L6235

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SLOW DECAY MODE

Figure 11 shows the operation of the bridge in the Slow Decay mode during the Off Time. At any time only two

legs of the three-phase bridge are active, therefore only the two active legs of the bridge are shown in the figure

and the third leg will be off. At the start of the Off Time, the lower power MOS is switched off and the current

recirculates around the upper half of the bridge. Since the voltage across the coil is low, the current decays slow-

ly. After the Dead Time the upper power MOS is operated in the synchronous rectification mode reducing the

impendence of the freewheeling diode and the related conducting losses. When the monostable times out, up-

per MOS that was operating the synchronous mode turns off and the lower power MOS is turned on again after

some delay set by the Dead Time to prevent cross conduction.

Figure 11. Slow Decay Mode Output Stage Configurations

DECODING LOGIC

The Decoding Logic section is a combinatory logic that provides the appropriate driving of the three-phase

bridge outputs according to the signals coming from the three Hall Sensors that detect rotor position in a 3-

phase BLDC motor. This novel combinatory logic discriminates between the actual sensor positions for sensors

spaced at 60, 120, 240 and 300 electrical degrees. This decoding method allows the implementation of a uni-

versal IC without dedicating pins to select the sensor configuration.

There are eight possible input combinations for three sensor inputs. Six combinations are valid for rotor posi-

tions with 120 electrical degrees sensor phasing (see Figure 12, positions 1, 2, 3a, 4, 5 and 6a) and six combi-

nations are valid for rotor positions with 60 electrical degrees phasing (see Figure 14, positions 1, 2, 3b, 4, 5

and 6b). Four of them are in common (1, 2, 4 and 5) whereas there are two combinations used only in 120 elec-

trical degrees sensor phasing (3a and 6a) and two combinations used only in 60 electrical degrees sensor phas-

ing (3b and 6b).

The decoder can drive motors with different sensor configuration simply by following the Table 2. For any input

tion 3a is the same than 3b and analogously output configuration 6a is the same than 6b.

The sequence of the Hall codes for 300 electrical degrees phasing is the reverse of 60 and the sequence of the

Hall codes for 240 phasing is the reverse of 120. So, by decoding the 60 and the 120 codes it is possible to drive

the motor with all the four conventions by changing the direction set.

A) ON TIME

B) 1

µs DEAD TIME

C) SYNCHRONOUS

RECTIFICATION

D) 1

µs DEAD TIME

D01IN1336