Multi-point temperature monitoring based on MAX6636 in the system

introduction

MAX6636 is a multi-channel precision temperature monitor that can not only monitor local temperature, but also connect up to 6 diodes externally. Each channel has a programmable low temperature alarm, and channels 1, 4, 5, and 6 also have programmable high temperature alarms. When the temperature measured by a channel reaches its preset limit, the corresponding bit of the status register will be set. The most significant advantage of MAX6636 is that it uses a micro 20-pin TSS OP package, which can monitor the temperature of the CPU and other 4 locations. It is mainly used in desktop computers, laptops, workstations, and servers.

1 MAX6636 Package and Performance Characteristics

1.1 Pin Function

The pin diagram of MAX6636 is shown in Figure 1. The functions of each pin of MAX6636 are as follows:

• DXPl~DXP6: Positive terminal of remote temperature sensor . When remote diode is not used, this pin is left empty or connected to V∝ pin. A 2200 pF capacitor should be connected between DXP and DXN to filter out noise.

• DXN1~DXN6: Negative terminal of remote temperature sensor . This pin is internally connected to ground.

• STBY: Standby mode input pin, low level is valid. At this time, the temperature value and threshold value will still be retained.

• NC: No pin. In the application circuit, 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 pre-set programmable over-temperature threshold, it can be used to slow down or shut down the fan, and control the CPU clock.

• VCC: Power input terminal. Bypass to ground with a 0.1μF capacitor.

• ALERT: Open-drain output. Used as an 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.

1.2 Performance characteristics

The main features of MAX6636 are as follows:

• 6-channel thermal diode input;

• Local temperature sensor;

• The remote measurement accuracy is 1°C within the range of +60°C to +100°C;

• Temperature monitoring starts at POR to achieve fail-safe system protection;

• OVERT and ALERT outputs are used to interrupt, slow down or shut down;

• The STBY input is used for hardware shutdown mode;

• Small 20-pin TSSOP package;

• 2-wire SMBus interface.

2 Working Principle

The MAX6636 can monitor its own temperature and 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 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. [page]

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 is operating normally, the on-chip A/D converter operates 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 the local or remote temperature value register.

2.1 Temperature data format

The lowest bit of the ADC on the MAX6636 chip corresponds to 0.125°C, so the ADC can measure the range from 0°C to 127.875°C. Its temperature data format and extended temperature resolution are listed in Tables 1 and 2.

2.2 MAX6636 Registers

The MAX6636 registers are used to store remote and local temperature results, extreme high and low temperatures, and to set up and control the device.

(1) Local temperature register

The local temperature register address is 07H, the POR state 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 01H to 06H. The remote temperature value of the corresponding channel is read through the SMBus bus.

(3) Architecture registers

The MAX6636 has three configuration registers.

Structure register 1 uses 5 bits: bit 7 is the standby mode control bit. If set to 1, MAX6636 stops switching and enters standby mode. Bit 6 is the reset bit. If set to 1, the device is reset. Bit 5 is the pause enable bit. If set to 0, the SMBus enters the pause state. Bit 4 is the channel 1 high-speed conversion bit. High level is effective. Bit 3 is the resistor cancel bit. If set to 1, the resistor in series with the thermal diode in channel 1 is canceled. The resistance range is 0 to 100Ω.

Structure register 2 uses 7 bits: Bit 6 is the local alarm mask bit, and setting it to 1 masks the local channel alarm signal; Bits 5 to 0 are the remote channel mask alarm interrupt output bits, and high level is valid.

Structure register 3 uses 4 bits: bits 5, 4, 3, and O are the over-temperature alarm mask interrupt bits for channels 6, 5, 4, and 1, respectively, and high level is valid.

(4) Status Register

The MAX6636 also has three status registers.

Status register 1 describes the high temperature alarm bit of the local temperature or remote measured temperature. If the local temperature or remote measured temperature is higher than the high temperature threshold value set in the ALERT register, the corresponding bit is set to 1.

Status register 2 describes the over-temperature alarm bits of the remote measurement temperature in channels 1, 4, 5, and 6. If the remote measurement temperature of these four channels is higher than the over-temperature threshold set in the 0VERT register, the corresponding position 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 overtemperature limit registers. These registers can be read/written via the 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 +0.125℃, 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 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 out the address of a device through the bus, the device with that address will respond. The address of the MAX6636 is 4D (1001101). [page]

2.5 ALERT alarm response address

The SMBus interrupt alert response pointer provides a fast, default acknowledgement method for 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 send an address of the interrupt source, and the device with that address will respond.

The ALERT signal can respond to multiple different devices at the same time, similar to the I2C bus response. If more than one device's ALERT is waiting to be responded to, the device with the lowest address has priority according to the SMBus protocol. 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 the ALERT on the SMBus remains low, the master device will send interrupt requests again until all devices with ALERT signals going low are responded to.

2.6 OVERT over temperature alarm

MAX6636 has 4 remote over-temperature limit registers to store remote alarm output limit values. When the temperature value of a channel exceeds the limit value stored in its register, OVERT will present an alarm state, and this state will remain until its measured value drops below 4°C of its set value. This over-temperature alarm output can be used as an excitation source for cooling systems, an initialization clock source, or as a trigger switch for automatic system shutdown to avoid losses caused by overheating.

2.7 Sensors Fault detection

The MAX6636 has a fault detector on the DXP input that detects if 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 when the conversion is triggered, the comparator output is checked and bits 1 to 6 of status register 3 are set. For example, due to a shorted diode, the ADC output is 128 (1111 1111). Because the normal operating range of the device extends to +127°C, this output value will never occur, so it is an error state.

The MAX6636 detects the diode every 4 ms to see if there is a fault. Once a fault is detected, the next channel will be detected in the conversion sequence. A shorted diode may cause an alarm interrupt, so unused channel pins should not be connected.

3 Applications

3.1 Application Circuit

The typical application circuit of MAX6636 is shown in Figure 2, which is connected to a separate transistor through a shielded twisted-pair cable.

SMBCLK, SMBDATA, ALERT and OVERT need to be pulled to VCC through 4.7 kΩ resistors. 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, the port will make a 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 . Substrate transistors are generally PNP type with their collector connected to the substrate. Discrete transistors can be PNP or NPN connected in a diode type (base and collector shorted). If an NPN tube is used, the collector and base are connected to DXP, and the emitter is connected to DXN; if a PNP tube is used, 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 errors caused by their changes, the following factors need to be considered:

① Ideal factor n of transistor. The accuracy of remote temperature measurement mainly depends on the ideal factor n of the remote sensing diode. The ideal factor nN value of MAX6636 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 used in a CPU with an ideality factor of 1.002, assuming that the sensing diode has no series resistor, the actual temperature is

For an actual temperature of +85°C, the measured temperature is approximately +83.91°C, with an error of approximately -1.09°C.

② When the sensor is a discrete transistor, the collector and base must be connected together. This 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 forward voltage should be greater than 0.25 V/10μA and the minimum should be less than 0.95 V/100μA, so high-power transistors cannot be used in the application. In addition, ensure that the base resistance is less than 100Ω.

3.2.2 Thermal inertia and self-heating

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 board on the PCB. When using the on-chip sensor to measure the temperature of a CPU or other IC, thermal inertia actually has little effect on it, and the measured temperature value is very close to the actual value within a conversion cycle. When using discrete remote transistors to measure temperature, small packages such as SOT-23 or SC-70 will achieve the best thermal response time. Careful consideration should be given to the thermal slope between the heat source and the sensor to ensure that the surrounding air current passing through the sensor package does not affect the accuracy of the measurement. To a large extent, self-heating does 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 in an electrically noisy environment, and the voltage measured by the MAX6636 from the 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:

① Place the MAX6636 as close to the remote sensing diode as possible. If there are no noise sources (such as clock generators, data/address buses, and CRTs), the best distance is 10.2 to 20.4 cm.

② When wiring, do not place the DXP and DXN signal lines close to the CRT-related pads, and do not choose the wiring path in the high-speed digital signal area.

③DXP and DXN are placed in parallel and close to each other. Due to the leakage current of the PCB, if DXP is connected to the ground through a 20 MΩ path, a temperature rise error of +1°C will occur. Therefore, it is best to set ground wires on both sides of DXP and DXN during wiring, and if possible, set a ground layer under the printed line.

④Try to reduce the number of copper and solder joints that may cause 热电偶\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> the thermocouple effect. At the copper and solder joints, ensure that DXP and DXN are on the same path and at the same temperature, and the thermocouple effect can be ignored.

⑤ Use wide leads to reduce induction and noise. The line width and line spacing should preferably be 10 nail (mil is an illegal unit of measurement, 1000 mil="25"．4 mm).

Conclusion

The most notable feature of the multi-channel temperature monitor MAX6636 is that it uses a miniature 20-pin TSS OP package, which can be widely used in places with strict requirements on chip size. MAX6636 will appear in notebook computers and monitor the next generation of CPUs, which has a good application prospect.