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AN1625 APPLICATION NOTE

The limits of this system depend on the fact that the speed information is obtained by an analog integration of

erence voltage. The introduced pole, in conjunction with the very low frequency mechanical pole of the motor,

strongly affects loop stability, system bandwidth and static speed regulation error: to preserve stability a DC loop

to reduce the frequency of the integrator pole, cutting the bandwidth and reducing the phase margin.

A full digital approach to convert the tacho frequency in a voltage can give further improvement in static and

dynamic speed regulation: a microcontroller can measure the frequency of the tacho output and provide the ref-

erence voltage to the L6235 through a D/A converter or a low-pass filtered (see Reference Voltage section)

PWM output (whose frequency can be much higher than the hall effect signals frequency, resulting in a strongly

reduced Vref ripple).

Another possibility is using a PLL to generate a voltage proportional to the speed (or used directly, taking a fre-

quency input as the command).

2.16 Brake

In general, motor braking can be achieved making a short circuit across the windings: the BEMF forces a cur-

rent, proportional to the braking torque, that flows in the opposite direction than in normal running mode. For

high BEMF and inertia moment the current may reach very high values: a power resistor is often used to reduce

the maximum braking current and dissipate the motor energy.

L6235 Brake pin can be used to quickly stop the motor while it is running: providing a low logic level to this pin

all the high-side DMOS switch on, making a short-circuit across the motor windings.

A power resistor is not used: while the motor is braking, both Thermal and Over Current protections still work,

avoiding BEMF to cause a current exceeding the device's maximum ratings.

Connecting EN and DIAG pins together and using a RC network (see Over Current Detection section) a disable

time between each over current event can be set, reducing the maximum r.m.s. value of the current.

Figure 34 and Figure 35 show what happens if the current exceeds the OCD threshold while the motor is brak-

ing: as soon as the current in one of the three motor phases reaches the OCD threshold (5.6 A typ.) the open

drain mosfet internally connected to the DIAG pin discharges the external capacity; the EN pin voltage falls to

GND and all the bridges of the device are disabled for a time that depends on the RC network values. During

this disable time the current forced by the BEMF decreases, and so the braking torque; when the current be-

comes zero (because the motor inductances have been fully discharged), if the BEMF is less than the supply

voltage there is no braking effect (since the freewheeling diodes cannot be turned on) until the disable time ex-

pires and all the high side PowerDMOS turn on again