L8229 datasheet(33/42 Pages) STMICROELECTRONICS

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9.3

Decay Modes and Synchronous Rectification

For stepping motor drive, the recirculation mode during Toff could be slow, fast or mixed

(that is fast followed by slow decay): W2, by means of bit 3 (for Out1) and bit 4 (for Out2),

allows to select MIXED or SLOW decay for bridge 1 and bridge 2 respectively; by selecting

(with W1, bits 9 to 12) Tfast longer than Toff, it is possible to select FAST decay for all the

Toff duration.

For DC motor drive, W2 bit 15 allows to choose slow or fast decay.

During the decay, it is possible for the current to flow through the intrinsic body diodes of the

LDMOS or through switched on LDMOS channel (in this case the current conduction will be

reversed from Source to Drain). This switching on of the active device in parallel with the

diode is called SYNCHRONOUS rectification and means that the recirculation path is not

obtained only through the internal diodes but also by turning on the DMOS in parallel with

the diode needed to recirculate current; because of the low RDSon of this LDMOS the

dissipation is reduced.

For stepping motors only with W1, by means of bits 13 and 14, it is possible to choose

several kinds of SYNCHRONOUS recirculation modes: ACTIVE recirculation, PASSIVE

recirculation, SYNC OFF recirculation, and LOW SIDE recirculation.

ACTIVE means that the current is monitored to avoid reverse current in the load: this could

happen because of FAST SYNC recirculation (that reverse the conduction of the bridge).

When a reverse current is detected, synchronous rectification is turned off, so the outputs

are placed in Hi-Z state until the end of Toff. Please note that Active Synchronous

rectification can't be used when in Slow Decay mode.

PASSIVE means that the current is not monitored to avoid a reverse current in the load. In

this case the current, during Toff, could be inverted and no action is taken until the current

reaches the current limit selected by the user by means of Vref and Rsense. When this

Table 33.

Stepping decay mode selected by W2

Bit 3 (4)

Mode

Mixed decay

Slow decay

Table 34.

DC decay mode selected by W2

Bit 15

Mode

0Slow

1Fast

Table 35.

Sync rectification selected by W1

Bit 14

Bit 13

Mode

Active(1)

Only for Mixed Decay mode.

Passive

Off

Low side

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