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MAX4051AEEE+T 其他数据使用手册 - Maxim Integrated（美信）
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MAX4051AEEE+T数据手册
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though a power-dissipating device is being used, there is still an overall power savings when fan speed
is reduced. See Figure 6.
Figure 5. Power dissipation in a linear-regulator pass element versus fan supply voltage.
Figure 6. Total power consumption of a linearly regulated fan circuit.
Startup and stall issues are related. Fans require a certain voltage before they will start. This is called
"startup voltage." Once a fan is already spinning, decreasing the voltage below the stall voltage will
cause the fan to stop. The startup voltage is equal to or (usually) greater than the stall voltage. Typically
they are 25% to 50% of the rated voltage for the fan. When linear regulation is used without speed
monitoring, there is no way of knowing if a fan has stalled or even started.
There are several solutions to this problem. One is to prevent voltages across the fan from going lower
than the startup voltage. Although this is easily accomplished in software, selecting the correct voltage to
ensure proper startup for all fans and accounting for aging can limit the useful range of speed control.
You might have to choose a minimum worst-case voltage of 60% nominal to make sure all fans will start.
This can be wasteful, considering that the average fan might easily be controlled down to 40%. Another
solution is to use a fan with a tachometer. The tachometer can now be monitored by a microcontroller,
allowing software to know when a fan has not started or if it has stalled. Although this method is
significantly more robust and less wasteful, it requires design time and additional hardware/software
resources.
DC-DC Regulation
DC-DC regulation is similar to linear regulation in that it controls the speed of the fan by adjusting the
DC voltage across it. However, unlike a linear regulator, a DC-DC regulator uses a switch-mode power
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