1 MAX6636 package and performance characteristics 1. 1 pin function The pin diagram of MAX6636 is shown as in Fig. 1. The functions of each pin of MAX6636 are as follows: 1.2 Performance characteristics The main features of MAX6636 are as follows: 2 Working principle The MAX6636 can monitor its own temperature and the temperature of up to six diode-connected external transistors. All temperature channels have programmable alarm thresholds, and channels 1, 4, 5, and 6 also have programmable overtemperature thresholds. When the temperature measured by a channel exceeds its respective threshold, the status bit in 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 reading temperature data and alarm threshold programming. When the MAX6636 works 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 the remote sensor to measure the remote temperature. These signals are digitized by the ADC, and the results are stored in local or remote temperature value registers. 2.1 Temperature data format The lowest bit of the MAX6636 on-chip ADC corresponds to O. 125 ℃, so ADC can measure the range is 0 ℃ ~ 127.875 ℃, its temperature data format and extended temperature resolution are listed in Table 1 and Table 2. 2. 2 registers of MAX6636 The MAX6636 register is used to store remote and local temperature results, limit high and low temperatures, and set and control devices. (1) Local temperature register The local temperature register address is 07H, the POR status is 00, and the local temperature value is read through the SMBus bus. (2) Remote temperature register The MAX6636 has 6 remote temperature registers with addresses 01H to 06H, and reads the remote temperature value of the corresponding channel through the SMBus bus. (3) Structure register The MAX6636 has three structural registers. The structure register l uses 5 of them: Bit 7 is the standby mode control bit. When set to 1, the MAX6636 stops conversion and enters standby mode; Bit 6 is the reset bit, and set to 1 to reset the device; Bit 5 is the pause enable bit, set O then the SMBus bus enters the suspended state; bit 4 is channel 1. Shaanxi speed conversion bit, the high level is effective; bit 3 is the resistance cancellation bit, set to 1 to cancel the resistance in series with the thermal diode in channel 1, the resistance value range is O ~ 100Ω. Structure register 2 uses 7 of them: bit 6 is the local alarm mask bit, set to 1 to mask the local channel alarm signal; bit 5 ~ bit 0 is the remote channel mask alarm interrupt output bit, high level is effective. The structure register 3 uses 4 of them: bits 5, 4, 3, and O are the channel 6, 5, 4, and 1 overtemperature alarm mask interrupt bits, respectively, and the high level is valid. (4) Status register The MAX6636 also has three status registers. Status register 1 describes the local temperature or remote measurement temperature high temperature alarm bit. If the local temperature or remote measurement 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 for remotely measuring the temperature in channels 1, 4, 5, and 6. If the remote measured temperature of these 4 channels is higher than the overtemperature threshold set in the 0VERT register, then the corresponding bit is set to 1. Status register 3 describes the remote sensing diode fault bit. If the remote measurement channel senses that the diode is open or shorted, the corresponding bit is set to 1. (5) Limit register 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, the least significant bit is + O. 125 ℃; while the other channels provide 8 data bits, the least significant bit is +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 Addressing of the device In general, each SMBus device has a 7-bit address (except for some extended addresses of 10 bits). When the master device sends out the address of a device through the bus, the device with that address will respond. The address of MAX6636 is 4D (1001101). 2.5 ALERT alarm response address The SMBus interrupt alarm response pointer provides a quick, default confirmation method for those simple slave devices. For devices that lack complex logic, they need to be connected through a hub. After receiving an interrupt signal, the host will issue an address of the interrupt source, and the device with this address will respond. The ALERT signal can respond to multiple different devices at the same time, which is similar to the I2C bus response. If ALERT of more than one device is waiting to be responded to, according to the SMBus agreement, the device with the lowest address has priority. Once the MAX6636 responds to the warning response address, it will reset the ALERT output as long as the error condition that caused the ALERT output does not exist. If ALERT on SMBus remains low, the master device will send an interrupt request again until all devices whose ALERT signal goes low are responded. 2. 6 OVERT 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, OVERT will display an alarm state, and this state will remain until the measured value drops below 4 ℃ of its set value. This over-temperature alarm output can be used as the excitation source for the cooling system, to initialize the clock source, or as a trigger switch for automatic shutdown of the system to avoid losses caused by overheating of the machine. 2.7 Sensor fault detection At the DXP input, the MAX6636 has a fault detector that detects whether the external sensor diode 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 trigger conversion, check the comparator output and set the first to sixth bits of status register 3. For example, because the diode is shorted, the ADC outputs 128 (1111 1111). Since the normal operating range of the device extends to + 127 ° C, such an output value will never occur, so it is an error state. The MAX6636 detects the diode once every 4 ms to see if there is a fault. Once a fault is detected, the next channel will be detected in the conversion order. Short-circuit diodes may cause alarm interrupts, so unused channel pins should not be connected. 3 Application 3.1 Application circuit The typical application circuit of the MAX6636 is shown in Figure 2. It is connected to a separate transistor through a shielded twisted pair cable. SMBCLK, SMBDATA, ALERT and OVERT need to be pulled to VCC by 4.7 kΩ electric block respectively, SMBCLK and SMBDATA can be directly connected to SMBus of I / 0 controller (such as Intel 820). ALERT is connected to the interrupt input terminal of the controller. OVERT is generally connected to the fan control circuit. When there is a corresponding interrupt response, this port makes the corresponding deceleration or shutdown action. 3.2 Factors affecting accuracy 3.2.1 Remote sensing diode 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 transistors can be PNP or NPN connected in a diode type (the base and collector are shorted). If using an NPN tube, the collector and base are connected to DXP and the emitter is connected to DXN; if using a PNP tube, the collector and base are connected to DXN and the emitter is connected to DXP. Many CPUs have substrate transistors. In order to reduce the error caused by their changes, the following factors need to be considered: ①The 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 design is 1.015. For a sensing diode with an actual temperature of TA and an ideal factor of n, the measured temperature is: If the MAX6636 is applied to a CPU with an ideal factor of 1.002, assuming that the sensing diode is not connected to a series resistor, then the actual temperature is For the actual temperature of + 85 ℃, the measured temperature is about + 83.91 ℃, and the error is about 1.09 ℃. ②When the sensor is a discrete transistor, the collector and base must be connected together. Such a transistor must be a small signal and have a relatively high forward voltage, otherwise the A / D input voltage range will be affected. At the ideal temperature, the maximum value of the forward voltage should be greater than 0.25 V / 10A, and the minimum value should be less than O. 95 V / 100μA, so high-power transistors cannot be used in applications. In addition, make sure that the base resistance is less than 100Ω. 3.2.2 Thermal inertia and self-heating The 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 the local temperature, the wire provides good thermal contact between the device on the PCB and the template. When using an on-chip sensor to measure the temperature of a certain CPU or other IC, the thermal inertia actually has little effect on him. During a conversion cycle, the measured temperature value is very close to the actual value. When measuring temperature with separate remote transistors, SOT-23 or SC-70 small package devices will get the best thermal response time. The thermal slope must be carefully considered between the heat source and the sensor to ensure that the surrounding air current through the sensor package does not affect the accuracy of the measurement. To a considerable extent, self-heating will not affect the accuracy of the measurement. The self-heating of the remote sensor depends on the diode current and can be ignored. 3.3 PCB layout considerations Digital circuit boards are often exposed to electrical noise, and the voltage measured by the MAX6636 from a remote temperature sensor is very small, so measures must be taken to minimize the noise induced at the sensor input. In order to reduce the remote temperature measurement error, it is recommended to follow the following layout and wiring principles: ①Place the MAX6636 as close to the remote sensing diode as possible. If there are no noise sources (such as a clock generator, data / address bus, and CRT), the distance is preferably from 10.2 to 20.4 cm. ② When wiring, do not put the DXP and DXN signal lines close to the pads related to the CRT, and do not choose the routing path in the high-speed digital signal area. ③ DXP and DXN are placed in parallel and close to each other. Due to the leakage current of the PCB, if DXP is connected to ground through a 20 MΩ path, a temperature rise error of + 1 ° C will occur. Therefore, when wiring, it is best to set the ground wire on both sides of DXP and DXN, and if possible, set a ground layer under the printed wire. ④ Minimize the number of copper and solder joints that may cause thermocouple effects. At the copper and solder joints, ensure that DXP and DXN have the same path and temperature, and the thermocouple effect is negligible. ⑤Use wide lead wires to reduce induction and reduce noise. Line width and line spacing are preferably 10 nails (mil is an illegal unit of measurement, 1000 mil = "25". 4 mm). The most significant feature of the multi-channel temperature monitor MAX6636 is the miniature 20-pin TSSOP package, which can be widely used in applications that have strict requirements on chip size. MAX6636 will appear in notebook computers and monitor the next-generation CPU, which has a very good application prospect.
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Multi-point temperature monitoring and working principle of MAX6636 in the system
The MAX6636 is a multi-channel precision temperature monitor. It can not only monitor the local temperature, but also connect up to 6 diodes externally. Each channel has a programmable low temperature alarm, and 1, 4, 5 and 6 channels also have a programmable high temperature alarm. When the temperature measured by a channel reaches its preset limit value, the corresponding bit of the status register will be set. The most significant advantage of the MAX6636 is that it uses a miniature 20-pin TSSOP package that can monitor the temperature of the CPU and other four locations. It is mainly used in desktop computers, notebook computers, workstations and servers.