MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
General Description
The MAX6654 is a precise digital thermometer that
reports the temperature of both a remote P-N junction
and its own die. The remote junction can be a diode-connected transistor—typically a low-cost, easily mounted
2N3904 NPN type or 2N3906 PNP type—that replaces
conventional thermistors or thermocouples. Remote
accuracy is ±1°C for multiple transistor manufacturers,
with no calibration needed. The remote junction can also
be a common-collector PNP, such as a substrate PNP of
a microprocessor (µP).
The 2-wire serial interface accepts standard System
Management Bus (SMBus), Write Byte, Read Byte, Send
Byte, and Receive Byte commands to program the alarm
thresholds and to read temperature data. Measurements
can be done automatically and autonomously, with the
conversion rate programmed by the user, or programmed
to operate in a single-shot mode. The adjustable conversion rate allows the user to optimize supply current and
temperature update rate to match system needs. When
the conversion rate is faster than 1Hz, the conversion
results are available as a 7-bit-plus-sign byte with a 1°C
LSB. When the conversion rate is 1Hz or slower, the
MAX6654 enters the extended mode. In this mode, 3
additional bits of temperature data are available in the
extended resolution register, providing 10-bit-plus-sign
resolution with a 0.125°C LSB. Single-shot conversions
also have 0.125°C per LSB resolution when the conversion rate is 1Hz or slower.
A parasitic resistance cancellation (PRC) mode can also
be invoked for conversion rates of 1Hz or slower by setting bit 4 of the configuration register to 1. In PRC mode,
the effect of series resistance on the leads of the external
diode is canceled. The 11-bit conversion in PRC mode is
performed in 3.15V.
Note 3: The conversion time doubles for the extended resolution mode. This causes the average operating current to approximately
double.
Note 4: The serial interface resets when SMBCLK is low for more than tTIMEOUT.
Note 5: Note that a transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SMBCLK’s
falling edge.
Maxim Integrated
3
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
Typical Operating Characteristics
(VCC = +3.3V to +5.5V, TA = +25°C, unless otherwise noted.)
0
-1
-2
-50 -30 -10 10 30 50 70 90 110 130 150
12
VIN = 500mVp-p REMOTE
8
4
0
100
10k
1M
100M
1k
10M
100k
FREQUENCY (Hz)
STANDBY SUPPLY CURRENT vs.
SUPPLY VOLTAGE
6
5
4
3
2
1
MAX6654 toc05
100
STANDBY SUPPLY CURRENT (μA)
MAX6654 toc04
VCC = +5V
7
TEMPERATURE ERROR (°C)
1
FREQUENCY (Hz)
TEMPERATURE ERROR vs.
DXP-DXN CAPACITANCE
90
80
70
60
50
40
0
0
10 20 30 40 50 60 70 80 90 100
DXP-DXN CAPACITANCE (nF)
4
2
0
1
TEMPERATURE (°C)
8
MAX6654 toc03
VIN = 500mVp-p LOCAL
TEMPERATURE ERROR (°C)
1
3
MAX6654 toc02
16
MAX6654 toc01
FAIRCHILD 2N3904
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
2
TEMPERATURE ERROR vs.
COMMON-MODE NOISE FREQUENCY
TEMPERATURE ERROR vs.
POWER-SUPPLY NOISE FREQUENCY
TEMPERATURE ERROR vs.
REMOTE-DIODE TEMPERATURE
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
SUPPLY VOLTAGE (V)
Maxim Integrated
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
Pin Description
PIN
NAME
1, 5, 9, 13, 16
N.C.
No Connection. Not internally connected. May be used for PCB trace routing.
2
VCC
Supply Voltage Input. +3.0V to +5.5V. Bypass to GND with a 0.1µF capacitor. A 200Ω series
resistor is recommended but not required for additional noise filtering.
3
DXP
Combined Current Source and ADC Positive Input for Remote-Junction Channel. If a remotesensing junction is not used, connect DXP to DXN.
4
DXN
Combined Current Sink and ADC Negative Input. DXN is internally biased to a diode voltage above
ground.
6
ADD1
SMBus Slave Address Select Input. ADD0 and ADD1 are sampled upon power-up.
7, 8
GND
Ground
10
ADD0
SMBus Slave Address Select Input. ADD0 and ADD1 are sampled upon power-up.
11
ALERT
SMBus Alert (Interrupt) Output. Open drain.
12
SMBDATA
14
SMBCLK
15
STBY
Maxim Integrated
FUNCTION
SMBus Serial-Data Input/Output. Open drain.
SMBus Serial-Clock Input
Hardware standby input. Temperature and comparison threshold data are retained in standby
mode. Low = standby mode, high = operating mode.
5
MAX6654
1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
Detailed Description
The MAX6654 is a temperature sensor that communicates through an SMBus/I2C-compatible interface with a
µP in thermal-management applications. Essentially an
11-bit serial analog-to-digital converter (ADC) with a
sophisticated front end, the MAX6654 measures the
change in diode voltage at different current levels to calculate temperature. It contains a current source, a multiplexer, an ADC, an SMBus interface, and associated
control logic (Figure 1). Temperature data from the ADC
is loaded into data registers, where it is automatically
compared with data previously stored in four
over/undertemperature alarm registers.
ADC and Multiplexer
The ADC is an averaging type that integrates over a
60ms period (each channel, typically, in the 8-bit “legacy” mode), with excellent noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes. The ADC and
associated circuitry measure their forward voltages and
compute their temperatures. Both channels are automatically converted once the conversion process has
started, either in free-running or single-shot mode. If
one of the two channels is not used, the device still performs both measurements, and the user can ignore the
results of the unused channel. If the remote-diode
channel is unused, connect DXP to DXN rather than
leave the pins open.
The DXN input is biased at 1VBE above ground by an
internal diode to set up the ADC inputs for a differential
measurement. The worst-case DXP-DXN differential
input voltage range is 0.28V to 0.9V.
Excess resistance in series with the remote diode causes about +1/2°C error per ohm when the parasitic resistance cancellation mode is not being used. When the
parasitic resistance cancellation mode is being used,
excess resistance of up to 100Ω will not cause any discernable error. A 200µV offset voltage forced on DXPDXN causes about 1°C error.
A/D Conversion Sequence
A conversion sequence consists of a local temperature
measurement and a remote temperature measurement.
Each time a conversion begins, whether initiated automatically in the free-running autoconvert mode
(RUN/STOP = 0) or by writing a “One-Shot” command,
both channels are converted, and the results of both
measurements are available after the end of conversion. A BUSY status bit in the status byte shows that the
device is actually performing a new conversion; however, even if the ADC is busy, the results of the previous
conversion are always available.
6
Table 1. Remote-Sensor Transistor
Manufacturers
MANUFACTURER
MODEL NUMBER
Central Semiconductor (USA)
CMPT3904
Fairchild Semiconductor (USA)
2N3904, 2N3906
ON Semiconductor (USA)
2N3904, 2N3906
Rohm Semiconductor (USA)
SST3904
Samsung (Korea)
KST3904-TF
Siemens (Germany)
SMBT3904
Zetex (England)
FMMT3904CT-ND
Note: Transistors must be diode connected (base shorted to
collector).
Remote-Diode Selection
The MAX6654 can directly measure the die temperature of CPUs and other ICs having on-board temperature-sensing diodes as shown in the Typical Operating
Circuit, or it can measure the temperature of a discrete
diode-connected transistor. For best accuracy, the discrete transistor should be a small-signal device with its
collector and base connected together. Accuracy has
been experimentally verified for all of the devices listed
in Table 1.
The transistor must be a small-signal type with a relatively high forward voltage; otherwise, the A/D input
voltage range can be violated. The forward voltage
must be >0.28V at 10µA; check to ensure this is true at
the highest expected temperature. The forward voltage
must be