Datasheet MAX6665 (Maxim) - 5

Fan Controller/Driver with Factory- Programmed Temperature Thresholds MAX6665
+4.5V TO +24V 100mA TO 250mA COOLING FAN +3.3V VDD FANOUT VDD 1µF MAX6665 100kΩ HYST WARN µP I/O FORCEON FANON I/O VDD 100kΩ OT SYSTEM POWER GND SHUTDOWN Figure 1. High-Reliability, Fail-Safe Fan Controller and Temperature Monitor increases by a maximum of:
Locating the MAX6665
51°C/W x 0.03125W = 1.59°C The location of the MAX6665 in the system affects its operation. Because the fan is turned on and off based Therefore, the effective hysteresis is about 1.59°C high- on the MAX6665’s die temperature, place the MAX6665 er than the hysteresis selected by the HYST pin. For close to major heat-generating components in the sys- example, setting the HYST pin for 8°C of hysteresis tem—a high-speed CPU or a power device, for exam- results in an effective hysteresis of about 9.6°C. ple. A higher supply voltage reduces the FANOUT A larger fan with a power-supply current of 250mA voltage, which reduces the self-heating effects. causes a maximum voltage drop of 0.6V at the output The die temperature of the MAX6665 tracks the tempera- pin. This results in 150mW power dissipation and the ture of its leads and the EP. If it is soldered to a PC board, die temperature increases by: it quickly reaches the temperature of the traces in that 51°C/W x 0.150W = 7.65°C section of the circuit board. Air temperature affects the die If the HYST pin has been set for 8°C of hysteresis, the temperature. Since the plastic package does not conduct total effective hysteresis will be about 15.7°C. heat as well as the leads, the effect of air temperature is much less than that of lead temperature. Using fans with somewhat higher operating current than 250mA results in higher voltage across the output
Layout Issues
transistor. The increased power dissipation caused by The MAX6665’s GND pin is ground return for the fan dri- the higher current and voltage levels will increase self- ver and the device. Large fan current induces noise heating, thereby increasing the effective hysteresis. (ground bounce) to the MAX6665. Bypass VDD to GND When using higher-power fans, be sure that the with a 1µF tantalum capacitor located as close to the MAX6665’s power dissipation does not cause so much MAX6665 as possible. For long VDD and GND lines, an self-heating that the MAX6665 stays on constantly. additional bypass capacitor may be needed. The bypass capacitor reduces GND noise. The EP is internally con- nected to the GND pin. Solder the EP to the ground plane for better electrical and thermal performance.
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