參數(shù)資料
型號(hào)： NCT7491RQR2G
廠商： ON Semiconductor
文件頁(yè)數(shù)： 26/80頁(yè)
文件大?。?/td> 844K
描述： IC REMOTE THERMAL MONITOR 24QSOP
標(biāo)準(zhǔn)包裝： 2,500
系列： *
NCT7491
http://onsemi.com
26
PWM
1
Temperature
2
3    4 5
1
2
3
4
5
100%
6,7,8
Figure 12.
0癈
255癈
Figure 12 shows a typical lookup table curve. The
temperatures are programmed as unsigned data. In this
example 5 of the 8 control points are used and the remaining
3 are set to the maximum value of 255癈. This curve applies
if relative PECI values are not
 assigned to control the PWM
channel.
PWM
1
Temperature
2
3    4 5
1
2
3
4
5
100%
6,7,8
Figure 13.
128癈
0癈
Figure 13 shows a typical lookup table curve that applies
when relative PECI values are assigned to control the PWM
channel. The temperatures are programmed as negative 2s
complement values. In this example 5 of the 8 control points
are used and the remaining 3 are set to the maximum value
of 0癈, as this is the maximum value for relative PECI
values.
Fan Override Settings
There are bits in the NCT7491 that allow the PWM
outputs to be overdriven so that the outputs go to maximum
speed (as programmed in the maximum PWM registers), to
go to full speed (100% duty) or to be shut off. These bits will
override all other fan control settings.
" Setting bit 1 of register 0x11 to 1 runs the fans at the
maximum programmed PWM duty cycle
" Setting bit 3 of register 0x40 to 1 runs the fans at 100%
duty cycle. This bit has precedence over all others.
" Setting bit 0 of register 0x87 to 1 turns off PWM1
" Setting bit 1 of register 0x87 to 1 turns off PWM2
" Setting bit 2 of register 0x87 to 1 turns off PWM3
THERM Override
Setting bit 5 of register 0x40 will allow assertions on any
pin configured as a THERM pin to drive the fans to 100%
duty cycle or Max PWM, deending on bits <4:2> of register
0x16. This will override all other fan settings. This allows an
external device to bypass the register settings of the
NCT7491 for fail safe operation.
Fan Drive
The NCT7491 uses pulse width modulation (PWM) to
control fan speed. This relies on varying the duty cycle (or
on/off ratio) of a square wave applied to the fan to vary the
fan speed. The external circuitry required to drive a fan using
PWM control is extremely simple. For 4wire fans, the
PWM drive may need only a pullup resistor. In many cases
the 4wire fan PWM input has an internal pullup resistor.
The NCT7491 PWM frequency can be set to a selection of
low frequencies or a single high PWM frequency. The low
frequency options are used for 3wire fans, while the high
frequency option is usually used with 4wire fans. For
3wire fans, a single Nchannel MOSFET is the only drive
device required. The specifications of the MOSFET depend
on the maximum current required by the fan being driven
and the input capacitance of the FET. Because a 10 k (or
greater) resistor must be used as a PWM pullup, an FET with
large input capacitance can cause the PWM output to
become distorted and adversely affect the fan control range.
This is a requirement only when using high frequency PWM
mode. Typical notebook fans draw a nominal 170 mA,
therefore, SOT devices can be used where board space is a
concern. In desktops, fans typically draw 250 mA to 300 mA
each. If several fans are driven in parallel from a single
PWM output or drive larger server fans, the MOSFET must
handle the higher current requirements. The only other
stipulation is that the MOSFET should have a gate voltage
drive, VGS < 3.3 V, for direct interfacing to the PWM output
pin.
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