LM90CIMMX [NSC]
3 Degree Celcious Accurate, Remote Diode and Local Digital Temperature Sensor with Two-Wire Interface; 3度Celcious精确，远程二极管和本地数字温度传感器，具有双线接口
| 型号： | LM90CIMMX |
| 厂家： | 
National Semiconductor |
| 描述： | 3 Degree Celcious Accurate, Remote Diode and Local Digital Temperature Sensor with Two-Wire Interface |
| 文件： | 总20页 (文件大小：385K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
February 2002
LM90
±
3˚C Accurate, Remote Diode and Local Digital
Temperature Sensor with Two-Wire Interface
n On-board local temperature sensing
n 10 bit plus sign remote diode temperature data format,
0.125 ˚C resolution
n Diode fault detection circuitry
n T_CRIT_A output useful for system shutdown (open
diode does not activate T_CRIT_A)
n ALERT output supports SMBus 2.0 protocol
n SMBus 2.0 compatible interface, supports TIMEOUT
n 8-pin MSOP packages
General Description
The LM90 is an 11-bit digital temperature sensor with a
2-wire System Management Bus (SMBus) serial interface.
The LM90 accurately measures its own temperature as well
as the temperature of an external device, such as processor
thermal diode or diode connected transistor such as the
2N3904. The temperature of any ASIC can be accurately
determined using the LM90 as long as a dedicated diode
(semiconductor junction) is available on the target die. The
±
LM90 remote sensor accuracy of 3˚C is factory trimmed for
™
the 1.008 typical non-ideality factor of the mobile Pentium
Key Specifications
III thermal diode. The LM90 has an Offset
register to allow measuring other diodes without
requiring continuous software management. Contact
j
j
j
Supply Voltage
Supply Current
3.0V to 3.6V
0.8mA (typ)
@
hardware.monitor.team nsc.com to obtain the latest data for
Local Temp Accuracy (includes quantization error)
new processors.
±
TA=25˚C to 125˚C
4.0˚C (max)
Activation of the ALERT output occurs when any tempera-
ture goes outside a preprogrammed window set by the HIGH
and LOW temperature limit registers or exceeds the T_CRIT
temperature limit. Activation of the T_CRIT_A occurs when
any temperature exceeds the T_CRIT programmed limit.
The LM90 is pin and register compatible with the LM86,
Analog Devices ADM1032 and Maxim MAX6657/8.
j
Remote Diode Temp Accuracy (includes quantization
error)
TA=30˚C to 50˚C, TD=60˚C to 100˚C
TA=0˚C to 85˚C, TD=25˚C to 125˚C
±
±
3.0˚C (max)
4.0˚C (max)
Applications
Features
n System Thermal Management
n Accurately senses die temperature of remote ICs or
diode junctions
(e.g. Laptop, Desktop, Workstations, Server)
n Electronic Test Equipment
n Office Electronics
n Offset register allows sensing a variety of thermal
diodes accurately
LM90 Simplified Block Diagram
20033701
™
Pentium is a trademark of Intel Corporation.
© 2002 National Semiconductor Corporation
DS200337
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Connection Diagram
MSOP-8
20033702
TOP VIEW
Ordering Information
Package
Marking
T11C
NS Package
Transport
Media
Number
MUA08A
(MSOP-8)
MUA08A
(MSOP-8)
LM90CIMM
1000 Units on
Tape and Reel
3500 Units on
Tape and Reel
LM90CIMMX
T11C
Pin Descriptions
#
Label
Pin
Function
Typical Connection
DC Voltage from 3.0 V to 3.6 V
Positive Supply Voltage
Input
VDD
1
Diode Current Source
To Diode Anode. Connected to remote discrete
diode conected transistor junction or to the diode
connected transistor junction on a remote IC whose
die temperature is being sensed.
D+
2
D−
T_CRIT_A
GND
3
4
5
6
Diode Return Current Sink
T_CRIT Alarm Output,
Open-Drain, Active-Low
Power Supply Ground
Interrupt Output,
To Diode Cathode.
Pull-Up Resistor, Controller Interrupt or Power
Supply Shutdown Control
Ground
Pull-Up Resistor, Controller Interrupt or Alert Line
ALERT
Open-Drain, Active-Low
SMBus Bi-Directional Data
Line, Open-Drain Output
SMBus Input
From and to Controller, Pull-Up Resistor
From Controller, Pull-Up Resistor
SMBData
SMBCLK
7
8
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2
Typical Application
20033703
3
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Absolute Maximum Ratings (Note 1)
Vapor Phase (60 seconds)
Infrared (15 seconds)
ESD Susceptibility (Note 4)
Human Body Model
215˚C
220˚C
Supply Voltage
−0.3 V to 6.0 V
Voltage at SMBData, SMBCLK,
ALERT, T_CRIT_A
2000 V
200 V
−0.5V to 6.0V
−0.3 V to
Machine Model
Voltage at Other Pins
(VDD + 0.3 V)
±
D− Input Current
1 mA
Operating Ratings
Input Current at All Other Pins (Note
2)
(Notes 1, 5)
±
5 mA
Operating Temperature Range
Electrical Characteristics
Temperature Range
LM90
0˚C to +125˚C
Package Input Current (Note 2)
SMBData, ALERT, T_CRIT_A Output
Sink Current
30 mA
T
MIN≤TA≤TMAX
0˚C≤TA≤+85˚C
+3.0V to +3.6V
10 mA
−65˚C to
+150˚C
Storage Temperature
Supply Voltage Range (VDD
)
Soldering Information, Lead Temperature
SOIC-8 or MSOP-8 Packages (Note
3)
Temperature-to-Digital Converter Characteristics
Unless otherwise noted, these specifications apply for VDD=+3.0Vdc to 3.6Vdc. Boldface limits apply for TA = TJ
MIN≤TA≤TMAX; all other limits TA= TJ=+25˚C, unless otherwise noted.
=
T
Parameter
Conditions
Typical
Limits
Units
(Note 6)
(Note 7)
(Limit)
±
±
±
Temperature Error Using Local Diode
Temperature Error Using Remote Diode of
mobile Pentium III with typical non-ideality of
1.008. For other processors email
TA = +25˚C to +125˚C, (Note 8)
1.5
4
3
˚C (max)
TA = +30˚C to
+50˚C
TD = +25˚C
to +100˚C
˚C (max)
˚C (max)
@
hardware.monitor.team nsc.com to obtain the
±
TA = +0˚C to
+85˚C
TD = +25˚C
to +125˚C
4
latest data. (TD is the Remote Diode Junction
Temperature)
Remote Diode Measurement Resolution
11
0.125
8
Bits
˚C
Local Diode Measurement Resolution
Bits
1
˚C
Conversion Time of All Temperatures at the
Fastest Setting
(Note 10)
31.25
34.4
1.7
ms (max)
Quiescent Current (Note 9)
SMBus Inactive, 16Hz conversion
0.8
mA (max)
rate
Shutdown
315
0.7
µA
D− Source Voltage
V
Diode Source Current
(D+ − D−)=+ 0.65V; high level
Low level
160
315
110
20
µA (max)
µA (min)
µA (max)
µA (min)
13
7
ALERT and T_CRIT_A Output Saturation
Voltage
IOUT = 6.0 mA
0.4
V (max)
Power-On Reset Threshold
Measure on VDD input, falling
2.4
1.8
V (max)
V (min)
˚C
edge
Local and Remote HIGH Default Temperature
(Note 11)
+70
0
settings
Local and Remote LOW Default Temperature
settings
(Note 11)
˚C
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4
Temperature-to-Digital Converter Characteristics (Continued)
Unless otherwise noted, these specifications apply for VDD=+3.0Vdc to 3.6Vdc. Boldface limits apply for TA = TJ
=
TMIN≤TA≤TMAX; all other limits TA= TJ=+25˚C, unless otherwise noted.
Parameter Conditions
Typical
(Note 6)
+85
Limits
Units
(Note 7)
(Limit)
Local and Remote T_CRIT Default
Temperature Setting
(Note 11)
˚C
Logic Electrical Characteristics
DIGITAL DC CHARACTERISTICS
Unless otherwise noted, these specifications apply for VDD=+3.0 to 3.6 Vdc. Boldface limits apply for TA = TJ = TMIN to
MAX; all other limits TA= TJ=+25˚C, unless otherwise noted.
T
Symbol Parameter Conditions
Typical
Limits
Units
(Note 6)
(Note 7)
(Limit)
SMBData, SMBCLK INPUTS
VIN(1)
VIN(0)
Logical “1” Input Voltage
2.1
0.8
V (min)
V (max)
mV
Logical “0”Input Voltage
SMBData and SMBCLK Digital
Input Hysteresis
VIN(HYST)
400
±
±
IIN(1)
IIN(0)
CIN
Logical “1” Input Current
Logical “0” Input Current
Input Capacitance
VIN = VDD
0.005
−0.005
5
10
10
µA (max)
µA (max)
pF
VIN = 0 V
ALL DIGITAL OUTPUTS
IOH
High Level Output Current
VOH = VDD
SMBus Low Level Output Voltage IOL = 4mA
IOL = 6mA
10
µA (max)
V (max)
VOL
0.4
0.6
SMBus DIGITAL SWITCHING CHARACTERISTICS
Unless otherwise noted, these specifications apply for VDD=+3.0 Vdc to +3.6 Vdc, CL (load capacitance) on output lines = 80
pF. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25˚C, unless otherwise noted.
The switching characteristics of the LM90 fully meet or exceed the published specifications of the SMBus version 2.0. The fol-
lowing parameters are the timing relationships between SMBCLK and SMBData signals related to the LM90. They adhere to
but are not necessarily the SMBus bus specifications.
Symbol
Parameter
SMBus Clock Frequency
SMBus Clock Low Time
Conditions
Typical
Limits
(Note 7)
100
Units
(Limit)
(Note 6)
fSMB
kHz (max)
kHz (min)
µs (min)
ms (max)
µs (min)
µs (max)
µs (max)
ns (max)
10
tLOW
from VIN(0)max to
VIN(0)max
4.7
25
tHIGH
tR,SMB
tF,SMB
tOF
SMBus Clock High Time
SMBus Rise Time
SMBus Fall Time
from VIN(1)min to VIN(1)min
(Note 12)
4.0
1
(Note 13)
0.3
Output Fall Time
CL = 400pF,
250
IO = 3mA, (Note 13)
tTIMEOUT
SMBData and SMBCLK Time Low for
Reset of Serial Interface (Note 14)
Data In Setup Time to SMBCLK High
Data Out Stable after SMBCLK Low
25
35
ms (min)
ms (max)
ns (min)
ns (min)
ns (max)
ns (min)
tSU;DAT
tHD;DAT
250
300
900
100
tHD;STA
Start Condition SMBData Low to SMBCLK
Low (Start condition hold before the first
clock falling edge)
tSU;STO
Stop Condition SMBCLK High to SMBData
Low (Stop Condition Setup)
100
ns (min)
5
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SMBus DIGITAL SWITCHING CHARACTERISTICS
Unless otherwise noted, these specifications apply for VDD=+3.0 Vdc to +3.6 Vdc, CL (load capacitance) on output lines = 80
pF. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25˚C, unless otherwise noted.
The switching characteristics of the LM90 fully meet or exceed the published specifications of the SMBus version 2.0. The fol-
lowing parameters are the timing relationships between SMBCLK and SMBData signals related to the LM90. They adhere to
but are not necessarily the SMBus bus specifications.
Symbol
tSU;STA
tBUF
Parameter
Conditions
Typical
Limits
(Note 7)
0.6
Units
(Limit)
µs (min)
(Note 6)
SMBus Repeated Start-Condition Setup
Time, SMBCLK High to SMBData Low
SMBus Free Time Between Stop and Start
Conditions
1.3
µs (min)
SMBus Communication
20033740
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its rated operating conditions.
<
>
V
Note 2: When the input voltage (V ) at any pin exceeds the power supplies (V
GND or V
), the current at that pin should be limited to 5 mA.
DD
I
I
I
Parasitic components and or ESD protection circuitry are shown in the figure below for the LM90’s pins. The nominal breakdown voltage of D3 is 6.5 V. Care should
be taken not to forward bias the parasitic diode, D1, present on pins: D+, D−. Doing so by more than 50 mV may corrupt a temperature measurements.
#
Pin Name
VDD
PIN
1
D1
D2
D3
D4
D5
D6
D7
R1
SNP
ESD CLAMP
x
x
x
D+
2
x
x
x
x
x
x
x
x
x
x
x
x
D−
3
x
T_CRIT_A
ALERT
SMBData
SMBCLK
4
x
x
x
x
x
x
x
6
7
8
Note: An “x” indicates that the diode exists.
20033713
FIGURE 1. ESD Protection Input Structure
Note 3: See the URL ”http://www.national.com/packaging/“ for other recommendations and methods of soldering surface mount devices.
Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor. Machine model, 200pF discharged directly into each pin.
Note 5: Thermal resistance junction-to-ambient when attached to a printed circuit board with 2 oz. foil:
– MSOP-8 = 210˚C/W
Note 6: Typicals are at T = 25˚C and represent most likely parametric norm.
A
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6
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: Local temperature accuracy does not include the effects of self-heating. The rise in temperature due to self-heating is the product of the internal power
dissipation of the LM90 and the thermal resistance. See (Note 5) for the thermal resistance to be used in the self-heating calculation.
Note 9: Quiescent current will not increase substantially with an SMBus.
Note 10: This specification is provided only to indicate how often temperature data is updated. The LM90 can be read at any time without regard to conversion state
(and will yield last conversion result).
Note 11: Default values set at power up.
Note 12: The output rise time is measured from (V
max + 0.15V) to (V
min − 0.15V).
IN(0)
IN(1)
Note 13: The output fall time is measured from (V
min - 0.15V) to (V
min + 0.15V).
IN(1)
IN(1)
Note 14: Holding the SMBData and/or SMBCLK lines Low for a time interval greater than t
will reset the LM90’s SMBus state machine, therefore setting
TIMEOUT
SMBData and SMBCLK pins to a high impedance state.
1.0 Functional Description
The LM90 temperature sensor incorporates a delta VBE
based temperature sensor using a Local or Remote and a
10-bit plus sign ADC (Delta-Sigma Analog-to-Digital Con-
verter). The LM90 is compatible with the serial SMBus ver-
sion 2.0 two-wire interface. Digital comparators compare the
measured Local Temperature (LT) to the Local High (LHS),
Local
Low
(LLS)
and
Local
T_CRIT
(LCS)
user-programmable temperature limit registers. The mea-
sured Remote Temperature (RT) is digitally compared to the
Remote High (RHS), Remote Low (RLS) and Remote
T_CRIT (RCS) user-programmable temperature limit regis-
ters. Activation of the ALERT output indicates that a com-
parison is greater than the limit preset in a T_CRIT or HIGH
limit register or less than the limit preset in a LOW limit
register. The T_CRIT_A output responds as a true compara-
tor with built in hysteresis. The hysteresis is set by the value
placed in the Hysteresis register (TH). Activation of
T_CRIT_A occurs when the temperature is above the
T_CRIT setpoint. T_CRIT_A remains activated until the tem-
perature goes below the setpoint calculated by T_CRIT −
TH. The hysteresis register impacts both the remote tem-
perature and local temperature readings.
20033739
FIGURE 2. Conversion Rate Effect on Power Supply
Current
The LM90 may be placed in a low power consumption
(Shutdown) mode by setting the RUN/STOP bit found in the
Configuration register. In the Shutdown mode, the LM90’s
SMBus interface remains while all circuitry not required is
turned off.
1.2 THE ALERT OUTPUT
The LM90’s ALERT pin is an active-low open-drain output
that is triggered by a temperature conversion that is outside
the limits defined by the temperature setpoint registers. Re-
set of the ALERT output is dependent upon the selected
method of use. The LM90’s ALERT pin is versatile and will
accommodate three different methods of use to best serve
the system designer: as a temperature comparator, as a
temperature based interrupt flag, and as part of an SMBus
ALERT system. The three methods of use are further de-
scribed below. The ALERT and interrupt methods are differ-
ent only in how the user interacts with the LM90.
The Local temperature reading and setpoint data registers
are 8-bits wide. The format of the 11-bit remote temperature
data is a 16-bit left justified word. Two 8-bit registers, high
and low bytes, are provided for each setpoint as well as the
temperature reading. Two offset registers (RTOLB and
RTOHB) can be used to compensate for non_ideality error,
discussed further in Section 4.1 DIODE NON-IDEALITY.
The remote temperature reading reported is adjusted by
subtracting from or adding to the actual temperature reading
the value placed in the offset registers.
Each temperature reading (LT and RT) is associated with a
T_CRIT setpoint register (LCS, RCS), a HIGH setpoint reg-
ister (LHS and RHS) and a LOW setpoint register (LLS and
RLS). At the end of every temperature reading, a digital
comparison determines whether that reading is above its
HIGH or T_CRIT setpoint or below its LOW setpoint. If so,
the corresponding bit in the STATUS REGISTER is set. If the
ALERT mask bit is not high, any bit set in the STATUS
REGISTER, with the exception of Busy (D7) and OPEN
(D2), will cause the ALERT output to be pulled low. Any
temperature conversion that is out of the limits defined by the
temperature setpoint registers will trigger an ALERT. Addi-
tionally, the ALERT mask bit in the Configuration register
must be cleared to trigger an ALERT in all modes.
1.1 CONVERSION SEQUENCE
The LM90 takes approximately 31.25 ms to convert the
Local Temperature (LT), Remote Temperature (RT), and to
update all of its registers. Only during the conversion pro-
cess the busy bit (D7) in the Status register (02h) is high.
These conversions are addressed in a round robin se-
quence. The conversion rate may be modified by the Con-
version Rate Register (04h). When the conversion rate is
modified a delay is inserted between conversions, the actual
conversion time remains at 31.25ms. Different conversion
rates will cause the LM90 to draw different amounts of
supply current as shown in Figure 2.
1.2.1 ALERT Output as a Temperature Comparator
When the LM90 is implemented in a system in which it is not
serviced by an interrupt routine, the ALERT output could be
used as a temperature comparator. Under this method of
7
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1.0 Functional Description (Continued)
use, once the condition that triggered the ALERT to go low is
no longer present, the ALERT is de-asserted (Figure 3). For
example, if the ALERT output was activated by the compari-
>
son of LT
LHS, when this condition is no longer true the
ALERT will return HIGH. This mode allows operation without
software intervention, once all registers are configured dur-
ing set-up. In order for the ALERT to be used as a tempera-
ture comparator, bit D0 (the ALERT configure bit) in the
FILTER and ALERT CONFIGURE REGISTER (xBF) must
be set high. This is not the power on default default state.
20033728
FIGURE 4. ALERT Output as an Interrupt Temperature
Response Diagram
1.2.3 ALERT Output as an SMBus ALERT
When the ALERT output is connected to one or more ALERT
outputs of other SMBus compatible devices and to a master,
an SMBus alert line is created. Under this implementation,
the LM90’s ALERT should be operated using the ARA (Alert
Response Address) protocol. The SMBus 2.0 ARA protocol,
defined in the SMBus specification 2.0, is a procedure de-
signed to assist the master in resolving which part generated
an interrupt and service that interrupt while impeding system
operation as little as possible.
20033731
FIGURE 3. ALERT Comparator Temperature Response
Diagram
The SMBus alert line is connected to the open-drain ports of
all devices on the bus thereby AND’ing them together. The
ARA is a method by which with one command the SMBus
master may identify which part is pulling the SMBus alert line
LOW and prevent it from pulling it LOW again for the same
triggering condition. When an ARA command is received by
all devices on the bus, the devices pulling the SMBus alert
line LOW, first, send their address to the master and second,
release the SMBus alert line after recognizing a successful
transmission of their address.
1.2.2 ALERT Output as an Interrupt
The LM90’s ALERT output can be implemented as a simple
interrupt signal when it is used to trigger an interrupt service
routine. In such systems it is undesirable for the interrupt flag
to repeatedly trigger during or before the interrupt service
routine has been completed. Under this method of operation,
during a read of the STATUS REGISTER the LM90 will set
the ALERT mask bit (D7 of the Configuration register) if any
bit in the STATUS REGISTER is set, with the exception of
Busy (D7) and OPEN (D2). This prevents further ALERT
triggering until the master has reset the ALERT mask bit, at
the end of the interrupt service routine. The STATUS REG-
ISTER bits are cleared only upon a read command from the
master (see Figure 4) and will be re-asserted at the end of
the next conversion if the triggering condition(s) persist(s). In
order for the ALERT to be used as a dedicated interrupt
signal, bit D0 (the ALERT configure bit) in the FILTER and
ALERT CONFIGURE REGISTER (xBF) must be set low.
This is the power on default state.
The SMBus 1.1 and 2.0 specification state that in response
to an ARA (Alert Response Address) “after acknowledging
the slave address the device must disengage its SMBALERT
pulldown”. Furthermore, “if the host still sees SMBALERT
low when the message transfer is complete, it knows to read
the ARA again”. This SMBus “disengaging of SMBALERT”
requirement prevents locking up the SMBus alert line. Com-
petitive parts may address this “disengaging of SMBALERT”
requirement differently than the LM90 or not at all. SMBus
systems that implement the ARA protocol as suggested for
the LM90 will be fully compatible with all competitive parts.
The following sequence describes the response of a system
that uses the ALERT output pin as a interrupt flag:
The LM90 fulfills “disengaging of SMBALERT” by setting the
ALERT mask bit (bit D7 in the Configuration register, at
address 09h) after successfully sending out its address in
response to an ARA and releasing the ALERT output pin.
Once the ALERT mask bit is activated, the ALERT output pin
will be disabled until enabled by software. In order to enable
the ALERT the master must read the STATUS REGISTER,
at address 02h, during the interrupt service routine and then
reset the ALERT mask bit in the Configuration register to 0 at
the end of the interrupt service routine.
1. Master Senses ALERT low
2. Master reads the LM90 STATUS REGISTER to deter-
mine what caused the ALERT
3. LM90 clears STATUS REGISTER, resets the ALERT
HIGH and sets the ALERT mask bit (D7 in the Configu-
ration register).
4. Master attends to conditions that caused the ALERT to
be triggered. The fan is started, setpoint limits are ad-
justed, etc.
The following sequence describes the ARA response proto-
col.
5. Master resets the ALERT mask (D7 in the Configuration
register).
1. Master Senses SMBus alert line low
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8
1.0 Functional Description (Continued)
2. Master sends a START followed by the Alert Response
Address (ARA) with a Read Command.
3. Alerting Device(s) send ACK.
4. Alerting Device(s) send their Address. While transmitting
their address, alerting devices sense whether their ad-
dress has been transmitted correctly. (The LM90 will
reset its ALERT output and set the ALERT mask bit once
its complete address has been transmitted successfully.)
5. Master/slave NoACK
6. Master sends STOP
7. Master attends to conditions that caused the ALERT to
be triggered. The STATUS REGISTER is read and fan
started, setpoint limits adjusted, etc.
20033706
FIGURE 6. T_CRIT_A Temperature Response Diagram
8. Master resets the ALERT mask (D7 in the Configuration
register).
1.4 POWER ON RESET DEFAULT STATES
The ARA, 000 1100, is a general call address. No device
should ever be assigned this address.
LM90 always powers up to these known default states. The
LM90 remains in these states until after the first conversion.
Bit D0 (the ALERT configure bit) in the FILTER and ALERT
CONFIGURE REGISTER (xBF) must be set low in order for
the LM90 to respond to the ARA command.
1. Command Register set to 00h
2. Local Temperature set to 0˚C
The ALERT output can be disabled by setting the ALERT
mask bit, D7, of the Configuration register. The power on
default is to have the ALERT mask bit and the ALERT
configure bit low.
3. Remote Diode Temperature set to 0˚C until the end of
the first conversion.
4. Status Register set to 00h.
5. Configuration register set to 00h; ALERT enabled, Re-
mote T_CRIT alarm enabled and Local T_CRIT alarm
enabled
6. 85˚C Local and Remote T_CRIT temperature setpoints
7. 70˚C Local and Remote HIGH temperature setpoints
8. 0˚C Local and Remote LOW temperature setpoints
9. Filter and Alert Configure Register set to 00h; filter dis-
abled, ALERT output set as an SMBus ALERT
10. Conversion Rate Register set to 8h; conversion rate set
to 16 conv./sec.
1.5 SMBus INTERFACE
The LM90 operates as a slave on the SMBus, so the
SMBCLK line is an input and the SMBData line is
bi-directional. The LM90 never drives the SMBCLK line and
it does not support clock stretching. According to SMBus
specifications, the LM90 has a 7-bit slave address. All bits A6
through A0 are internally programmed and can not be
changed by software or hardware.
20033729
FIGURE 5. ALERT Output as an SMBus ALERT
Temperature Response Diagram
The complete slave address is:
1.3 T_CRIT_A OUTPUT and T_CRIT LIMIT
A6
1
A5
0
A4
0
A3
1
A2
1
A1
0
A0
0
T_CRIT_A is activated when any temperature reading is
greater than the limit preset in the critical temperature set-
point register (T_CRIT), as shown in Figure 6. The Status
Register can be read to determine which event caused the
alarm. A bit in the Status Register is set high to indicate
which temperature reading exceeded the T_CRIT setpoint
temperature and caused the alarm, see Section 2.3.
1.6 TEMPERATURE DATA FORMAT
Temperature data can only be read from the Local and
Remote Temperature registers; the setpoint registers
(T_CRIT, LOW, HIGH) are read/write.
Local and remote temperature diodes are sampled in se-
quence by the A/D converter. The T_CRIT_A output and the
Status Register flags are updated after every Local and
Remote temperature conversion. T_CRT_A follows the state
of the comparison, it is reset when the temperature falls
below the setpoint RCS-TH. The Status Register flags are
reset only after the Status Register is read and if a tempera-
ture conversion(s) is/are below the T_CRIT setpoint, as
shown in . Figure 6
Remote temperature data is represented by an 11-bit, two’s
complement word with an LSB (Least Significant Bit) equal
to 0.125˚C. The data format is a left justified 16-bit word
available in two 8-bit registers:
Temperature
Digital Output
Binary
0111 1101 0000 0000
Hex
+125˚C
7D00h
9
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itself will not activate the ALERT or T_CRIT_A outputs. If the
remote temperature reading is greater than its T_CRIT level
when the OPEN bit is set the T_CRIT_A will remain inactive.
1.0 Functional Description (Continued)
Temperature
Digital Output
Binary
In the event that the D+ pin is shorted to ground or D−, the
Remote Temperature High Byte (RTHB) register is loaded
with −128˚C (1000 0000) and the OPEN bit (D2) in the status
register will not be set. Since operating the LM90 at −128˚C
is beyond it’s operational limits, this temperature reading
represents this shorted fault condition. If the value in the
Remote Low Setpoint High Byte Register (RLSHB) is more
than −128˚C and the Alert Mask is disabled, ALERT will be
pulled low.
Hex
+25˚C
+1˚C
0001 1001 0000 0000
0000 0001 0000 0000
0000 0000 0010 0000
0000 0000 0000 0000
1111 1111 1110 0000
1111 1111 0000 0000
1110 0111 0000 0000
1100 1001 0000 0000
1900h
0100h
0020h
0000h
FFE0h
FF00h
E700h
C900h
+0.125˚C
0˚C
−0.125˚C
−1˚C
−25˚C
−55˚C
Remote diode temperature sensors that have been previ-
ously released and are competitive with the LM90 output a
code of 0˚C if the external diode is short-circuited. This
change is an improvement that allows a reading of 0˚C to be
truly interpreted as a genuine 0˚C reading and not a fault
condition.
Local Temperature data is represented by an 8-bit, two’s
complement byte with an LSB (Least Significant Bit) equal to
1˚C:
1.9 COMMUNICATING with the LM90
The data registers in the LM90 are selected by the Com-
mand Register. At power-up the Command Register is set to
“00”, the location for the Read Local Temperature Register.
The Command Register latches the last location it was set
to. Each data register in the LM90 falls into one of four types
of user accessibility:
Temperature
Digital Output
Binary
Hex
7Dh
19h
01h
00h
FFh
E7h
C9h
+125˚C
+25˚C
+1˚C
0111 1101
0001 1001
0000 0001
0000 0000
1111 1111
1110 0111
1100 1001
1. Read only
0˚C
2. Write only
−1˚C
3. Read/Write same address
4. Read/Write different address
−25˚C
−55˚C
A Write to the LM90 will always include the address byte and
the command byte. A write to any register requires one data
byte.
1.7 OPEN-DRAIN OUTPUTS
Reading the LM90 can take place either of two ways:
The SMBData, ALERT and T_CRIT_A outputs are
open-drain outputs and do not have internal pull-ups. A
“high” level will not be observed on these pins until pull-up
current is provided by some external source, typically a
pull-up resistor. Choice of resistor value depends on many
system factors but, in general, the pull-up resistor should be
as large as possible. This will minimize any internal tempera-
ture reading errors due to internal heating of the LM90. The
maximum resistance of the pull-up to provide a 2.1V high
level, based on LM90 specification for High Level Output
Current with the supply voltage at 3.0V, is 82kΩ(5%) or
88.7kΩ(1%).
1. If the location latched in the Command Register is cor-
rect (most of the time it is expected that the Command
Register will point to one of the Read Temperature Reg-
isters because that will be the data most frequently read
from the LM90), then the read can simply consist of an
address byte, followed by retrieving the data byte.
2. If the Command Register needs to be set, then an
address byte, command byte, repeat start, and another
address byte will accomplish a read.
The data byte has the most significant bit first. At the end of
a read, the LM90 can accept either acknowledge or No
Acknowledge from the Master (No Acknowledge is typically
used as a signal for the slave that the Master has read its
last byte). It takes the LM90 31.25ms to measure the tem-
perature of the remote diode and internal diode. When re-
trieving all 10 bits from a previous remote diode temperature
measurement, the master must insure that all 10 bits are
from the same temperature conversion. This may be
achieved by using one-shot mode or by setting the conver-
sion rate and monitoring the busy bit such that no conversion
occurs in between reading the MSB and LSB of the last
temperature conversion.
1.8 DIODE FAULT DETECTION
The LM90 is equipped with operational circuitry designed to
detect fault conditions concerning the remote diode. In the
event that the D+ pin is detected as shorted to VDD or
floating, the Remote Temperature High Byte (RTHB) register
is loaded with +127˚C, the Remote Temperature Low Byte
(RTLB) register is loaded with 0, and the OPEN bit (D2) in
the status register is set. As a result, if the Remote T_CRIT
setpoint register (RCS) is set to a value less than +127˚C the
ALERT output pin will be pulled low, if the Alert Mask is
disabled. If the Remote HIGH Setpoint High Byte Register
(RHSHB) is set to a value less than +127˚C then ALERT will
be pulled low, if the Alert Mask is disabled. The OPEN bit
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10
1.9.1 SMBus Timing Diagrams
1.0 Functional Description (Continued)
20033710
(a) Serial Bus Write to the internal Command Register followed by a the Data Byte
20033711
(b) Serial Bus Write to the Internal Command Register
20033712
(c) Serial Bus Read from a Register with the Internal Command Register preset to desired value.
FIGURE 7. SMBus Timing Diagrams
1.10 SERIAL INTERFACE RESET
2. When SMBData is HIGH, have the master initiate an
SMBus start. The LM90 will respond properly to an
SMBus start condition at any point during the communi-
cation. After the start the LM90 will expect an SMBus
Address address byte.
In the event that the SMBus Master is RESET while the
LM90 is transmitting on the SMBData line, the LM90 must be
returned to a known state in the communication protocol.
This may be done in one of two ways:
1. When SMBData is LOW, the LM90 SMBus state ma-
chine resets to the SMBus idle state if either SMBData
or SMBCLK are held low for more than 35ms (tTIMEOUT).
Note that according to SMBus specification 2.0 all de-
vices are to timeout when either the SMBCLK or SMB-
Data lines are held low for 25-35ms. Therefore, to insure
a timeout of all devices on the bus the SMBCLK or
SMBData lines must be held low for at least 35ms.
1.11 DIGITAL FILTER
In order to suppress erroneous remote temperature readings
due to noise, the LM90 incorporates a user-configured digital
filter. The filter is accessed in the FILTER and ALERT CON-
FIGURE REGISTER at BFh. The filter can be set according
to the following table.
11
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Level 2 sets maximum filtering.
1.0 Functional Description (Continued)
Figure 8 depict the filter output to in response to a step input
and an impulse input. Figure 9 depicts the digital filter in use
in a Pentium 4 processor system. Note that the two curves,
with filter and without, have been purposely offset so that
both responses can be clearly seen. Inserting the filter does
not induce an offset as shown.
D2
0
D1
0
Filter
No Filter
Level 1
Level 1
Level 2
0
1
1
0
1
1
20033725
20033726
a)Step Response
b)Impulse Response
FIGURE 8. Filter Output Response to a Step Input
20033727
FIGURE 9. Digital Filter Response in a Pentium 4 processor System. The filter on and off curves were purposely
offset to better show noise performance.
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12
1.0 Functional Description (Continued)
1.12 Fault Queue
In order to suppress erroneous ALERT or T_CRIT triggering
the LM90 incorporates a Fault Queue. The Fault Queue acts
to insure a remote temperature measurement is genuinely
beyond a HIGH, LOW or T_CRIT setpoint by not triggering
until three consecutive out of limit measurements have been
made, see Figure 10. The fault queue defaults off upon
power-up and may be activated by setting bit D0 in the
Configuration register (09h) to “1”.
20033730
FIGURE 10. Fault Queue Temperature Response
Diagram
1.13 One-Shot Register
The One-Shot register is used to initiate a single conversion
and comparison cycle when the device is in standby mode,
after which the device returns to standby. This is not a data
register and it is the write operation that causes the one-shot
conversion. The data written to this address is irrelevant and
is not stored. A zero will always be read from this register.
2.0 LM90 REGISTERS
2.1 COMMAND REGISTER
Selects which registers will be read from or written to. Data for this register should be transmitted during the Command Byte of
the SMBus write communication.
P7
P6
P5
P4
P3
P2
P1
P0
Command Select
P0-P7: Command Select
Command Select Address
Read Address Write Address
Power On Default State
Register
Name
Register Function
<
>
<
>
D7:D0
D7:D0 binary
<
>
<
>
P7:P0 hex
P7:P0 hex
decimal
00h
01h
02h
03h
04h
05h
06h
07h
NA
NA
0000 0000
0000 0000
0000 0000
0000 0000
0000 1000
0100 0110
0000 0000
0100 0110
0
LT
RTHB
SR
Local Temperature
0
Remote Temperature High Byte
Status Register
NA
0
09h
0Ah
0Bh
0Ch
0Dh
0
C
Configuration
8 (16 conv./sec)
CR
Conversion Rate
70
0
LHS
LLS
Local HIGH Setpoint
Local LOW Setpoint
Remote HIGH Setpoint High
Byte
70
RHSHB
08h
NA
0Eh
0Fh
0000 0000
0
RLSHB
Remote LOW Setpoint High
Byte
One Shot
Writing to this register will
initiate a one shot conversion
Remote Temperature Low Byte
Remote Temperature Offset
High Byte
10h
11h
NA
0000 0000
0000 0000
0
0
RTLB
11h
RTOHB
12h
12h
0000 0000
0
RTOLB
Remote Temperature Offset
Low Byte
13
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2.0 LM90 REGISTERS (Continued)
Command Select Address
Read Address Write Address
Power On Default State
Register
Name
Register Function
<
>
<
>
D7:D0
D7:D0 binary
<
>
<
>
P7:P0 hex
P7:P0 hex
decimal
13h
13h
0000 0000
0000 0000
0
RHSLB
RLSLB
Remote HIGH Setpoint Low
Byte
14h
14h
0
Remote LOW Setpoint Low
Byte
19h
20h
19h
20h
0101 0101
0101 0101
0000 1010
85
85
10
RCS
LCS
TH
Remote T_CRIT Setpoint
Local T_CRIT Setpoint
T_CRIT Hysteresis
Manufacturers Test Registers
Remote Diode Temperature
Filter
21h
21h
B0h-BEh
BFh
B0h-BEh
BFh
0000 0000
0
RDTF
FEh
FFh
NA
NA
0000 0001
0010 0001
1
RMID
RDR
Read Manufacturer’s ID
Read Stepping or Die Revision
Code
33
2.2 LOCAL and REMOTE TEMPERATURE REGISTERS (LT, RTHB, RTLB)
(Read Only Address 00h, 01h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
SIGN
64
32
16
8
4
2
1
For LT and RTHB D7–D0: Temperature Data. LSB = 1˚C. Two’s complement format.
(Read Only Address 10h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
0.5
0.25 0.125
0
0
0
0
0
For RTLB D7–D5: Temperature Data. LSB = 0.125˚C. Two’s complement format.
The maximum value available from the Local Temperature register is 127; the minimum value available from the Local
Temperature register is -128. The maximum value available from the Remote Temperature register is 127.875; the minimum value
available from the Remote Temperature registers is −128.875.
2.3 STATUS REGISTER (SR)
(Read Only Address 02h):
D7
D6
D5
D4
D3
D2
D1
D0
Busy
LHIGH
LLOW
RHIGH
RLOW
OPEN
RCRIT
LCRIT
Power up default is with all bits “0” (zero).
D0: LCRIT: When set to “1” indicates a Local Critical Temperature alarm.
D1: RCRIT: When set to “1” indicates a Remote Diode Critical Temperature alarm.
D2: OPEN: When set to “1” indicates a Remote Diode disconnect.
D3: RLOW: When set to “1” indicates a Remote Diode LOW Temperature alarm
D4: RHIGH: When set to “1” indicates a Remote Diode HIGH Temperature alarm.
D5: LLOW: When set to “1” indicates a Local LOW Temperature alarm.
D6: LHIGH: When set to “1” indicates a Local HIGH Temperature alarm.
D7: Busy: When set to “1” ADC is busy converting.
2.4 CONFIGURATION REGISTER
(Read Address 03h /Write Address 09h):
D7
D6
D5
D4
D3
D2
D1
D0
ALERT mask
RUN/STOP
0
Remote
T_CRIT_A
mask
0
Local
0
Fault Queue
T_CRIT_A
mask
Power up default is with all bits “0” (zero)
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14
2.0 LM90 REGISTERS (Continued)
D7: ALERT mask: When set to “1” ALERT interrupts are masked.
D6: RUN/STOP: When set to “1” SHUTDOWN is enabled.
D5: is not defined and defaults to “0”.
D4: Remote T_CRIT mask: When set to “1” a diode temperature reading that exceeds T_CRIT setpoint will not activate the
T_CRIT_A pin.
D3: is not defined and defaults to “0”.
D2: Local T_CRIT mask: When set to “1” a Local temperature reading that exceeds T_CRIT setpoint will not activate the
T_CRIT_A pin.
D1: is not defined and defaults to “0”.
D0: Fault Queue: when set to “1” three consecutive remote temperature measurements outside the HIGH, LOW, or T_CRIT
setpoints will trigger an “Outside Limit” condition resulting in setting of status bits and associated output pins..
2.5 CONVERSION RATE REGISTER
(Read Address 04h /Write
Address 0Ah)
(Read Address 04h /Write
Address 0Ah)
Value
Conversion
Value
Conversion
Rate
62.5 mHz
125 mHz
250 mHz
500 mHz
1 Hz
Rate
4 Hz
00
01
02
03
04
05
06
07
8 Hz
08
16 Hz
32 Hz
Undefined
09
10-255
2 Hz
2.6 LOCAL and REMOTE HIGH SETPOINT REGISTERS (LHS, RHSHB, and RHSLB)
(Read Address 05h, 07h /Write Address 0Bh, 0Dh):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
SIGN
64
32
16
8
4
2
1
For LHS and RHSHB: HIGH setpoint temperature data. Power up default is LHIGH = RHIGH = 70˚C. 1LSB = 1˚C. Two’s
complement format.
(Read/Write Address 13h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
0.5
0.25 0.125
0
0
0
0
0
For RHSLB: Remote HIGH Setpoint Low Byte temperature data. Power up default is 0˚C. 1LSB = 0.125˚C. Two’s complement
format.
2.7 LOCAL and REMOTE LOW SETPOINT REGISTERS (LLS, RLSHB, and RLSLB)
(Read Address 06h, 08h, /Write Address 0Ch, 0Eh):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
SIGN
64
32
16
8
4
2
1
For LLS and RLSHB: HIGH setpoint temperature data. Power up default is LHIGH = RHIGH = 0˚C. 1LSB = 1˚C. Two’s
complement format.
(Read/Write Address 14h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
0.5
0.25 0.125
0
0
0
0
0
For RLSLB: Remote HIGH Setpoint Low Byte temperature data. Power up default is 0˚C. 1LSB = 0.125˚C. Two’s complement
format.
2.8 REMOTE TEMPERATURE OFFSET REGISTERS (RTOHB and RTOLB)
(Read/Write Address 11h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
SIGN
64
32
16
8
4
2
1
15
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2.0 LM90 REGISTERS (Continued)
For RTOHB: Remote Temperature Offset High Byte. Power up default is LHIGH = RHIGH = 0˚C. 1LSB = 1˚C. Two’s complement
format.
(Read/Write Address 12h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
0.5
0.25 0.125
0
0
0
0
0
For RTOLB: Remote Temperature Offset High Byte. Power up default is 0˚C. 1LSB = 0.125˚C. Two’s complement format.
The offset value written to these registers will automatically be added to or subtracted from the remote temperature measurement
that will be reported in the Remote Temperature registers.
2.9 LOCAL and REMOTE T_CRIT REGISTERS (RCS and LCS)
(Read/Write Address 20h, 19h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
SIGN
64
32
16
8
4
2
1
D7–D0: T_CRIT setpoint temperature data. Power up default is T_CRIT = 85˚C. 1 LSB = 1˚C, two’s complement format.
2.10 T_CRIT HYSTERESIS REGISTER (TH)
(Read and Write Address 21h):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
16
8
4
2
1
D7–D0: T_CRIT Hysteresis temperature. Power up default is TH = 10˚C. 1 LSB = 1˚C, maximum value = 31.
2.11 FILTER and ALERT CONFIGURE REGISTER
(Read and Write Address BFh):
BIT
D7
D6
D5
D4
D3
D2
D1
D0
Value
0
0
0
0
0
Filter Level
ALERT
Configure
D7-D3: is not defined defaults to ’0’.
D2-D1: input filter setting as defined the table below:
D2
0
D1
Filter Level
No Filter
Level 1
0
1
0
1
0
1
Level 1
1
Level 2
Level 2 sets maximum filtering.
D0: when set to ’1’ comparator mode is enabled.
2.12 MANUFACTURERS ID REGISTER
(Read Address FEh) Default value 01h.
2.13 DIE REVISION CODE REGISTER
(Read Address FFh) Default value 21h. This register will increment by 1 every time there is a revision to the die by National
Semiconductor.
surface temperature, the actual temperature of the of the
LM90 die will be at an intermediate temperature between the
4.0 Application Hints
The LM90 can be applied easily in the same way as other
surface and air temperatures. Again, the primary thermal
integrated-circuit temperature sensors, and its remote diode
conduction path is through the leads, so the circuit board
sensing capability allows it to be used in new ways as well.
temperature will contribute to the die temperature much
It can be soldered to a printed circuit board, and because the
more strongly than will the air temperature.
path of best thermal conductivity is between the die and the
To measure temperature external to the LM90’s die, use a
pins, its temperature will effectively be that of the printed
remote diode. This diode can be located on the die of a
circuit board lands and traces soldered to the LM90’s pins.
target IC, allowing measurement of the IC’s temperature,
This presumes that the ambient air temperature is almost the
independent of the LM90’s temperature. The LM90 has been
same as the surface temperature of the printed circuit board;
optimized to measure the remote diode of a Pentium III
if the air temperature is much higher or lower than the
processor as shown in Figure 11. A discrete diode can also
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16
In the above equation, η and IS 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 IS term. Solving for the forward
voltage difference yields the relationship:
4.0 Application Hints (Continued)
be used to sense the temperature of external objects or
ambient air. Remember that a discrete diode’s temperature
will be affected, and often dominated, by the temperature of
its leads.
The non-ideality factor, η, is the only other parameter not
accounted for and depends on the diode that is used for
measurement. Since ∆VBE is proportional to both η and T,
the variations in η cannot be distinguished from variations in
temperature. Since the non-ideality factor is not controlled by
the temperature sensor, it will directly add to the inaccuracy
±
of the sensor. For the Pentium III Intel specifies a 1%
variation in η from part to part. As an example, assume a
±
temperature sensor has an accuracy specification of 3˚C at
20033715
Mobile Pentium III or 3904 Temperature vs LM90
Temperature Reading
room temperature of 25 ˚C and the process used to manu-
facture the diode has a non-ideality variation of 1%. The
resulting accuracy of the temperature sensor at room tem-
perature will be:
±
± ± ±
3˚C + ( 1% of 298 ˚K) = 6 ˚C
TACC
=
FIGURE 11.
The additional inaccuracy in the temperature measurement
caused by η, can be eliminated if each temperature sensor is
calibrated with the remote diode that it will be paired with.
The following table shows the variations in non-ideality for a
variety of processors.
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 avail-
able on a Pentium III microprocessor for temperature mea-
surement. Therefore, the LM90 can sense the temperature
of this diode effectively.
Processor Family
η, non-ideality
min
1
typ
max
Pentium II
1.0065 1.0173
1.0065 1.0125
Pentium III CPUID 67h
Pentium III CPUID
68h/PGA370Socket/Celeron
Pentium 4, 423 pin
Pentium 4, 478 pin
MMBT3904
1
4.1 DIODE NON-IDEALITY
1.0057 1.008 1.0125
4.1.1 Diode Non-Ideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following
relationship holds for variables VBE, T and If:
0.9933 1.0045 1.0368
0.9933 1.0045 1.0368
1.003
AMD Athlon MP model 6
1.002
1.008
1.016
4.1.2 Compensating for Diode Non-Ideality
where:
In order to compensate for the errors introduced by
non-ideality, the temperature sensor is calibrated for a par-
ticular processor. National Semiconductor temperature sen-
sors are always calibrated to the typical non-ideality of a
given processor type. The LM90 is calibrated for the
non-ideality of a mobile Pentium III processor, 1.008. When
a temperature sensor calibrated for a particular processor
type is used with a different processor type or a given
processor type has a non-ideality that strays from the typical,
errors are introduced. Figure 12 shows the minimum and
maximum errors introduced to a temperature sensor cali-
brated specifically to the typical value of the processor type
it is connected to. The errors in this figure are attributed only
to the variation in non-ideality from the typical value. In
Figure 13 is a plot of the errors that result from using a
temperature sensor calibrated for a Pentium II, the LM84,
with a typical Pentium 4 or AMD Athlon MP Model 6.
•
•
•
•
q = 1.6x10−19 Coulombs (the electron charge),
T = Absolute Temperature in Kelvin
k = 1.38x10−23joules/K (Boltzmann’s constant),
η is the non-ideality factor of the process the diode is
manufactured on,
•
•
•
IS = Saturation Current and is process dependent,
If= Forward Current through the base emitter junction
VBE = Base Emitter Voltage drop
In the active region, the -1 term is negligible and may be
eliminated, yielding the following equation
17
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4.0 Application Hints (Continued)
20033736
Errors Induced when Temperature Sensor is Not
Calibrated to Typical Non-Ideality
20033737
Error Caused by Non-Ideality Factor
FIGURE 13.
FIGURE 12.
20033738
Compensating for an Untargeted Non-Ideality Factor
FIGURE 14.
Temperature errors associated with non-ideality may be re-
14. This offset has resulted in an error of less than 0.05˚C for
the temperatures measured in the critical range between 60
to 100˚C. This method yeilds a first order correction factor.
duced in a specific temperature range of concern through
use of the offset registers (11h and 12h). Figure 14 shows
how the offset register may be used to compensate for the
non-ideality errors shown in Figure 13. For the case of
non-ideality=1.008, the offset register was set to −0.5˚C
resulting in the calculated residual error as shown in Figure
@
Please send an email to hardware.monitor.team nsc.com
requesting further information on our recommended setting
of the offset register for different processor types.
www.national.com
18
compensated by using the Remote Temperature Offset
Registers, since the value placed in these registers will
automatically be subtracted from or added to the remote
temperature reading.
4.0 Application Hints (Continued)
4.2 PCB LAYOUT for MINIMIZING NOISE
4. Diode traces should be surrounded by a GND guard ring
to either side, above and below if possible. This GND
guard should not be between the D+ and D− lines. In the
event that noise does couple to the diode lines it would
be ideal if it is coupled common mode. That is equally to
the D+ and D− lines.
5. Avoid routing diode traces in close proximity to power
supply switching or filtering inductors.
6. Avoid running diode traces close to or parallel to high
speed digital and bus lines. Diode traces should be kept
at least 2cm apart from the high speed digital traces.
20033717
FIGURE 15. Ideal Diode Trace Layout
7. If it is necessary to cross high speed digital traces, the
diode traces and the high speed digital traces should
cross at a 90 degree angle.
In a noisy environment, such as a processor mother board,
layout considerations are very critical. Noise induced on
traces running between the remote temperature diode sen-
sor and the LM90 can cause temperature conversion errors.
Keep in mind that the signal level the LM90 is trying to
measure is in microvolts. The following guidelines should be
followed:
8. The ideal place to connect the LM90’s GND pin is as
close as possible to the Processors GND associated
with the sense diode.
9. Leakage current between D+ and GND should be kept
to a minimum. One nano-ampere of leakage can cause
as much as 1˚C of error in the diode temperature read-
ing. Keeping the printed circuit board as clean as pos-
sible will minimize leakage current.
1. Place a 0.1 µF power supply bypass capacitor as close
as possible to the VDDpin and the recommended 2.2 nF
capacitor as close as possible to the LM90’s D+ and D−
pins. Make sure the traces to the 2.2nF capacitor are
matched.
Noise coupling into the digital lines greater than 400mVp-p
(typical hysteresis) and undershoot less than 500mV below
GND, may prevent successful SMBus communication with
the LM90. SMBus no acknowledge is the most common
symptom, causing unnecessary traffic on the bus. Although
the SMBus maximum frequency of communication is rather
low (100kHz max), care still needs to be taken to ensure
proper termination within a system with multiple parts on the
bus and long printed circuit board traces. An RC lowpass
filter with a 3db corner frequency of about 40MHz is included
on the LM90’s SMBCLK input. Additional resistance can be
added in series with the SMBData and SMBCLK lines to
further help filter noise and ringing. Minimize noise coupling
by keeping digital traces out of switching power supply areas
as well as ensuring that digital lines containing high speed
data communications cross at right angles to the SMBData
and SMBCLK lines.
2. The recommended 2.2nF diode bypass capacitor actu-
ally has a range of TBDpF to 3.3nF. The average tem-
perature accuracy will not degrade. Increasing the ca-
pacitance will lower the corner frequency where
differential noise error affects the temperature reading
thus producing a reading that is more stable. Con-
versely, lowering the capacitance will increase the cor-
ner frequency where differential noise error affects the
temperature reading thus producing a reading that is
less stable.
3. Ideally, the LM90 should be placed within 10cm of the
Processor diode pins with the traces being as straight,
short and identical as possible. Trace resistance of 1Ω
can cause as much as 1˚C of error. This error can be
19
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Physical Dimensions inches (millimeters)
unless otherwise noted
8-Lead Molded Mini-Small-Outline Package (MSOP),
JEDEC Registration Number MO-187
Order Number LM90CIMM or LM90CIMMX
NS Package Number MUA08A
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