LTC3350 datasheet(20/46 Pages) LINER

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LTC3350

3350fc

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operaTion

Details of the mon_status and msk_mon_status registers

can be found in the Register Descriptions section of this

data sheet.

Charge Status Register

TheLTC3350chargerstatusregister(chrg_status)contains

data about the state of the charger, switcher, shunts, and

balancers. Details of this register may be found in the

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

levelofanenabledlimit,theassociatedbitinthealarm_reg

ThisinformstheI2C/SMBushostalimithasbeenexceeded.

The alarms register may then be read to determine exactly

which programmed limits have been exceeded.

AsingleADCissharedbetweenthe11channelswithabout

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

consecutiveADCmeasurementswithoutsettingthealarm.

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

valueexceedstheprogrammedlevelforthatalarmthealarm

is set. That alarm may be cleared by writing a one to the

appropriate bit of the clr_alarms register or by writing a

zero to the appropriate bit to the msk_alarms register. All

alarms that have been set and have not yet been cleared

may be read in the alarm_reg register.

Alloftheindividualmeasuredvoltageshaveacorresponding

undervoltage (uv) and overvoltage (ov) alarm level. All of

theindividualcapacitorvoltagesarecomparedtothesame

alarmlevels,setinthecap_ov_lvlandcap_uv_lvlregisters.

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

settingthedtemp_cold_lvland/ordtemp_hot_lvlregisters

and enabling their respective alarms in the msk_alarms

to degrees Celsius use the following:

TDIE (°C) = 0.028 • meas_dtemp – 251.4

General Purpose Input

Thegeneralpurposeinput(GPI)pincanbeusedtomeasure

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.Thisbufferisenabledbysettingthectl_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

programmedusingthegpi_uv_lvlandgpi_ov_lvlregisters.

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 INTVCC to GPI and a thermistor is required

from GPI to ground. Connect GPI to SGND if not used.

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