L6235D datasheet(14/25 Pages) STMICROELECTRONICS

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L6235

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TACHO

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

14. For component values refer to Application Information section.

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 pin TACHO is turned off for a constant time

tPULSE (see Figure 15). The off time tPULSE can be set using the external RC network (RPUL, CPUL) connected

to the pin RCPULSE. Figure 16 gives the relation between tPULSE and CPUL, RPUL. We have approximately:

tPULSE = 0.6 · RPUL · CPUL

where CPUL should be chosen in the range 1nF … 100nF and RPUL in the range 20KΩ … 100KΩ.

By connecting the tachometer pin to an external pull-up resistor, the output signal average value VM is propor-

tional to the frequency of the Hall Effect signal and, therefore, to the motor speed. This realizes a simple Fre-

quency-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.

Figure 14. Tacho Operation Waveforms.

PULS E

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

⋅

t PULSE

VTACHO

VDD

L6235D Datenblatt(PDF) 14 Page - STMicroelectronics