LTC4015_15 Datasheet, PDF (21/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Operation
When input power is absent, the measurement system can
be sampled periodically at reduced battery load using the
following procedure as an example:
a. Write en_meas_sys_valid_alert = 1
b. Write force_meas_sys_on = 1
c. Wait for SMBALERT to go low (typically 20ms from
force_meas_sys_on = 1)
d. Upon SMBALERT going low, perform ARA command. If
there are multiple slave devices, verify that the LTC4015
is asserting the alert.
e. Verify meas_sys_valid_alert = 1
f. Write en_meas_sys_valid_alert = 0
g. Write force_meas_sys_on = 0
h. Read updated measurement system data from the
LTC4015
This procedure can be repeated at desired intervals (for
example, once per second) in order to periodically moni-
tor the system.
Thermistor/NTC Measurement
NTCBIAS
NTC
R_NTCBIAS
THERMISTOR T
RNTC
4015 F01
Figure 1. NTC Bias Configuration
Battery temperature is sensed by using an external NTC
(negative temperature coefficient) thermistor, R_NTC. R_NTC
is normally located in the battery pack. Connect R_NTC
between the NTC pin and ground. A bias resistor, R_NTCBIAS,
is connected between NTCBIAS and NTC. R_NTCBIAS should
be a 1% resistor with a value equal to the value of the
chosen thermistor at 25°C (r25). NTC_RATIO (0x40) is
available via the serial port, except when the ship mode
feature has been activated.
The LTC4015 measurement system is configured to directly
calculate NTC_RATIO, where:
NTC_RATIO= 21845 •
R_NTC
R_NTCBIAS + R_NTC
R_NTC
= R_NTCBIAS
•
NTC_RATIO
21845 –NTC_RATIO
NTC_RATIO has a bit weight of 1/21845 = 4.5777^–5/LSB.
For a NTC_RATIO of 0.5, where R_NTC = R_NTCBIAS, the
value of NTC_RATIO reported by the serial port would be
approximately 10922.
The data in the first two columns in the following table
are from a Vishay NTC thermistor with a R₂₅ of 10k
and β value of 3490k, such as provided by a Vishay
NTCS0402E3103FLT or NTHS0402N02N1002JE. The
NTC_RATIO value is21845 • [R_NTC/( R_NTC + R_NTCBIAS )],
where R_NTCBIAS = R₂₅ = 10k.
TEMPERATURE
10.0
15.0
20.0
25.0
30.0
35.0
40.0
R_NTC
18290
14867
12157
10000
8272
6879
5751
NTC_RATIO
14122
13059
11985
10922
9889
8902
7975
Steps to find NTC resistor temperature given a NTC_RATIO value;
1. Retrieve value for the NTC_RATIO from the LTC4015 via the I²C port
2. Calculate R_NTC;
R_NTC
=
R_NTCBIAS
NTC_RATIO
21845 –NTC_RATIO
3. Using the calculated R_NTC, use NTC resistor manufacturers data to
determine temperature.
For more information www.linear.com/LTC4015
4015f
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