AN-1262| Application Note

Power Supply Effects on Noise Performance

Four-Speed Fan Control Using Simple Remote Diode Temperature Sensor
The circuit shown in Figure 1controls the speed of a 12V DC fan using an LM88 Remote Diode Temperature Sensor (RDTS) IC. The LM88 is a dual remote diode temperature sensor with 3 digital comparators and has 3 open-drain outputs (O_SP0, O_SP1 and O_CRIT) that can be used as interrupts or to signal system shutdown. The digital comparators can be programmed independently to make a greater than or less than comparison. When programmed for a greater than comparison: O_SP0 and O_SP1 activate when the temperatures measured by D0 or D1 exceed the associated setpoints of T_SP0 or T_SP1. O_CRIT activates when the temperature measured by either D0 or D1 exceeds set point T_CRIT. T_CRIT can be set at 1 C intervals from -40 C to +125 C. T_SP0 and T_SP1 can be set at 4 C intervals in the range of T_CRIT, 100 C. In the circuit shown in Figure 1 the two D+ inputs have been wired in parallel to allow all three set points to be evaluated against a single temperature measurement. The hysteresis of each comparator is inte
rnally set to 1 C, allowing the set point values to be placed very close together without any interaction. The three outputs of the LM88 are connected to resistors forming a crude 2-bit DAC. The output of this DAC is fed to a PNP emitter follower, controlling the voltage on the negative pin of the fan from 1.25V to 5.7V. The output voltage (VOUT) decreases as the temperature reading increases, when SP0 < SP1 < CRIT.

National Semiconductor Application Note 1262 September 2002

The equations shown in Figure 1 describe the behavior of VOUT . The maximum speed of the fan is dependent on the minimum VOUT . The minimum VOUT is dependent on the drain to source on resistance (Rds) of the O_CRIT output, the MPSW51's beta and base emitter voltage when R5 is set to 0 (as shown in Figure 1). The MPSW51 beta variation will introduce an error term that cannot be accounted for. Therefore, it is tempting to make the current through the resistors as high as possible. Increasing this current is a "Catch 22", because the minimum VOUT level will increase as the current increases, because of O_CRIT's Rds that is typical 100 and worst case .4V/3 mA = 133. A compromise would be to set this current 10 times the MPSW51 base current. O_SP0, O_SP1 and O_CRIT have a maximum voltage limit of 5V. This sets the ratio of R2/(R2+R1) = 5/12 = 0.41666. The current through R1 and R2 should be set such that the base current of the MPSW51 is negligible. The current through the fan with (12 - 5.7) 6.3V is about 65mA o
r so. That makes the base current about 65mA/130 = 0.5 mA. Since the beta will vary slightly as the collector current changes, it's best to set the current through R1/R2 ten times greater than 0.5 mA. Therefore: (R1+R2)= 12V/5mA = 2400 Since R2/(R2+R1)=5/12 R2= (5/12)*(2400)=1000 and R1=1400

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if TD < SP0 VOUTmin = 5.7V (fan min. on) if TD < SP1 VOUTin1 = ((Rp23/(R1+Rp23))12V)+0.7V=3.61V if TD < CRIT VOUTint2 = ((Rp234/(R1+Rp234))12V)+0.7V=2.28V if TD < CRIT VOUTmax = ((Rp2345/(R1+Rp2345))12V)+0.7V=1.25V where SP0 < SP1 < CRIT and TD=diode temperature, see text for values of Rp23, Rp234 and Rp2345

AN-1262

FIGURE 1. Low Cost Remote Diode Temperature Fan Speed Control

2002 National Semiconductor Corporation

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