Introduction The MAX6636 is a multi-channel precision temperature monitor that can monitor local temperatures and connect up to six external diodes. Each channel has a programmable undertemperature alarm, and channels 1, 4, 5, and 6 also have programmable overtemperature alarms. When the temperature measured by a channel reaches its preset limit, the corresponding bit in the status register is set. The most significant advantage of the MAX6636 is its use of a miniature 20-pin TSSOP package, which allows it to monitor the temperature of the CPU and four other locations. It is mainly used in desktop computers, laptops, workstations, and servers. [b]1 MAX6636 Package and Performance Characteristics 1.1 Pin Functions[/b] The pin diagram of the MAX6636 is shown in Figure 1. The functions of each pin of the MAX6636 are as follows: DXP1~DXP6: Positive terminals of the remote temperature sensor. When no remote diode is used, this pin should be left unconnected or connected to the V∝ pin. A 2200 pF capacitor should be connected between DXP and DXN to filter noise. [img=225,223]http://21ic.com/news/upload/2008_11/081117095034781.jpg[/img] DXN1～DXN6: Negative terminals of the remote temperature sensor. This pin is internally connected to ground. STBY: Standby mode input pin, active low. The temperature value and threshold value are still retained. NC: No connection. In application circuits, this pin must be connected to ground. OVERT: Open-drain output. In practical applications, when a temperature value in channels 1, 4, 5, or 6 exceeds its preset programmable over-temperature threshold, it can be used to slow down or shut down the fan, and control the CPU clock. VCC: Power input. Bypass to ground with a 0.1μF capacitor. ALERT: Open-drain output. Used for interrupt or SMBus (System Management Bus) alarm. SMBDATA: SMBus serial data input/output. A pull-up resistor is required. SMBCLK: SMBus serial clock input. A pull-up resistor is required. GND: Power ground. [b]1.2 Performance Characteristics[/b] The main features of the MAX6636 are as follows: ◆ 6-channel thermal diode input; ◆ Local temperature sensor; ◆ Remote measurement accuracy of 1℃ within the range of +60℃ to +100℃; ◆ Temperature monitoring starts at POR for fail-safe system protection; ◆ OVERT and ALERT outputs for interrupt, deceleration, or shutdown; ◆ STBY input for hardware stop mode; ◆ Small 20-pin TSSOP package; ◆ 2-wire SMBus interface. 2. Operating Principle The MAX6636 can monitor its own temperature, as well as the temperature of up to six external diode-connected transistors. All temperature channels have programmable alarm thresholds, and channels 1, 4, 5, and 6 also have programmable over-temperature thresholds. When the temperature measured by a channel exceeds its respective threshold, the status register is set. The two open-drain outputs, OVERT and ALERT, will go low according to these bits in the status register. Its 2-wire serial interface supports the standard SMBus protocol: write byte, read byte, send byte, and receive byte to complete temperature data reading and alarm threshold programming. When the MAX6636 is operating normally, the on-chip A/D converter works normally. The analog input multiplexer selects the on-chip temperature sensor to measure the local temperature or selects a remote sensor to measure the remote temperature. These signals are digitized by the ADC, and the results are stored in the local or remote temperature value register. 2.1 Temperature Data Format The least significant bit of the MAX6636's on-chip ADC corresponds to 0.125℃, so the ADC's measurable range is 0℃ to 127.875℃. Its temperature data format and extended temperature resolution are listed in Tables 1 and 2. [img=368,205]http://21ic.com/news/upload/2008_11/081117094854922.jpg[/img] 2.2 Registers of MAX6636 The MAX6636 registers are used to store remote and local temperature results, extreme high and low temperatures, and settings and control devices. (1) Local Temperature Register The local temperature register address is 07H, POR status is 00, and the local temperature value is read through the SMBus bus. (2) Remote Temperature Register MAX6636 has 6 remote temperature registers, with addresses from 01H to 06H, and the remote temperature value of the corresponding channel is read through the SMBus bus. (3) Structure Register MAX6636 has 3 structure registers. The structure register 1 uses 5 bits: bit 7 is the standby mode control bit, setting it to 1 will stop the MAX6636 from switching and enter standby mode; bit 6 is the reset bit, setting it to 1 will reset the device; bit 5 is the pause enable bit, setting it to 0 will put the SMBus bus into pause state; bit 4 is the channel 1 speed conversion bit, active high; bit 3 is the resistor cancellation bit, setting it to 1 will cancel the resistor connected in series with the thermal diode in channel 1, with a resistance range of 0 to 100Ω. The structure register 2 uses 7 bits: bit 6 is the local alarm mask bit, setting it to 1 will mask the local channel alarm signal; bits 5 to 0 are the remote channel mask alarm interrupt output bits, active high. The structure register 3 uses 4 bits: bits 5, 4, 3, and 0 are the over-temperature alarm mask interrupt bits for channels 6, 5, 4, and 1, respectively, active high. (4) Status Registers The MAX6636 also has 3 status registers. Status register 1 describes the local temperature or remote measured temperature high temperature alarm bit. If the local temperature or remote measured temperature is higher than the high temperature threshold set in the ALERT register, the corresponding bit is set to 1. Status register 2 describes the over-temperature alarm bit of the remote measurement channels 1, 4, 5, and 6. If the remote measured temperature of these four channels is higher than the over-temperature threshold set in the 0VERT register, the corresponding bit is set to 1. Status register 3 describes the remote sensing diode fault bit. If the remote measurement channel senses an open circuit or short circuit in the diode, the corresponding bit is set to 1. (5) Limit registers The MAX6636 has 11 limit registers, including 1 local high temperature alarm limit register, 6 remote high temperature alarm limit registers, and 4 remote over-temperature limit registers. These registers can be read/written via SMBus. 2.3 Serial bus interface The MAX6636 is connected to the serial bus as a slave device and is controlled by the master device. It should be noted that: Remote measurement channel 1 provides 11 data bits, and the least significant bit is +0. 125℃; while the other channels provide 8 data bits, with the least significant bit being +1℃. The 8 most important data bits are read from the local or remote temperature register, and the other 3 data bits in the remote measurement channel can be read from the extended temperature register. 2.4 Device Addressing Generally, each SMBus device has a 7-bit address (except for some extended addresses which are 10 bits). When the master device sends the address of a device via the bus, the device with that address will respond. The address of the MAX6636 is 4D (1001101). 2.5 ALERT Alarm Response Address The SMBus interrupt alarm response pointer provides a fast, default acknowledgment method for simple slave devices. For devices lacking complex logic, a hub is required for connection. Upon receiving an interrupt signal, the master sends the address of the interrupt source, and the device with that address will respond. The ALERT signal can respond to multiple different devices simultaneously, similar to I2C bus responses. If more than one device's ALERT is waiting to be responded to, according to the SMBus protocol, the device with the least significant address has priority. Once the MAX6636 responds to the warning response address, it will reset the ALERT output as long as the error state causing the ALERT output does not exist. If the ALERT on the SMBus remains low, the master device will send an interrupt request again until all devices with low ALERT signals are responded to. 2.6 Over-temperature alarm The MAX6636 has four remote over-temperature limit registers to store remote alarm output limit values. When the measured temperature value of a channel exceeds the limit value stored in its register, the OVERT will display an alarm state, and this state will remain until the measured value drops below 4°C of its set value. This over-temperature alarm output can be used as the excitation source of a cooling system, the initialization clock source, or as a trigger switch for automatic system shutdown to avoid damage caused by overheating. [b]2.7 Sensor fault detection[/b] At the DXP input, the MAX6636 has a fault detector that can detect whether the diode of an external sensor is open. This is a simple voltage comparator that triggers when the DXP voltage exceeds (VCC - 1V). If a fault is detected at the start of the transition, the comparator output is checked and bits 1 through 6 of status register 3 are set. For example, due to a diode short circuit, the ADC output is 128 (1111 1111). Since the device's normal operating range extends to +127°C, such an output value should never occur, making it an error condition. The MAX6636 probes the diode approximately every 4 ms for faults, and once a fault is detected, it will proceed to the next channel probe in the transition sequence. A shorted diode may cause an alarm interrupt, so unused channel pins should not be connected. [b]3 Applications 3.1 Application Circuit[/b] A typical application circuit for the MAX6636 is shown in Figure 2, connected to a discrete transistor via a shielded twisted-pair cable. [img=378,200]http://21ic.com/news/upload/2008_11/081117094854923.jpg[/img] SMBCLK, SMBDATA, ALERT, and OVERT need to be pulled to VCC through a 4.7 kΩ resistor. SMBCLK and SMBDATA can be directly connected to the SMBus of the I/O controller (such as Intel 820). ALERT is connected to the interrupt input of the controller. OVERT is generally connected to the fan control circuit. When there is a corresponding interrupt response, this port performs the corresponding deceleration or shutdown action. [b]3.2 Factors Affecting Accuracy 3.2.1 Remote Sensing Diode[/b] The MAX6636 works with substrate transistors or discrete transistors embedded in the CPU. Among them, the substrate transistor is generally PNP type, and its collector is connected to the substrate. The discrete transistor can be PNP or NPN connected as a diode (base and collector shorted). If using an NPN transistor, the collector and base are connected to the DXP diode, and the emitter is connected to the DXN diode; if using a PNP transistor, the collector and base are connected to the DXN diode, and the emitter is connected to the DXP diode. Many CPUs contain substrate transistors. To reduce errors caused by variations in these transistors, the following factors need to be considered: ① The ideal factor n of the transistor. The accuracy of remote temperature measurement mainly depends on the ideal factor n of the remote sensing diode. The ideal factor nN of the MAX6636 is designed to be 1.015. For a sensing diode with an actual temperature of TA and an ideality factor of n, the measured temperature is: [img=126,41]http://21ic.com/news/upload/2008_11/081117094854924.jpg[/img] If the MAX6636 is applied to a CPU with an ideality factor of 1.002, assuming the sensing diode is not connected in series, then the actual temperature is [img=325,43]http://21ic.com/news/upload/2008_11/081117094633355.jpg[/img] For an actual temperature of +85℃, the measured temperature is approximately +83.91℃, with an error of approximately -1.09℃. ② When the sensor is a discrete transistor, the collector and base must be connected together. This transistor must be a small-signal transistor with a relatively high forward voltage; otherwise, the A/D input voltage range will be affected. At ideal temperatures, the maximum forward voltage should be greater than 0.25 V/10μA, and the minimum should be less than 0.95 V/100μA. Therefore, high-power transistors cannot be used in this application. Additionally, the base resistance should be less than 100Ω. [b]3.2.2 Thermal Inertia and Self-Heating[/b] Accuracy depends not only on the temperature of the remote sensing diode and the internal temperature sensor, but also on other factors. When the MAX6636 measures local temperature, the leads provide good thermal contact between the device and the PCB. When using an on-chip sensor to measure the temperature of a CPU or other IC, thermal inertia has little effect; the measured temperature is very close to the actual value within one conversion cycle. When measuring temperature with discrete remote transistors, small packages such as SOT-23 or SC-70 provide the best thermal response time. Careful consideration must be given to the thermal gradient between the heat source and the sensor to ensure that the air current flowing through the sensor package does not affect the measurement accuracy. To a considerable extent, self-heating does not affect the measurement accuracy; the self-heating of the remote sensor depends on the diode current and is negligible. **3.3 PCB Routing Considerations** Digital circuit boards are often exposed to electrical noise. Since the voltage measured by the MAX6636 from the remote temperature sensor is very small, measures must be taken to minimize the noise induced at the sensor input. To reduce remote temperature measurement errors, the following layout and routing principles are recommended: ① Place the MAX6636 as close as possible to the remote sensing diode. Ideally, this distance should be 10.2–20.4 cm if there are no noise sources (such as clock generators, data/address buses, and CRTs). ② When routing, do not place DXP and DXN signal lines near CRT-related pads, and do not select routing paths in high-speed digital signal areas. ③ Place DXP and DXN parallel and close to each other. Due to PCB leakage current, if DXP is connected to ground via a 20 MΩ path, a temperature rise error of +1°C will occur. Therefore, it is best to place ground lines on both sides of DXP and DXN, and if possible, place a ground plane on the printed circuit board. ④ Minimize the number of copper and solder joints that may cause thermocouple effects. At the copper and solder joints, ensure that the DXP and DXN are on the same path and at the same temperature; thermocouple effects can be ignored. ⑤ Use wide leads to reduce induction and noise; the lead width and spacing should ideally both be 10 nails (mil is a non-legal unit of measurement; 1000 mil = 25.4 mm). Conclusion The most significant feature of the MAX6636 multi-channel temperature monitor is its use of a miniature 20-pin TSSOP package, making it widely applicable in situations with strict requirements on chip size. The MAX6636 will appear in laptops and monitor next-generation CPUs, showing great promise for future applications.