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ADM1030ARQZ View Datasheet(PDF) - ON Semiconductor

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ADM1030ARQZ Datasheet PDF : 29 Pages
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ADM1030
Tek PreVu
T
T
D: 250mV
@: –258mV
CLOCK
CONFIG 2
REG. BIT 2
1
FAN
INPUT
START OF
MONITORING
CYCLE
FAN
MEASUREMENT
PERIOD
Figure 21. Fan Speed Measurement
4
CH1 100mV
CH3 50.0mV
CH2 5.00mV M 4.00ms A CH1 –2.00mV
CH4 50.0mV
Figure 20. Fan Speed Sensing Waveform at TACH/AIN Pin
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 revolu-
tion is measured by gating an on-chip 11.25 kHz oscillator into
the input of an 8-bit counter. The fan speed measuring circuit is
initialized on the rising edge of a PWM high output if fan speed
measurement is enabled (Bit 2 of Configuration Register 2 =
1). It then starts counting on the rising edge of the second tach
pulse and counts for two fan tach periods, until the rising edge of
the fourth tach pulse, or until the counter overranges if the fan
tach period is too long. The measurement cycle will repeat until
monitoring is disabled. The fan speed measurement is stored in
the Fan Speed Reading register at address 0x08.
The fan speed count is given by:
Count = (f ¥ 60)/R ¥ N
where:
f = 11.25 kHz
R = Fan Speed in RPM.
N = Speed Range (Either 1, 2, 4, or 8)
The frequency of the oscillator can be adjusted to suit the expected
running speed of the fan by varying N, the Speed Range. The
oscillator frequency is set by Bits 7 and 6 of Fan Characteristics
Register 1 (20h) as shown in Table XI. Figure 21 shows how the
fan measurements relate to the PWM_OUT pulse trains.
Bit 7
0
0
1
1
Table XI. Oscillator Frequencies
Bit 6
N
Oscillator
Frequency (kHz)
0
1
11.25
1
2
5.625
0
4
2.812
1
8
1.406
In situations where different output drive circuits are used for
fan drive, it may be desirable to invert the PWM drive signal.
Setting Bit 3 of Configuration Register 1 (0x00) to 1, inverts the
PWM_OUT signal. This makes the PWM_OUT pin high for
100% duty cycle. Bit 3 of Configuration Register 1 should gen-
erally be set to 1, when using an n-MOS device to drive the fan.
If using a p-MOS device, Bit 3 of Configuration Register 1
should be cleared to 0.
FAN FAULTS
The FAN_FAULT output (Pin 8) is an active-low, open-drain
output used to signal fan failure to the system processor. Writing a
Logic 1 to Bit 4 of Configuration Register 1 (0x00) enables the
FAN_FAULT output pin. The FAN_FAULT output is enabled
by default. The FAN_FAULT output asserts low only when
five consecutive interrupts are generated by the ADM1030 device
due to the fan running underspeed, or if the fan is completely
stalled. Note that the Fan Tach High Limit must be exceeded
by at least one before a FAN_FAULT can be generated. For
example, if we are only interested in getting a FAN_FAULT if
the fan stalls, then the fan speed value will be 0xFF for a failed
fan. Therefore, we should make the Fan Tach High Limit =
0xFE to allow FAN_FAULT to be asserted after five consecu-
tive fan tach failures.
Figure 22 shows the relationship between INT, FAN_FAULT,
and the PWM drive channel. The PWM_OUT channel is driv-
ing a fan at some PWM duty cycle, say 50%, and the fan’s tach
signal (or fan current for a 2-wire fan) is being monitored at the
TACH/AIN pin. Tach pulses are being generated by the fan,
during the high time of the PWM duty cycle train. The tach is
pulled high during the off time of the PWM train because the
fan is connected high-side to the n-MOS device.
Suppose the fan has already failed its fan speed measurement
twice previously. Looking at Figure 22, PWM_OUT is brought
high for two seconds, to restart the fan if it has stalled. Some-
time later a third tach failure occurs. This is evident by the tach
signal being low during the high time of the PWM pulse, causing
the Fan Speed Reading register to reach its maximum count of
255. Since the tach limit has been exceeded, an interrupt is
generated on the INT pin. The Fan Fault bit (Bit 1) of Inter-
rupt Status Register 1 (Register 0x02) will also be asserted.
Once the processor has acknowledged the INT by reading the
status register, the INT is cleared. PWM_OUT is then brought
high for another 2 seconds to restart the fan. Subsequent fan
failures cause INT to be reasserted and the PWM_OUT signal
is brought high for 2 seconds (fan spin-up default) each time to
restart the fan. Once the fifth tach failure occurs, the failure is
deemed to be catastrophic, and the FAN_FAULT pin is asserted
low. PWM_OUT is brought high to attempt to restart the fan.
REV. A
Rev. 2 | Page 21 of 29 | www.onsemi.com
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