Applications Note AN36
Issue1 - JULY 2002
When driving the SPD pin from a signal or
thermistor remote from the fan or controller
there may be a need to provide noise
protection at the SPD pin. In order to maintain
a smooth speed control characteristic this
will take the form of a capacitor of between
100nF and 1F connected between the SPD
and Gnd pins somewhere close to the device.
This should suffice for most purposes and
can be determined for each particular
application.
A capacitor on this pin also has an effect on
the start-up characteristic of the motor as the
charging of the capacitor will lag behind the
rise of the supply voltage. As a low voltage on
the SPD pin represents a higher speed the
effect at start-up, if the SPD value is not set for
full speed, will be for the motor speed to
increase past its allotted point before settling
back to that required. Whilst for the capacitor
values stated above this effect may not be
noticeable, for values greater than 1F it can
be used to advantage. In situations where the
motor will be regularly started at a speed
lower than its practical start value it can help
to get motors started, in effect giving the
motor a ‘kick’ to start it.
Figure 5
showing resistive divider used with <100k⍀ thermistors
One problem with using an external potential
divider (R1 & R2 in Figure 5) is that the motor
becomes susceptible to supply voltage
changes. Say the supply voltage falls,
normally the speed would drop due to less
voltage across the windings, however as the
potential divider is across the supply, as the
voltage falls so the voltage at the SPD pin will
fall. As a lower SPD voltage represents a
higher speed so a lowering of the supply
therefore causes the fan to speed up. To
overcome this it is probably best to use a
Zener regulated supply to the ZXBM200x.
Table 1
MOSFET
R1 R2
3.3k⍀ 2.2k⍀ 3.3k⍀ 1.5k⍀
Bipolar
Thermistor Supply
Value
Voltage
R1 R2
The following Table 1 gives an indication of
the values required for R1 and R2 against
different thermistor values. It should be
noted that these are only for guidance as the
final value will depend upon the thermistor
and motor characteristics and so will need
fine adjustment by experimentation. If the
ZXBM200x device is to be run from a Zener
regulated supply to control the speed verses
supply voltage variation so R1 and R2 will
need allowances made for the voltage value
of Zener chosen.
5V
4.7k⍀
12V 6.8k⍀ 1.2k⍀ 6.8k⍀ 1.0k⍀
24V
5V
15k⍀ 1.3k⍀ 15k⍀ 1.0k⍀
5.6k⍀ 3.3k⍀ 5.6k⍀ 2.4k⍀
13k⍀ 2.2k⍀ 13k⍀ 1.8k⍀
36k⍀ 3.3k⍀ 33k⍀ 2.2k⍀
24k⍀ 12k⍀ 22k⍀ 8.6k⍀
56k⍀ 9.1k⍀ 56k⍀ 7.5k⍀
10k⍀
47k⍀
12V
24V
5V
12V
24V 130k⍀ 10k⍀ 130k⍀ 8.6k⍀
As the Bipolar drivers have a slower turn-off
characteristics when driven from the
ZXCB200x series slightly different values of
R2 are required over those for MOSFETs.
5V
*
*
*
*
*
*
*
*
*
*
*
*
100k⍀
12V
24V
* Note: Not normally required - dependent upon
thermistor and speed range needed.
AN 36 - 5
ZETEX [ ZETEX SEMICONDUCTORS ]
