AN1625 STMicroelectronics, AN1625 Datasheet - Page 31

• Manufacturer Part Number: AN1625
• Description: L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER
• Manufacturer: STMicroelectronics

AN1625 APPLICATION NOTE
The limits of this system depend on the fact that the speed information is obtained by an analog integration of
the TACHO output. This operation introduces a low frequency pole in the Gl
function, and a ripple in the ref-
oop
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
gain (G
(0)) diminution may be needed. At the same time, decreasing the reference voltage ripple requires
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
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