AN4339 datasheet(21/27 Pages) STMICROELECTRONICS

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AN4339

Description

1.4.4

TACHO

It allows developing an easy speed control loop by using an external op-amp. This is an

open drain output. At each rising edge of the Hall effect sensors H1, the monostable is

triggered and the MOSFET connected to the pin TACHO is turned off for a constant time

tPULSE (as shown in Figure 12). The off time tPULSE can be set using the external RC

network (RPUL, CPUL) connected to the pin RCPULSE. Figure 12 gives the relation between

tPULSE and CPUL, RPUL. We have approximately:

Equation 30

tPULSE = 0.6 · RPUL · CPUL

Figure 12. tPULSE versus CPUL and RPUL

Where CPUL should be chosen in the range of 1 nF to 100 nF and RPUL in the range

of 20 K

 to 100 K. By connecting the tachometer pin to an external pull-up resistor, the

output signal average value VM is proportional to the frequency of the Hall effect signal and,

therefore, to the motor speed. This realizes a simple frequency-to-voltage converter. An

op-amp, configured as an integrator, filters the signal and compares it with a reference

voltage VREF, which sets the speed of the motor. So we can also connect the non-inverting

pin of op-amp by using a potentiometer at VREF pin to vary the speed of motor.

Equation 31

tPULSE

------------------ V



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