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MAX1618MUB

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18
introduces errors due to the rise time of the switched-
current source.
PC Board Layout
1) Place the MAX1618 as close as practical to the
remote diode. In a noisy environment, such as a
computer motherboard, this distance can be 4 inch-
es to 8 inches (typ) or more, as long as the worst
noise sources (such as CRTs, clock generators,
memory buses, and ISA/PCI buses) are avoided.
2) Do not route the DXP–DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across a fast memory bus, which can easily
introduce +30°C error, even with good filtering.
Otherwise, most noise sources are fairly benign.
3) Route the DXP and DXN traces parallel and close to
each other, away from any high-voltage traces such
as +12V
DC
. Avoid leakage currents from PC board
contamination. A 20MΩ leakage path from DXP to
ground causes approximately +1°C error.
4) Connect guard traces to GND on either side of the
DXP-DXN traces (Figure 5). With guard traces in
place, routing near high-voltage traces is no longer
an issue.
5) Route as few vias and crossunders as possible to
minimize copper/solder thermocouple effects.
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
______________________________________________________________________________________ 13
Figure 5. SMBus Read Timing Diagram
Figure 4. SMBus Write Timing Diagram
SMBCLK
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
AB CD
E
FG H
I
J
SMBDATA
t
SU:STA
t
HD:STA
t
LOW
t
HIGH
t
SU:DAT
t
SU:STO
t
BUF
K
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
14 ______________________________________________________________________________________
6) When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. In general, PC board-induced ther-
mocouples are not a serious problem. A copper-
solder thermocouple exhibits 3µV/°C, and it takes
approximately 200µV of voltage error at DXP-DXN to
cause a +1°C measurement error, so most parasitic
thermocouple errors are swamped out.
7) Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10mil
widths and spacings recommended in Figure 5 are
not absolutely necessary (as they offer only a minor
improvement in leakage and noise), but try to use
them where practical.
8) Note that copper cannot be used as an EMI shield.
Use only ferrous materials such as steel. Placing a
copper ground plane between the DXP-DXN traces
and traces carrying high-frequency noise signals
does not help reduce EMI.
Twisted Pair and Shielded Cables
For remote-sensor distances longer than 8 inches, or in
particularly noisy environments, a twisted pair is recom-
mended. Its practical length is 6 feet to 12 feet (typ)
before noise becomes a problem, as tested in a noisy
electronics laboratory. For longer distances, the best
solution is a shielded twisted pair like that used for audio
microphones. For example, Belden #8451 works well for
distances up to 100 feet in a noisy environment. Connect
the twisted pair to DXP and DXN and the shield to GND,
and leave the shield’s remote end unterminated.
Excess capacitance at DX_ limits practical remote-sen-
sor distances (see Typical Operating Characteristics).
For very long cable runs, the cable's parasitic capaci-
tance often provides noise filtering, so the recommended
2200pF capacitor can often be removed or reduced in
value.
Cable resistance also affects remote-sensor accuracy. A
1Ω series resistance introduces about +1/2°C error.
Programming Example:
Clock-Throttling Control for CPUs
Listing 1 gives an untested example of pseudocode for
proportional temperature control of Intel mobile CPUs
through a power-management microcontroller. This pro-
gram consists of two main parts: an initialization routine
and an interrupt handler. The initialization routine checks
for SMBus communications problems and sets up the
MAX1618 configuration. The interrupt handler responds
to ALERT signals by reading the current temperature and
setting a CPU clock duty factor proportional to that tem-
perature. The relationship between clock duty and tem-
perature is fixed in a look-up table contained in the
microcontroller code.
Note: Thermal management decisions should be made
based on the latest external temperature obtained from
the MAX1618 rather than the value of the Status Byte.
The MAX1618 responds very quickly to changes in its
environment due to its sensitivity and its small thermal
mass. High and low alarm conditions can exist at the
same time in the Status Byte, because the MAX1618 is
correctly reporting environmental changes around it.
MINIMUM
10 MILS
10 MILS
10 MILS
10 MILS
GND
DXN
DXP
GND
Figure 6. Recommended DXP/DXN PC Traces
Chip Information
TRANSISTOR COUNT: 9911
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
______________________________________________________________________________________ 15
/* Beginning of the header file which sets the constants */
int NumStates = 10;
int RRTE = 1; /* 0x01, command for reading remote temp
register */
int WCA = 9; /* 0x09, command for writing configuration
register */
int RSL = 2; /* 0x02, command for reading status register */
int WRHA = 13; /* 0x0D, command for writing remote THIGH limit
register */
int WRLN = 14; /* 0x0E, command for writing remote TLOW limit
register */
int NoError = 0;
int Nobody = 0;
int MAX1618Addr = 84; /* 0x54, default address for MAX1618,
ADD0,ADD1=open */
int InitConfig
= 8; /* 0x08, configure MAX1618 to MASK=0 and
RUN/STOP=0 */
int HighAdder = 2; /* 2oC offset for calculating THIGH limit */
int LowSubtracter = 4; /* 4oC offset for calculating TLOW limit
*/
int CollisionMask = 1; /* 0x01, mask for status bit that
indicates collision */
int DiodeFaultMask = 4; /* 0x04, mask for the OPEN diode fault
status bit */
int TempChangeMask = 24; /* 0x18, mask for RHIGH and RLOW status
bits */
array State[0..NumStates] of int;
State[0] = -65 oC /* At or above this temperature CPU duty cycle is
100% */
State[1] = 72 oC /* At or above this temperature CPU duty cycle is
87.5% */
State[2] = 74 oC /* At or above this temperature CPU duty cycle is 75%
*/
State[3] = 76 oC /* At or above this temperature CPU duty cycle is
62.5% */
State[4] = 78 oC /* At or above this temperature CPU duty cycle is 50%
*/
State[5] = 80 oC /* At or above this temperature CPU duty cycle is
37.5% */
State[6] = 82 oC /* At or above this temperature CPU duty cycle is 25%
*/
State[7] = 84 oC /* At or above this temperature CPU duty cycle is
12.5% */
State[8] = 86 oC /* At or above this temperature CPU duty cycle is
0.0% */
State[9] = 88 oC /* At or above this temperature SHUT SYSTEM OFF! */
State[10] = 127 oC /* Extra array location so looping is easier */
/* End of the header file */
Listing 1. Pseudocode Example
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18

MAX1618MUB

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Manufacturer:
Maxim Integrated
Description:
Board Mount Temperature Sensors Remote Temperature Sensor with SMBus Serial Interface
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