L6235PD datasheet(20/35 Pages) STMICROELECTRONICS

20 / 35 page

Tachometer

L6235

20/35

DocID7618 Rev 2

Tachometer

A tachometer function consists of a monostable, with constant off time (tPULSE), whose input

is one hall effect signal (H1). It allows developing an easy speed control loop by using an

external op amp, as shown in Figure 16. For component values refer to Section 11:

Application information on page 25.

The monostable output drives an open drain output pin (TACHO). 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 (see Figure 17). The off time tPULSE can be

set using the external RC network (RPUL, CPUL) connected to the pin RCPULSE. Figure 18

gives the relation between tPULSE and CPUL, RPUL. We have approximately:

Equation 5

tPULSE = 0.6 · RPUL · CPUL

where CPUL should be chosen in the range 1nF to 100 nF and RPUL in the range 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.

Equation 6

Figure 16. TACHO operation waveforms

tPULSE

----------------- VDD



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