure
an additional system parameter. The voltage on this pin is
directly digitized by the ADC. For high impedance inputs,
an internal buffer may be selected and used to drive the
ADC. This buffer is enabled by setting the ctl_gpi_buffer_en
bit in the ctl_reg register. With this buffer, the input range
is limited from 0V to 3.5V. If this buffer is not used, the
range is from 0V to 5V, however, the input stage of the
ADC will draw about 0.4µA per volt from this pin. The ADC
input is a switched capacitor amplifier running at about
1MHz, so this current draw will be at that frequency. The
pin current can be eliminated at the cost of reduced range
and increased offset by enabling the buffer.
Alarms are available for this pin voltage with levels
programmed using the gpi_uv_lvl and gpi_ov_lvl registers.
These alarms are enabled using the msk_gpi_uv and
msk_gpi_ov bits in the msk_alarms register.
To monitor the temperature of the supercapacitor stack,
the GPI pin can be connected to a negative temperature
coefficient (NTC) thermistor. 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.
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