19-2337; Rev 0; 1/02 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface Features ♦ Integrated Thermal Measurement and Fan Regulation ♦ Programmable Fan Threshold Temperature ♦ Programmable Temperature Range for Full-Scale Fan Speed ♦ Accurate Closed-Loop Fan-Speed Regulation ♦ On-Chip Power Device Drives Fans Rated Up to 250mA ♦ Programmable Under/Overtemperature Alarms ♦ SPI-Compatible Serial Interface ♦ ±1°C (+60°C to +100°C) Thermal-Sensing Accuracy Ordering Information PART TEMP RANGE MAX6661AEE PIN-PACKAGE -40°C to +125°C 16 QSOP Typical Operating Circuit 12V Applications 3V TO 5.5V 0.1µF 50Ω Telecom Systems 10kΩ EACH Servers VFAN Workstations Electronic Instruments VCC 5kΩ 1µF TACH IN FAN FAN OVERT TO SYSTEM SHUTDOWN SC 2200pF DXN Pin Configuration appears at end of data sheet. INTERRUPT TO µP MAX6661 DXP PENTIUM ALERT SPI CLOCK SDIN SPI DATA IN DOUT SPI DATA OUT CS SPI CHIP SELECT AGND SPI is a trademark of Motorola, Inc. PGND ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX6661 General Description The MAX6661 is a remote temperature sensor and fanspeed regulator that provides complete closed-loop fan control. The remote temperature sensor is typically a common-collector PNP, such as a substrate PNP of a microprocessor, or a diode-connected transistor, typically a low-cost, easily mounted 2N3904 NPN type or 2N3906 PNP type. The device also incorporates a closed-loop fan controller that regulates fan speed with tachometer feedback. The MAX6661 compares temperature data to a fan threshold temperature and gain setting, both programmed over the SPI™ bus by the user. The result is automatic fan control that is proportional to the remotejunction temperature. The temperature feedback loop can be broken at any time for system control over the speed of the fan. Fan speed is voltage controlled as opposed to PWM controlled, greatly reducing acoustic noise and maximizing fan reliability. An on-chip power device drives fans rated up to 250mA. Temperature data is updated every 500ms and is readable at any time over the SPI interface. The MAX6661 is accurate to 1°C (max) when the remote junction is between +60°C to +100°C. Data is formatted as a 10bit + sign word with 0.125°C resolution. The MAX6661 is specified between -40°C to +125°C and is available in a 16-pin QSOP package. MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface ABSOLUTE MAXIMUM RATINGS VCC, ALERT, OVERT ...............................................-0.3V to +6V VFAN, TACH IN, FAN .............................................-0.3V to +16V DXP, CS, SDOUT, GAIN, SCL, SDIN..........-0.3V to (VCC + 0.3V) DXN ..........................................................................-0.3V to +1V SDOUT Current ...................................................-1mA to +50mA DXN Current ......................................................................±1mA FAN Out Current ..............................................................500mA Continuous Power Dissipation (TA = +70°C) 16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW Operating Temperature Range ........................ -40°C to +125°C Junction Temperature .....................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) ................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VCC = 3V to 5.5V, VFAN = 12V, TA = -40°C to +125°C, unless otherwise specified. Typical values are at VCC = 3.3V and TA = +25°C.) (Notes 1 and 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS ADC AND POWER SUPPLY 0.125 Temperature Resolution (Note 3) Remote-Junction Temperature Measurement Error (Note 4) °C 11 TE TA = +85°C, VCC = 3.3V Bits TRJ = +60°C to +100°C -1 +1 °C TRJ = +25°C to +125°C -3 +3 °C TRJ = -40°C to +125°C -5 +5 °C Fan-Speed Measurement Accuracy ±25 % VCC Supply Voltage Range VCC 3.0 5.5 V VFAN Supply Voltage Range VFAN 4.5 13.5 V +25 % 2.95 V Conversion Time 0.25 Conversion Rate Timing Error -25 Undervoltage Lockout (UVLO) Threshold VUVLO UVLO Threshold Hysteresis VHYST POR Threshold (VCC) VCC falling 2.50 2.80 VCC rising 1.4 2.0 90 POR Threshold Hysteresis Standby Supply Current s mV 2.5 90 20 µA Fan off 450 700 µA TACH Input Transition Level VFAN = 12V 10.5 V TACH Input Hysteresis VFAN = 12V 190 mV DXN Source Voltage ICC Shutdown, configuration bit 6 = 1 mV 3 Operating Supply Current ISHDN V VDXN 0.7 TACH Input Resistance 250 Fan Output Current IF Fan Output Current Limit IL Fan Output On-Resistance 2 V RONF kΩ 250 (Note 5) 250mA load mA 320 4 _______________________________________________________________________________________ 410 mA Ω Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface (VCC = 3V to 5.5V, VFAN = 12V, TA = -40°C to +125°C, unless otherwise specified. Typical values are at VCC = 3.3V and TA = +25°C.) (Notes 1 and 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 2.5 MHz INTERFACE PINS (SDIN, SC, CS, DOUT, ALERT, OVERT) Serial Bus Maximum Clock Frequency (Note 5) SC Logic Input High Voltage Logic Input Low Voltage VCC = 3V 2.2 VCC = 5.5V 2.4 V VCC = 3V to 5V 0.8 VCC 0.4V V Logic Output High-Voltage DOUT VCC = 3V, ISOURCE = 6mA (Note 5) Logic Output Low-Voltage DOUT VCC = 3V, ISINK = 6mA (Note 5) 0.4 V Logic Output Low-Voltage ALERT, OVERT VCC = 3V, ISINK = 6mA (Note 5) 0.4 V ALERT, OVERT Output High Leakage Current ALERT, OVERT forced to 5.5V 1 µA Logic Input Current Logic inputs forced to VCC or GND 2 µA 200 ns V -2 SPI AC TIMING (Figure 5) CS High to DOUT Three-State tTR CLOAD = 100pF, RGS = 10kΩ (Note 5) CS to SC Setup Time tCSS (Note 5) SC Fall to DOUT Valid tDO CLOAD = 100pF DIN to SC Setup Time tDS DIN to SC Hold Time tDH (Note 5) 200 ns 200 ns 200 ns 200 ns SC Period tCP 400 ns SC High Time tCH 200 ns SC Low Time tCL 200 ns CS High Pulse Width Output Rise Time Output Fall Time SC Falling Edge to CS Rising tCSW (Note 5) tR CLOAD = 100pF tF CLOAD = 100pF tSCS (Note 5) 400 200 ns 10 ns 10 ns ns Note 1: TA = TJ. This implies zero dissipation in pass transistor (no load, or fan turned off). Note 2: All parameters are 100% production tested at a single temperature, unless otherwise indicated. Parameter values through temperature are guaranteed by design. Note 3: The fan control section of the MAX6661 and temperature comparisons use only 9 bits of the 11-bit temperature measurement with a 0.5°C LSB. Note 4: Wide-range accuracy is guaranteed by design, not production tested. Note 5: Guaranteed by design. _______________________________________________________________________________________ 3 MAX6661 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) TEMPERATURE ERROR vs. REMOTE-DIODE TEMPERATURE 0 -5 -10 PATH = DXP TO VCC (5V) -15 3 2 1 0 -1 -2 -20 -3 -25 -4 -30 10 5 VIN = 250mVP-P 0 -5 -10 -15 VIN = 100mVP-P -20 -30 -50 100 10 -25 -5 1 VIN = SQUARE WAVE APPLIED TO VCC WITH NO 0.1µF VCC CAPACITOR 15 TEMPERATURE ERROR (°C) PATH = DXP TO GND 5 20 MAX6661 toc02 10 4 TEMPERATURE ERROR (°C) 15 0 50 100 150 1 10 100 1k 10k 100k 1M 10M 100M LEAKAGE RESISTANCE (MΩ) TEMPERATURE (°C) FREQUENCY (Hz) TEMPERATURE ERROR vs. COMMON-MODE NOISE FREQUENCY TEMPERATURE ERROR vs. DXP-DXN CAPACITANCE STANDBY SUPPLY CURRENT vs. SUPPLY VOLTAGE 2.5 VIN = 100mVP-P 2.0 1.5 1.0 0.5 VIN = 50mVP-P 0 -0.5 -1.0 -1 -2 -3 -4 -5 -6 100 1k MAX6661 toc06 3 2 CONFIG BIT 6 = 1 1 0 -8 10 4 -7 VIN = 25mVP-P -1.5 1 5 STANDBY SUPPLY CURRENT (µA) 3.0 1 0 TEMPERATURE ERROR (°C) VIN = SQUARE WAVE AC-COUPLED TO DXN 3.5 MAX6661 toc04 4.0 MAX6661 toc05 TEMPERATURE ERROR (°C) 5 MAX6661 toc01 20 TEMPERATURE ERROR vs. POWER-SUPPLY NOISE FREQUENCY 3.0 0 10 20 30 40 50 60 70 80 90 100 10k 100k 1M 10M 100M FREQUENCY (Hz) 3.5 4.0 MAX6661 toc07 AVERAGE SUPPLY CURRENT (µA) 450 3.0 3.3 3.6 3.9 4.2 4.5 4.8 4.5 SUPPLY VOLTAGE (V) DXP-DXN CAPACITANCE (nF) AVERAGE SUPPLY CURRENT vs. SUPPLY VOLTAGE 5.1 5.4 SUPPLY VOLTAGE (V) 4 MAX6661 toc03 TEMPERATURE ERROR vs. PC BOARD RESISTANCE TEMPERATURE ERROR (°C) MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface _______________________________________________________________________________________ 5.0 5.5 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface PIN NAME FUNCTION 1 VFAN Power Supply for Fan Drive: 4.5V to 13.5V 2 VCC Power Supply: 3V to 5.5V. Bypass with a 0.1µF capacitor to GND. 3 DXP Input: Remote-Junction Anode. Place a 2200pF capacitor between DXP and DXN for noise filtering. 4 DXN Input: Remote-Junction Cathode. DXN is internally biased to a diode voltage above ground. 5 FAN Output to Fan Low Side 6 N.C. No External Connection. Must be left floating. 7 PGND 8 AGND Power Ground Analog Ground 9 OVERT Output to System Shutdown. Active-low output, programmable for active high, if desired. Open drain. 10 CS 11 ALERT 12 DOUT SPI Data Output. High-Z when not being read. GAIN Leave open if tachometer feedback is being used. Connect an external resistor to VCC to reduce the gain of the current sense. 14 SCL SPI Clock 15 SDIN SPI Data In 16 TACH IN 13 SPI Chip Select. Active low. Open-Drain Active-Low Output Fan Tachometer Input. 13.5V tolerant, pullup from VCC to 13.5V is allowed on this line. Detailed Description The MAX6661 is a remote temperature sensor and fan controller with an SPI interface. The MAX6661 converts the temperature of a remote PN junction to a 10-bit + sign digital word. The remote PN junction can be a diode-connected transistor, such as a 2N3906, or the type normally found on the substrate of many processors’ ICs. The temperature information is provided to the fan-speed regulator and is read over the SPI interface. The temperature data, through the SPI interface, can be read as a 10-bit + sign two’s complement word with a 0.125°C resolution (LSB) and is updated every 0.5s. The MAX6661 incorporates a closed-loop fan controller that regulates the fan speed with tachometer feedback. The temperature information is compared to a threshold and range setting, which enables the MAX6661 to automatically set fan speed proportional to temperature. Full control of the fan is available by being able to open either the thermal control loop or the fan control loop. Figure 1 shows a simplified block diagram. ADC The ADC is an averaging type that integrates the signal input over a 125ms period with excellent noise rejection. A bias current is steered through the remote diode, where the forward voltage is measured, and the temperature is computed. The DXN pin is the cathode of the remote diode and is biased at 0.7V above ground by an internal diode to set up the ADC inputs for a differential measurement. The worst-case DXPDXN differential input voltage range is 0.25V to 0.95V. Excess resistance in series with the remote diode causes about 1/2°C error per ohm. Likewise, 200mV of offset voltage forced on DXP-DXN causes approximately 1°C error. A/D Conversion Sequence A temperature-conversion sequence is initiated every 500ms in the free-running autoconvert mode (bit 6 = 0 in the configuration register) or immediately by writing a one-shot command. The result of the new measurement is available after the end of conversion. The results of the previous conversion sequence are still available when the ADC is converting. Remote-Diode Selection Temperature accuracy depends on having a goodquality, diode-connected, small-signal transistor. Accuracy has been experimentally verified for all devices listed in Table 1. The MAX6661 can also directly measure the die temperature of CPUs and other ICs that have on-board temperature-sensing diodes. _______________________________________________________________________________________ 5 MAX6661 Pin Description MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface VFAN TACH IN FAN-SPEED REGULATOR FAN FAN N REGISTERS TMAX DXP MUX ADC THYST COMPARAT0R OVERT DXN REMOTE TEMPERATURE DATA CONTROL LOGIC ALERT THIGH SC SDIN DOUT SPI INTERFACE TLOW CS CONFIGURATION FAN TACHOMETER DIVISOR (FTD) TFAN (FT) THERMAL OPEN/ CLOSE LOOP FAN OPEN/ CLOSE LOOP FAN CONTROL CIRCUIT FAN GAIN (FG) FULL SCALE (FS) FAN TACHOMETER PERIOD LIMIT (FTPL) MODE (M) FAN-CONVERSION RATE (FCR) FAN-SPEED CONTROL (FSC) STATUS FAN TACHOMETER PERIOD (FTP) Figure 1. MAX6661 Block Diagram 6 _______________________________________________________________________________________ Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface MANUFACTURER MODEL NO. Central Semiconductor (USA) 2N3904, 2N3906 Fairchild Semiconductor (USA) 2N3904, 2N3906 Rohm Semiconductor (Japan) Samsung (Korea) SST3904 KST3904-TF Siemens (Germany) SMBT3904 Zetex (England) FMMT3904CT-ND Note: Transistors must be diode connected (base shorted to collector). The transistor must be a small-signal type with a relatively high forward voltage. Otherwise, the A/D input range could be violated. The forward voltage must be greater than 0.25V at 10µA. Check to ensure this is true at the highest expected temperature. The forward voltage must be less than 0.95V at 100µA. Check to ensure that this is true at the lowest expected temperature. Large power transistors, power diodes, or small-signal diodes must not be used. Also, ensure that the base resistance is less than 100Ω. Tight specifications for forward-current gain (50 < ß <150, for example) indicate that the manufacturer has good process controls and that the devices have consistent VBE characteristics. Bits 5–2 of the mode register can be used to adjust the ADC gain to achieve accurate temperature measurements with diodes not included in the recommended list or to calibrate individually the MAX6661 for use in specific control systems. Thermal Mass and Self-Heating When measuring the temperature of a CPU or other IC with an on-chip sense junction, the thermal mass of the sensor has virtually no effect; the measured temperature of the junction tracks the actual temperature within GND 10mils 10mils DXP MINIMUM 10mils DXN 10mils GND a conversion cycle. When measuring temperature with discrete remote sensors, smaller packages (e.g., a SOT23) yield the best thermal response times. Take care to account for thermal gradients between the heat source and the sensor, and ensure that stray air currents across the sensor package do not interfere with measurement accuracy. Sensor self-heating, caused by the diode current source, is negligible. ADC Noise Filtering The ADC is an integrating type with inherently good noise rejection, especially of low-frequency noise such as 60Hz line interference. Micropower operation places constraints on high-frequency noise rejection; therefore, careful PC board layout and proper external noise filtering are required for high-accuracy remote measurements in electrically noisy environments. High-frequency EMI is best filtered at DXP and DXN with an external 2200pF capacitor. This value can be increased to about 3300pF (max), including cable capacitance. Capacitance higher than 3300pF introduces errors due to the rise time of the switched current source. Nearly all noise sources tested cause the ADC measurements to be higher than the actual temperature, typically by 1°C to 10°C, depending on the frequency and amplitude (see Typical Operating Characteristics). PC Board Layout Follow these guidelines to reduce the measurement error of the temperature sensors: 1) Place the MAX6661 as close as practical to the remote diode. In noisy environments, such as a computer motherboard, this distance can be 4in to 8in (typ). This length can be increased if the worst noise sources are avoided. Noise sources include CRTs, clock generators, memory buses, and ISA/PCI buses. 2) Do not route the DXP-DXN lines next to the deflection coils of a CRT. Also, do not route the traces across fast digital signals, which can easily introduce a 30°C error, even with good filtering. 3) Route the DXP and DXN traces in parallel and in close proximity to each other, away from any higher voltage traces, such as 12VDC. Leakage currents from PC board contamination must be dealt with carefully since a 20MΩ leakage path from DXP to ground causes about a 1°C error. If high-voltage traces are unavoidable, connect guard traces to GND on either side of the DXP-DXN traces (Figure 2). 4) Route through as few vias and crossunders as possible to minimize copper/solder thermocouple effects. Figure 2. Recommended DXP-DXN PC Trace _______________________________________________________________________________________ 7 MAX6661 Table 1. Remote-Sensor Transistors MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface 5) When introducing a thermocouple by inserting different metals in the connection path, make sure that both the DXP and the DXN paths have matching thermocouples, i.e., the connection paths are symmetrical. A copper-solder thermocouple exhibits 3µV/°C. Adding a few thermocouples causes a negligible error. 6) The 10mil widths and spacings that are recommended in Figure 2 are not absolutely necessary, as they offer only a minor improvement in leakage and noise over narrow traces. Use wider traces when practical. 7) Add a 5Ω resistor in series with VCC for best noise filtering (see Typical Operating Circuit). PC Board Layout Checklist • Place the MAX6661 close to the remote-sense junction. • Keep traces away from high voltages (12V bus). • Keep traces away from fast data buses and CRTs. • Use recommended trace widths and spacings. • Place a ground plane under the traces. • Use guard traces connected to GND flanking DXP and DXN. • Place the noise filter and the 0.1µF V CC bypass capacitors close to the MAX6661. Twisted-Pair and Shielded Cables Use a twisted-pair cable to connect the remote sensor for distances longer than 8in or in very noisy environments. Twisted-pair cable lengths can be between 6ft and 12ft before noise introduces excessive errors. 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 100ft in a noisy environment. At the device, connect the twisted pair to DXP and DXN and the shield to GND. Leave the shield unconnected at the remote sensor. For very long cable runs, the cable’s parasitic capacitance often provides noise filtering, so the 2200pF capacitor can often be removed or reduced in value. Cable resistance also affects remote-sensor accuracy. For every ohm of series resistance, the error is approximately 1/2°C. Low-Power Standby Mode Standby mode reduces the supply current to less than 10µA (typ) by disabling the ADC, the control loop, and the fan driver. Enter standby mode by setting the RUN/STOP bit to 1 (bit 6) in the configuration byte register. In standby mode, all data is retained in memory, and the SPI interface is alive and listening for SPI commands. In standby mode, the one-shot command initiates a conversion. Activity on the SPI bus causes the device to draw extra supply current. If a standby command is received while a conversion is in progress, the conversion cycle is interrupted, and the temperature registers are not updated. The previous data is not changed and remains available. SPI Interface The data interface for the MAX6661 is compatible with SPI, QSPI™, and MICROWIRE™ devices. For SPI/QSPI, ensure that the CPU serial interface runs in master mode so that it generates the serial clock signal. Select a 2.5MHz clock frequency or lower, and set zero values for clock polarity (CPOL) and phase (CPHA) in the µP control registers. Data is clocked into the MAX6661 at SDIN on the rising edge of SC when CS is low. The first byte is the command byte and the second byte is the data byte. The command byte can be either a read byte or a write byte (Table 2). The last bit READ/WRITE (LSB) of the command byte tells the MAX6661 whether it is a read or a write operation, where a high signifies a read, and a low signifies a write. When CS is high, the MAX6661 does not respond to any activity on the SPI bus. All valid communications on the SPI should have 16 bits except for the SPOR and the OSHT. During a READ operation, the DOUT line goes low on the falling clock edge after the READ/WRITE bit (8th bit). The data in the shift register is moved to the DOUT line during the 8th to 15th falling-clock edges and the MSB of the data is available to be read at the rising edge of the 9th clock pulse. The remaining clock pulses in the READ operation shift the register contents on the negative clock edge so that they can be latched into the master on the positive edge. Any READ operation with less than 16 bits results in truncated data. Figure 3 shows the read cycle. For a WRITE operation, the command byte is decoded during the 8th clock pulse. Then data is loaded into the shift register on the positive edges of the 9th to 16th clock pulses and transferred to the appropriate register on the negative edge of the 16th clock period. Any WRITE operation that does not have the 16th clock edge does not get shifted out of the shift register and thus is ignored. Since returning CS high resets the SPI interface at the end of a transfer, this cannot be done until after the 16th falling clock edge. If CS is returned high before this 16th falling clock edge, the appropriate QSPI is a trademark of Motorola, Inc. MICROWIRE is a trademark of National Semiconductor Corp. 8 _______________________________________________________________________________________ Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface REGISTERS COMMAND POR STATE RRL 81h 00000000 FUNCTION Read Remote Temperature Low Byte (3MSBs) RRH 83h 00000000 Read Remote Temperature High Byte (Sign Bit and First 7 Bits) RSL 85h 00000000 Read Status Byte RCL/WCL 87h/92h 00000000 Read/Write Configuration Byte RFCR/WFCR 89h/94h 00000010 Read/Write Fan-Conversion Rate Byte RTMAX/WTMAX A1h/A4h 01100100 (+100°C) RTHYST/WTHYST A3h/A6h 01011111 (+95°C) Read/Write Remote THYST RTHIGH/WTHIGH 8Fh/9Ah 01111111 (+127°C) Read/Write Remote THIGH RTLOW/WTLOW 91h/9Ch 11001001 (-55°C) Read/Write Remote TLOW SPOR F8h N/A Read/Write Remote TMAX Write Software POR OSHT 9Eh N/A RTFAN/WTFAN A9h/B2h 00111100 (+60°C) Write One-Shot Temperature Conversion RFSC/WFSC ABh/B4h 00000000 Read/Write Fan-Speed Control RFG/WFG ADh/B6h 10000000 Read/Write Fan Gain Read/Write Fan-Control Threshold Temperature TFAN RFTP AFh 00000000 Read Fan Tachometer Period RFTCL/WFTPLP B1h/B8h 11111111 Read/Write Fan Tachometer Period Limit (Fan-Failure Limit) RFTD/WFTD BBh/BCh 00000001 Read/Write Fan Tachometer Divisor RFS/WFS BFh/C0h 11111111 Read/Write Full-Scale Register RM/WM F5h/F6h 00000000 Read/Write Mode Register ID CODE FDh 01001101 Read Manufacturer ID Code ID CODE FFh 00001001 Read Device ID Code register is not loaded. DOUT is high impedance during a WRITE operation. Figure 4 shows the write cycle. For single byte commands such as OSHT and SPOR, the operation need only be 7 bits long where the READ/WRITE bit is omitted. Here the command is loaded into the shift register on the rising edge of SC and the command is decoded during the high period of the 7th clock pulse. The 7th falling edge of SC shifts the command from the shift register to the appropriate register. CS can then go high after the SC low to CS high hold time tCSH (see SPI AC Timing, Electrical Characteristics). Figure 5 shows the timing waveforms for the MAX6661’s SPI interface. Remote Temperature Data Register Two registers, at addresses 81h and 83h, store the measured temperature data from the remote diode. The data format for the remote-diode temperature is 10 bits + sign, with each LSB corresponding to 0.125°C, in two’s complement format (Table 3). Register 83h contains the sign bit and the first 7 bits. Bits 7, 6, and 5 of register 81h are the 3LSBs. If the two registers are not read immediately, one after the other, their contents may be the result of two different temperature measurements, leading to erroneous temperature data. For this reason, a parity bit has been added to the 81h register. Bit 4 of this is zero if the data in 81h and 83h are from the same temperature conversion and 83h is read first. Otherwise, bit 4 is one. The remaining bits are don’t cares. When reading temperature data, register 83h must be read first. Alarm Threshold Registers The MAX6661 provides four alarm threshold registers that can be programmed with a two’s complement temperature value with each LSB corresponding to 1°C. The registers are THIGH, TLOW, TMAX, and THYST. If the measured temperature equals or exceeds THIGH, or is less than TLOW, an ALERT interrupt is asserted. If the measured temperature equals or exceeds TMAX, the OVERT output is asserted (see the Overtemperature Output OVERT section). The POR state for THIGH is _______________________________________________________________________________________ 9 MAX6661 Table 2. MAX6661 Command-Byte Bit Assignments MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface SC CS D15 DIN D8 D15 IS START BIT ALWAYS HIGH D8 IS READ/WRITE BIT HIGH FOR READ COMMAND BYTE D7–D0 DATA BYTE DOUT THREE-STATE D7 D6 THREE-STATE D0 Figure 3. Read Cycle SC CS D15 DIN D8 D15 IS START BIT ALWAYS HIGH DOUT D7 D8 IS READ/WRITE BIT LOW FOR WRITE D15–D8 COMMAND BYTE D0 D7–D0 DATA BYTE THREE-STATE THREE-STATE Figure 4. Write Cycle tCSW CS tCSS tSCS tCH tCL SC tCP tDS tDH SDIN tDO DOUT Figure 5. Serial Interface Timing 10 ______________________________________________________________________________________ tTR Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface Overtemperature Output (OVERT) The MAX6661 has an overtemperature output (OVERT) that is set when the remote-diode temperature crosses the limits set in the TMAX register. It is always active if the remote-diode temperature exceeds T MAX . The OVERT line clears when the temperature drops below THYST. Bit 1 of the configuration register can be used to mask the OVERT output. Typically, the OVERT output is connected to a power-supply shutdown line to turn system power off. At power-up, OVERT defaults to low when activated but the logic can be reversed by setting bit 5 of the configuration register. If reversed, OVERT is a logic one when the tMAX register temperature value is exceeded. The OVERT line can be taken active, either by the MAX6661 or driven by an external source. OVERT also acts as an input when set to go low when activated (default). If OVERT is driven or forced low externally, the fan loop forces the fan to full speed and bit 1 of the status register is set. The OVERT input can be masked out by bit 2 of the configuration register. Diode Fault Alarm A continuity fault detector at DXP detects an open circuit between DXP and DXN. If an open or short circuit exists, register 83h is loaded with 1000 0000. Additionally, if the fault is an open circuit, bit 2 of the status byte is set to 1 and the ALERT condition is activated at the end of the conversion. Immediately after POR, the status register indicates that no fault is present until the end of the first conversion. ALERT Interrupts The ALERT interrupt output signal is activated (unless it is masked by bit 7 in the configuration register) whenever the remote-diode’s temperature is below TLOW or exceeds THIGH. A disconnected remote diode (for continuity detection), a shorted diode, or an active OVERT also activates the ALERT signal. The activation of the ALERT signal sets the corresponding bits in the status register. Once activated, ALERT is latched until cleared. To clear the ALERT, read the status register. The interrupt does not halt automatic conversions. New temperature data continues to be available over the SPI interface after ALERT is asserted. ALERT is an activelow open-drain output so that devices can share a common interrupt line. The interrupt is updated at the end of each temperature conversion so, after being cleared, it reappears after the next temperature conversion if the cause of the fault has not been removed. MAX6661 +127°C, for TLOW is -55°C, for TMAX is +100°C, and for THYST is +95°C. Table 3. Temperature Data Format (Two’s Complement) TEMP (°C) DIGITAL OUTPUT +127 0111 1111 111 +125.00 0111 1101 000 +25 0001 1001 000 +0.125 0000 0000 001 0 0000 0000 000 -0.125 1111 1111 111 -25 1110 0111 111 -40 1101 1000111 By setting bit 0 in the configuration register to 1, the ALERT line always remains high. Prior to taking corrective action, always check to ensure that an interrupt is valid by reading the current temperature and the status register. Example: The remote temperature reading crosses T HIGH, activating ALERT. The host responds to the interrupt by reading the status register, clearing the interrupt. If the condition persists, the interrupt reappears. One Shot The one-shot command immediately forces a new conversion cycle to begin. In software standby mode (RUN/STOP bit = high), a new conversion is begun by writing an OSHT (9Eh) command. After the conversion, the device returns to standby mode. If a conversion is in progress when a one-shot command is received, the command is ignored. If a one-shot command is between conversions in autoconvert mode (RUN/STOP bit = low), a new conversion begins immediately. Configuration Register Functions The configuration register table (Table 4) describes this register’s bit assignments. Status Register Functions The status byte (Table 5) reports several fault conditions. It indicates when the fan driver transistor of the MAX6661 has overheated and/or in thermal shutdown, when the temperature thresholds, TLOW and THIGH, have been exceeded, and whether there is an open circuit in the DXP-DXN path. The register also reports the state of the ALERT and OVERT lines and indicates when the fan driver is fully on. The final bit in the status register indicates when a fan failure has occurred. After POR, the normal state of the flag bits is zero, assuming no alert or overtemperature conditions are ______________________________________________________________________________________ 11 MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface Table 4. Configuration Register Bit Assignments BIT NAME POR STATE 7(MSB) ALERT Mask 0 DESCRIPTION When set to 1, ALERT is masked from internally generated errors. 6 Run/Stop 0 When set to 1, the MAX6661 enters low-power standby. 5 OVERT Polarity 0 0 provides active low, 1 provides active high. 0 When set to 1, Write Protect is in effect for the following applicable registers: 1. Configuration register bits 6, 5, 4, 3 2. TMAX register 3. THYST register 4. Fan conversion rate register 4 Write Protect 3 Thermal Closed/ Open Loop 0 When set to 1, the thermal loop is open. The fan speed control retains the last closed-loop value unless overwritten by a bus command (in closed loop, the fan speed control is read only). If fan mode is set to open loop by writing a 1 to bit 0 of the fan gain register, then this bit is automatically set. 2 OVERT Input Inhibit 0 When set to 1, an external signal on OVERT is masked from bit 1 of the status register. 0 Mask the OVERT output from an internally generated overtemperature error. 0 Not used. 1 0 Mask OVERT Output N/A Table 5. Status Register Bit Assignments BIT NAME POR STATE DESCRIPTION 7(MSB) MAX6661 Overheat 0 When high, indicates that the fan driver transistor of the MAX6661 has overheated (temperature > +150°C) and is in thermal shutdown. The fan driver remains disabled until temperature falls below +140°C. 6 ALERT 0 When high, indicates ALERT has been activated (pulled low), regardless of cause (internal or external). 5 Fan Driver Full Scale 0 When high, indicates the fan driver is at full scale. Only valid in fan closed-loop mode. 4 Remote High 0 When high, the remote-junction temperature exceeds the temperature in the remote high register. 3 Remote Low 0 When low, the remote-junction temperature is lower than the temperature in the remote low register. 2 Diode Open 0 When high, the remote-junction diode is open. 1 OVERT 0 When active, indicates that OVERT has been activated, regardless of cause (internal or external). 0 Fan Failure 0 When high, indicates the count in the fan tachometer period register is higher than the limit set in the fan tachometer period limit register. present. Bits 2 through 6 of the status register are cleared by any successful read of the status register, unless the fault persists. The ALERT output follows the status flag bit. Both are cleared when successfully read, but if the condition still exists, the ALERT and the 12 corresponding status bit are reasserted at the end of the next conversion. When autoconverting, if the THIGH and TLOW limits are close together, it is possible for both high-temperature and low-temperature status bits to be set, depending ______________________________________________________________________________________ Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface Manufacturer and Device ID Codes Two ROM registers provide manufacturer and device ID codes. Reading the manufacturer ID returns 4D, which is the ASCII code M (for Maxim). Reading the device ID returns 09h, indicating the MAX6661 device. POR and UVLO The MAX6661 has a volatile memory. To prevent unreliable power-supply conditions from corrupting the data in memory and causing erratic behavior, a POR voltage detector monitors VCC and clears the memory if VCC falls below 1.91V (see Electrical Characteristics). When power is first applied and VCC rises above 2.0V (typ), the logic blocks begin operating, although reads and writes at VCC levels below 3.0V are not recommended. A second VCC comparator, the ADC UVLO comparator prevents the ADC from converting until there is sufficient headroom (VCC = 2.89V typ). The software POR (SPOR) command can force a power-on reset of the MAX6661 registers through the serial interface. This can be done by writing F8h to the MAX6661. Power-up defaults include: • Interrupt latch is cleared. • ADC begins autoconverting. • Command register is set to 00h to facilitate quickinternal Receive Byte queries. • THIGH and TLOW registers are set to +127°C and -55°C, respectively. • T HYST and T MAX are set to +95°C and +100°C, respectively. Fan Control The fan-control function can be divided into the thermal loop, the fan-speed-regulation loop (fan loop), and the fan-failure sensor. The thermal loop sets the desired fan speed based on temperature while the fan-speed-regulation loop uses an internally divided down reference oscillator to regulate the fan speed. The fan-speed-regulation loop includes the fan driver and the tachometer sensor. The fan-failure sensor provides a FAN FAIL alarm that signals when the value in the fan tachometer period register is greater than the fan tachometer period limit register value, which corresponds to a fan going slower than the limit. The fan driver is an N-channel, 4Ω MOSFET with a 13.5V maximum VDS whose drain terminal connects to the low side of the fan. The tachometer sensor (TACH IN) of the MAX6661 is driven from the tachometer output of the fan and provides the feedback signal to the fan-speed regulation loop for controlling the fan speed. For fans without tachometer outputs, the MAX6661 can generate its own tachometer pulses by monitoring the commutating current pulses (see the Commutating Current Pulses section). Thermal Loop Thermal Closed Loop The MAX6661 can be operated in a complete closedloop mode, with both the thermal and fan loops closed, where the remote-diode sensor temperature directly controls fan speed. Setting bit 3 of the configuration register to zero places the MAX6661 in thermal closed loop (Figure 6). The remote-diode temperature sensor is updated every 500ms. The value is stored in a temporary register (TEMPDATA) and compared to the programmed temperature values in the T HIGH , T LOW , THYST, TMAX, and TFAN registers to produce the error outputs OVERT and ALERT. The fan conversion rate (FCR) register (Table 6) can be programmed to update the TEMPDATA register every 0.5s to 32s. This enables control over timing of the thermal feedback loop to optimize stability. The fan threshold (TFAN) register value is subtracted from the UPDATE register value. If UPDATE exceeds TFAN temperature, then the fan-speed control (FSC) register (Table 7), stores the excess temperature in the form of a 7-bit word with an LSB of 0.5°C. If the difference between the TFAN and UPDATE registers is higher than 32°C, then bits 6-0 are set to 1. In thermal closed loop, the FSC register is READ ONLY. The fan gain (FG) register (Table 8) determines the number of bits used in the fan-speed control register. This gain can be set to 4, 5, or 6. If bits 6 and 5 are set to 10, all 6 bits of TEMPDATA are used directly to program the speed of the fan so that the thermal loop has a control range of 32°C with 64 temperature steps from fan off to full fan speed. If bits 6 and 5 are set to 01, the thermal control loop has a control range of 16°C with 32 temperature steps from fan off to full fan speed. If bits 6 and 5 are set to 00, the thermal control loop has a control range of 8°C with 16 temperature steps from fan off to full fan speed. Thermal Open Loop Setting bit 3 of the configuration register (Table 4) to 1 places the MAX6661 in thermal open loop. In thermal open-loop mode, the FSC register is read/write. ______________________________________________________________________________________ 13 MAX6661 on the amount of time between status read operations. In these circumstances, it is best not to rely on the status bits to indicate reversals in long-term temperature changes. Instead, use a current temperature reading to establish the trend direction. MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface Table 6. Fan Conversion Update Rate REMOTE SENSOR TEMPERATURE CONVERTER TEMP DATA UPDATE FAN CONVERSION RATE FCR 0.25s TO 16s FAN THRESHOLD TEMPERATURE (TFAN) FAN-SPEED CONTROL (FSC) Figure 6. MAX6661 Thermal Loop In thermal open-loop mode, the fan loop can operate in open or closed mode. In fan open loop, the FSC register programs fan voltage directly, accepting values from 0 to 64 (40h). For example, in fan open-loop mode, zero corresponds to no voltage across the fan and 40h corresponds to full fan voltage. Proportional control is available over the 0 to 63 (3Fh) range with 64 (40h) forcing unconditional full speed. In fan closed-loop mode, zero corresponds to zero fan speed. When the FG register is set to 4 bits, 10h corresponds to 100% fan speed; 100% fan speed is 20h at 5 bits, and 3Fh at 6 bits. Fan Loop The fan loop (Figure 7) is based on an up/down counter where a reference clock representing the desired fan speed drives the count up, while tachometer pulses drive it down. The reference clock frequency is divided down from the MAX6661 internal clock to a frequency of 8415Hz. This clock frequency is further divided by 14 BINARY FAN UPDATE RATE (Hz) SECONDS BETWEEN UPDATES 00h 00000000 0.0625 16 01h 00000001 0.125 8 02h 00000010 0.25 4 (POR) 03h 00000011 0.5 2 04h 00000100 1 1 05h 00000101 2 0.5 06h 00000110 4 0.25 the fan full-scale (FS) register (Table 9), which is limited to values between 127 to 255, for a range of reference clock full-scale frequencies from 33Hz to 66Hz. A further division is performed to set the actual desired fan speed. This value appears in the fan-speed control register in thermal closed-loop mode. If the thermal loop is open, but the fan-speed control loop is closed, this value is programmable in the FSC. When in fan openloop mode (which forces the thermal loop to open), the FSC register becomes a true DAC, programming the voltage across the fan from zero to nearly VFAN. The tachometer input (TACH IN) includes a programmable (1/2/4/8) prescaler. The divider ratio for the (1/2/4/8) prescaler is stored in the fan tachometer divisor (FTD) register (Table 10). In general, the values in FTD should be set such that the full-speed fan frequency divided by the prescaler fall in the 33Hz to 66Hz range. UPDATE TACH IN DATA The UP/DN counter has six stages that form the input of a 6-bit resistive ladder DAC whose voltage is divided down from V FAN . This DAC determines the voltage applied to the fan. Internal coding is structured such that when in fan closed-loop mode (which includes thermal closed loop), higher values in the 0 to 32 range correspond to higher fan speeds and greater voltage across the fan. In fan open-loop mode (which forces thermal open loop), acceptable values range from 0 to 63 (3Fh) for proportional control; a value of 64 (40h) commands unconditional full speed. Fan closed-loop mode is selected by setting bit 0 of the FG to zero; open-loop mode is selected by setting bit 0 to 1. In open-loop mode, the gain block is bypassed and the FSC register is used to program the fan voltage rather than the fan speed. When in fan open-loop mode, both the temperature feedback loop and fanspeed control loop are broken, which result in the TACH IN input becoming disabled. A direct voltage can be applied to the fan after reading the temperature, ______________________________________________________________________________________ Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface MAX6661 Table 7. Fan Speed Control Register (RFSC/WFSC) REGISTER/ADDRESS FSC (ABH = READ, B4H = WRITE) COMMAND READ/WRITE FAN DAC REGISTER 7 Not Used Label POR State 0 6 Overflow Bit 5 (MSB) 4 Data 3 Data 2 Data 1 Data 0 Data 0 0 0 0 0 0 0 Table 8. Fan Gain Register (RFG/WFG) REGISTER/ADDRESS FG (ADH = READ, B6H = WRITE) COMMAND READ/WRITE FAN GAIN REGISTER 7 Always Write a1 Label POR State 1 6 Fan Gain 0 5 Fan Gain 4 Always Write a0 3 Always Write a0 2 Always Write a0 1 Fan Driver Mode Bit 0 Fan Feedback Mode 0 0 0 0 0 0 Notes: Bit 0: Fan driver mode. When bit 0 is set to 1, the fan driver is in fan open-loop mode. In this mode, the fan DAC programs the fan voltage rather than the fan speed. Tachometer feedback is ignored, and the user must consider minimum fan drive and startup issues. Thermal open loop is automatically set to 1 (see configuration register). Fan Fail (bit 0 of the status register) is set to 1 in this mode and should be ignored. Bit 1: Fan feedback mode. When bit 1 is set to 1, the fan loop uses driver current sense rather than tachometer feedback. Bits 6, 5: Fan gain of the fan loop, where 00 = 8°C with resolution = 4 bits. This means that the fan reaches its full-scale (maximum) speed when there is an 8°C difference between the remote-diode temperature and the value stored in TFAN, 01 = +16°C, with a 5-bit resolution and 10 = +32°C with a 6-bit resolution. Bit 7: Writing a zero to bit 7 forces bits 6 and 5 to their POR values. Table 9. Fan Full-Scale Register (RFS/WFS) REGISTER/ADDRESS FS (BFH = READ, C0H = WRITE) COMMAND READ/WRITE MAXIMUM TEMPERATURE LIMIT BYTE Label 7 MSB 6 Data Bit 5 Data Bit 4 Data Bit 3 Data Bit 2 Data Bit 1 Data Bit 0 Data Bit POR State 1 1 1 1 1 1 1 1 Note: This register determines the maximum reference frequency at the input of the up/down counter. It controls a programmable divider that can be set anywhere between 127 and 255. The value in this register must be set in accordance with the procedure described in the TACH IN section (equivalent 8415/(Max Tachometer Frequency ✕ Fan Tachometer Divisor)). Programmed value below 127 defaults to 127. POR value is 255. Table 10. Fan Tachometer Divisor Register (RFTD/WFTD) REGISTER/ADDRESS FTD (BBH = READ, BCH = WRITE) COMMAND READ LIMIT/FAILURE REGISTER Label POR State 7 Not Used 0 6 Not Used 5 Not Used 4 Not Used 3 Not Used 2 Not Used 0 0 0 0 0 1 Divisor Bit 1 0 0 Divisor Bit 0 1 Note: This byte sets the prescalar division ratio for tachometer or current-sense feedback. (This register does not apply to the tach signal used in the fan-speed register.) Select this value such that the fan frequency (rpm / 60s x number of poles) divided by the FCD falls in the 33Hz to 66Hz range. See TACH IN section: Bits 1, 0: 00 = divide by 1, 01 = divide by 2, 10 = divide by 4, 11 = divide by 8. ______________________________________________________________________________________ 15 MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface using the FSC register. By selecting fan open-loop mode, the MAX6661 automatically invokes thermal open-loop mode. value in FTPL, a failure is indicated. In fan closed loop, a flag is activated when the fan is at full speed. Set the fan tachometer period limit byte to: Fan Conversion Rate Register fTACH = 8415 / [N ✕ f] where N = fan-fail ratio and fTACH = maximum frequency of the fan tachometer. The factor N is less than 1 and produces a fan-failure indication when the fan should be running at full speed, but is only reaching a factor of N of its expected frequency. The factor N is typically set to 0.75 for all fan speeds except at very low speeds where a fan failure is indicated by an overflow of the fan-speed counter. The overflow flag cannot be viewed separately in the status byte but is ORed with bit 0, the fan-fail bit. The FCR register (Table 6) programs the fan’s update time interval in free-running autonomous mode (RUN/STOP = 0). The conversion rate byte’s POR state is 02h (0.25Hz). The MAX6661 uses only the 3LSBs of this register. The 5MSBs are don’t cares. The update rate tolerance is ±25% (max) at any rate setting. Fan Driver The fan driver consists of an amplifier and low-side NMOS power device whose drain is connected to FAN and is the connection for the low side of the fan. There is an internal connection from the fan to the input of the amplifier. The FET has 4Ω on-resistance with 320mA (typ) current limit. The driver has a thermal shutdown sensor that senses the driver’s temperature. It shuts down the driver if the temperature exceeds +150°C. The driver is reactivated once the temperature has dropped below +140°C. TACH IN The TACH IN input connects directly to the tachometer output of a fan. Most commercially available fans have two tachometer pulses per revolution. The tachometer input is fully compatible with tachometer signals, which are pulled up to VFAN. Commutating Current Pulses When a fan does not come equipped with a tachometer output, the MAX6661 uses commutating generated current pulses for speed detection. This mode is entered by setting the FG register’s bit 1 to 1. An internal pulse is generated whenever a step increase occurs in the fan current. Connecting an external resistor between the GAIN pin and VCC can reduce the sensitivity of pulses to changes in fan current. In general, the lower the resistor value, the lower the sensitivity, and the fan is easier to turn ON and can use a smaller external capacitor across its terminals. A suitable resistor range is 1kΩ to 5kΩ. Fan-Failure Detection The MAX6661 detects fan failure by comparing the value in the fan tachometer period (FTP) register, a READ ONLY register, with a limit stored in the fan tachometer period limit (FTPL) register (Table 11). A counter counts the number of on-chip oscillator pulses between successive tachometer pulses and loads its value into the FTP register every time a tachometer pulse arrives. If the value in FTP is greater than the 16 Applications Information Mode Register Resistance in series with the remote-sensing junction causes conversion errors on the order of 0.5°C per ohm. The MAX6661 mode register gives the ability to eliminate the effects of external series resistance of up to several hundred ohms on the remote temperature measurement and to adjust the temperature-measuring ADC to suit different types of remote-diode sensor. For systems using external switches or long cables to connect to the remote sensor, a parasitic resistance cancellation mode can be entered by setting mode register bit 7 = 1. This mode requires a longer conversion time and so can only be used for fan conversion rates of 1Hz or slower. Bits 6, 1, and 0 of the mode register are not used. Use bits 5–2 to adjust the ADC gain to achieve accurate temperature measurements with diodes not included in the recommended list or to individually calibrate the MAX6661 for use in specific control systems. These bits adjust gain to set the temperature reading at +25°C, using two’s complement format reading. Bit 5 is the sign (1 = increase, 0 = decrease), bit 4 = 2°C shift, bit 3 = 1°C shift, bit 2 = 1/2°C shift. Origin of gain curve is referred to 0°K. To use this feature, the sensor must be calibrated by the user. General Programming Techniques The full-scale range of the fan-regulation loop is designed to accommodate fans operating between the 1000rpm to 8000rpm range of different fans. An onchip 8415Hz oscillator is used to generate the 33Hz to 66Hz reference frequency. Choose the FTD value such that the fan full-speed frequency divided by this value falls in the 33Hz to 66Hz range. The full-scale reference frequency is further divided by the value in the FSC ______________________________________________________________________________________ Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface MAX6661 TEMP DATA REF FREQ 8415Hz TACH IN FAN GAIN (FG) 8°C, 16°C, 32°C RANGE FAN TACHOMETER PERIOD (FTP) FAN TACHOMETER PERIOD LIMIT (FTPL) FAN-SPEED CONTROL 1 TO 63 COUNTER COMPARATOR FAN FULL SCALE (FS) 127 TO 255 FAN TACHOMETER DIVISOR (FTD) 1, 2, 4, 8 FAN OPEN/CLOSED LOOP FAN FAIL UP/DOWN VFAN TACH FAN DAC DRIVER N Figure 7. MAX6661 Fan Loop Functional Diagram Table 11. Fan Tachometer Period Limit (RFTCL/WWFTCL) REGISTER/ADDRESS FL (B1H = READ, B8H = WRITE) COMMAND Label POR State READ LIMIT/FAILURE REGISTER 7 (MSB) 1 6 Data Bit 5 Data Bit 4 Data Bit 3 Data Bit 2 Data Bit 1 Data Bit 0 Data Bit 1 1 1 1 1 1 1 Note: The fan tachometer period limit register is programmed with the maximum speed that is compared against the value in the FS register to produce an error output to the status register. register to the desired fan frequency [read: speed]. The 8415Hz is divided down from the MAX6661 internal clock, and has a ±25°C tolerance. 1) Determine the fan’s maximum tachometer frequency: f(TACH) Hz = (rpm/60s / min) ✕ number of poles Where poles = number of pulses per revolution. Most fans are two poles; therefore, they have two pulses per revolution. 2) Set the programmable FTD so that the frequency of the fan tachometer divided by the prescaler value in the FCD register falls in the 33Hz to 66Hz range. 3) Determine the value required for the fan FS register: FS = 8415 / (fTACH ✕ P) Where P is the prescaler division ratio of the FCD register. Example: Fan A has a 2500rpm rating and 2 poles: fTACH = 2500 / 60 ✕ 2 = 83.4Hz ______________________________________________________________________________________ 17 MAX6661 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface The 83.4Hz value is out of the 33Hz to 66Hz decrement/increment range. Set bits in the FTD register to divide the signal down within the 33Hz to 66Hz range. Bits 1, 0 = 10 (divide by 2: P = 2). The result is 83.4Hz/2 = 41.7Hz. 4) Set the FS register to yield approximately 42Hz: 42Hz = 8415Hz / FS (value) FS (value) ≈ 200 FS register = 11001000 5) In current-sense feedback, a pulse is generated whenever there is a step increase in fan current. The frequency of pulses is then not only determined by the fan rpms and the number of poles, but also by the update rate at which the fan driver forces an increase in voltage across the fan. The maximum pulse frequency is then given by: fC Hz = fTACH ✕ P / (P-1) Where f = (rpm/60) ✕ poles and P is the value in FTD. The value required for the fan FS register is: FS = 8415Hz / (fTACH / (P-1)) The fan speed limit in FTPL should be set to: fL = 8415Hz / (N ✕ fTACH) A value of P = 1 cannot be used in current-sense mode. Fan Selection For closed-loop operation and fan monitoring, the MAX6661 requires fans with tachometer outputs. A tachometer output is typically specified as an option on many fan models from a variety of manufacturers. Verify the nature of the tachometer output (open collector, totem pole) and the resultant levels and configure the connection to the MAX6661. For a fan with an opendrain/collector output, a pullup resistor of typically 5kΩ must be connected between TACH IN and VFAN. Note how many pulses per revolution are generated by the tachometer output (this varies from model to model and among manufacturers, though two pulses per revolution are the most common). Table 12 lists the representative fan manufacturers and the model they make available with tachometer outputs. Table 12. Fan Manufacturers MANUFACTURER FAN MODEL OPTION Comair Roton All DC brushless models can be ordered with optional tachometer output. EBM-Papst Tachometer output optional on some models. JMC Tachometer output optional. NMB All DC brushless models can be ordered with optional tachometer output. Panasonic Panaflo and flat unidirectional miniature fans can be ordered with tachometer output. Sunon Tachometer output optional on some models. electronics. If the voltage supplied to the fan is lowered too far, the internal electronics may no longer function properly. Some of the following symptoms are possible: • The fan may stop spinning. • The tachometer output may stop generating a signal. • The tachometer output may generate more than two pulses per revolution. The problems that occur and the supply voltages at which they occur depend on which fan is used. As a rule of thumb, 12V fans can be expected to experience problems somewhere around 1/4 and 1/2 their rated speed. Chip Information TRANSISTOR COUNT: 6479 PROCESS: BiCMOS Low-Speed Operation Brushless DC fans increase reliability by replacing mechanical commutation with electronic commutation. By lowering the voltage across the fan to reduce its speed, the MAX6661 is also lowering the supply voltage for the electronic commutation and tachometer 18 ______________________________________________________________________________________ Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface TOP VIEW 16 TACH IN VFAN 1 VCC 2 15 SDIN DXP 3 14 SCL DXN 4 MAX6661 13 GAIN FAN 5 12 DOUT N.C. 6 11 ALERT PGND 7 10 CS AGND 8 9 OVERT QSOP QSOP.EPS Package Information Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 19 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX6661 Pin Configuration