Datasheet
Table Of Contents
- FEATURES
- Applications
- Key Specifications
- DESCRIPTION
- Absolute Maximum Ratings
- Operating Ratings
- Temperature-to-Digital Converter Characteristics
- Logic Electrical Characteristics
- SMBus DIGITAL SWITCHING CHARACTERISTICS
- Functional Description
- CONVERSION SEQUENCE
- THE ALERT OUTPUT
- T_CRIT_A OUTPUT and T_CRIT LIMIT
- POWER ON RESET DEFAULT STATES
- SMBus INTERFACE
- TEMPERATURE DATA FORMAT
- OPEN-DRAIN OUTPUTS
- DIODE FAULT DETECTION
- COMMUNICATING with the LM90
- SERIAL INTERFACE RESET
- DIGITAL FILTER
- Fault Queue
- One-Shot Register
- LM90 REGISTERS
- COMMAND REGISTER
- LOCAL and REMOTE TEMPERATURE REGISTERS (LT, RTHB, RTLB)
- STATUS REGISTER (SR)
- CONFIGURATION REGISTER
- CONVERSION RATE REGISTER
- LOCAL and REMOTE HIGH SETPOINT REGISTERS (LHS, RHSHB, and RHSLB)
- LOCAL and REMOTE LOW SETPOINT REGISTERS (LLS, RLSHB, and RLSLB)
- REMOTE TEMPERATURE OFFSET REGISTERS (RTOHB and RTOLB)
- LOCAL and REMOTE T_CRIT REGISTERS (RCS and LCS)
- T_CRIT HYSTERESIS REGISTER (TH)
- FILTER and ALERT CONFIGURE REGISTER
- MANUFACTURERS ID REGISTER
- DIE REVISION CODE REGISTER
- Application Hints
- Revision History
LM90
www.ti.com
SNIS126A –MAY 2004–REVISED MARCH 2013
Figure 15. Mobile Pentium III or 3904 Temperature vs LM90 Temperature Reading
Most silicon diodes do not lend themselves well to this application. It is recommended that a 2N3904 transistor
base emitter junction be used with the collector tied to the base.
A diode connected 2N3904 approximates the junction available on a Pentium III microprocessor for temperature
measurement. Therefore, the LM90 can sense the temperature of this diode effectively.
DIODE NON-IDEALITY
Diode Non-Ideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following relationship holds for variables V
BE
, T and I
f
:
where
• (2)
• q = 1.6×10
−19
Coulombs (the electron charge),
• T = Absolute Temperature in Kelvin
• k = 1.38×10
−23
joules/K (Boltzmann's constant),
• η is the non-ideality factor of the process the diode is manufactured on,
• I
S
= Saturation Current and is process dependent,
• I
f
= Forward Current through the base emitter junction
• V
BE
= Base Emitter Voltage drop (2)
In the active region, the -1 term is negligible and may be eliminated, yielding the following equation
(3)
In the above equation, η and I
S
are dependant upon the process that was used in the fabrication of the particular
diode. By forcing two currents with a very controlled ration (N) and measuring the resulting voltage difference, it
is possible to eliminate the I
S
term. Solving for the forward voltage difference yields the relationship:
(4)
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