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ADM1026JSTZ-R7 Datasheet(PDF) 24 Page - ON Semiconductor |
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ADM1026JSTZ-R7 Datasheet(HTML) 24 Page - ON Semiconductor |
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24 / 55 page  ADM1026 http://onsemi.com 24 Figure 44. Fan with Tach Pullup to Voltage > VCC (e.g. 12 V), Clamped with Zener Diode 12 V FAN SPEED COUNTER FAN(0–7) PULLUP 4.7k TYP VCC * CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 x VCC If the fan has a strong pullup (less than 1 k W) to +12 V, or a totem pole output, a series resistor can be added to limit the Zener current, as shown in Figure 45. Alternatively, a resistive attenuator may be used, as shown in Figure 47. R1 and R2 should be chosen such that: (eq. 15) 2.0 V t VPULLUP R2 RPULLUP ) R1 ) R2 t 3.3 V STBY Figure 45. Fan with Strong Tach Pullup to >VCC or Totem Pole Output, Attenuated with R1/R2 12 V FAN SPEED COUNTER FAN(0–7) PULLUP TYP <1 k VCC * CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 x VCC Figure 46. Fan with Strong Tach Pullup to > VCC or Totem Pole Output, Clamped with Zener and Resistor 12 V FAN SPEED COUNTER FAN(0–7) <1 k TACH OUTPUT VCC R1* * SEE TEXT Fan Speed Measurement The fan counter does not count the fan tach output pulses directly because the fan speed may be less than 1000 RPM and it would take several seconds to accumulate a reasonably large and accurate count. Instead, the period of the fan revolution is measured by gating an on-chip 22.5 kHz oscillator into the input of an 8-bit counter for two periods of the fan tach output, as shown in Figure 47, so the accumulated count is actually proportional to the fan tach period and inversely proportional to the fan speed. Figure 47. Fan Speed Measurement 22.5kHz CLOCK CONFIGURATION REG. 1 BIT 0 FAN0 INPUT FAN0 MEASUREMENT PERIOD FAN1 MEASUREMENT PERIOD START OF MONITORING CYCLE 1 2 34 12 3 4 The monitoring cycle begins when a 1 is written to the monitor bit (Bit 0 of Configuration Register 1). The INT_Enable (Bit 1) should be set to 1 to enable the INT output. The fan speed counter starts counting as soon as the fan channel has been switched to. If the fan tach count reaches 0xFF, the fan has failed or is not connected. If a fan is connected and running, the counter is reset on the second tach rising edge, and oscillator pulses are actually counted from the second rising tach edge to the fourth rising edge. The measurement then switches to the next fan channel. Here again, the counter begins counting and is reset on the second tach rising edge, and oscillator pulses are counted from the second rising edge to the fourth rising edge. This is repeated for the other six fan channels. Note that fan speed measurement does not occur until 1.8 seconds after the monitor bit has been set. This is to allow the fans adequate time to spin up. Otherwise, the ADM1026 could generate false fan failure interrupts. During the 1.8 second fan spin-up time, all fan tach registers read 0x00. To accommodate fans of different speed and/or different numbers of output pulses per revolution, a prescaler (divisor) of 1, 2, 4, or 8 may be added before the counter. Divisor values for Fans 0 to 3 are contained in the Fan 0–3 divisor register (Address 02h) and those for Fans 4 to 7 in the Fan 4–7 divisor register (Address 03h). The default value is 2, which gives a count of 153 for a fan running at 4400 RPM producing two output pulses per revolution. The count is calculated by the equation: (eq. 16) Count + 22.5 103 60 RPM Divisor For constant-speed fans, fan failure is typically considered to have occurred when the speed drops below 70% of nominal, corresponding to a count of 219. Full scale (255) is reached if the fan speed fell to 60% of its nominal value. For temperature-controlled, variable-speed fans, the situation is different. |
Similar Part No. - ADM1026JSTZ-R7 |
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Similar Description - ADM1026JSTZ-R7 |
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