L7250 datasheet(24/46 Pages) STMICROELECTRONICS

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L7250

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2.7.2 Open Loop Commutation

After position sense is complete, the microcontroller commutates the motor following a constant acceleration

profile until sufficient BEMF is developed to reliably measure it.

The starting position of the open loop commutation, determined by the position sense routine, is set up by first

initializing the Memory Address Counter using LOADCP (*1), then clocking ADVANCE (*2) the appropriate

number of times (8 pulses per 6 state position). The spindle state will be OLCOAST while setting the initial state.

Then, drivers are enabled in either OL_SIX or OL_SIN modes (*3) , depending on whether 6 state or sine mode

open loop commutation is desired. Once the motor is accelerated up to an appropriate speed (*4) , the motor

is tri-stated by transitioning to the OLCOAST (*5) and then CLCOAST states, as described below, to synchro-

nize the Smoothdrive system to the motor.

Figure 5. Open Loop Commutation

2.7.3 Synchronization to Smoothdrive Commutation

When the open loop commutation is complete, the drivers are put in OLCOAST mode, and after a delay for set-

ting the Bemf sampling period, CLCOAST is asserted, so that a ZC Period (Tc, the time between two BEMF

zero crossings) can be detected and measured.

The BEMF sampling period is set in OLCOAST (*1) and after a delay (30 usec ) a Load CP (*2) is asserted.

After a delay of time Tc0 (300usec suggested) another Load CP is asserted (*3); this initializes the electrical

period for BEMF sampling. Once pregrammed the transition to CLCOAST (*4) , the BEMF is sampled at the

rate of Tc0 to look for two consecutive LOW readings (in anticipation of the LOW->HI zero crossing transition

(*5) ).

After the first ZC rising edge, the BEMF sampling period is refreshed to Tc0 value.

If two consecutive ZC edges are detected (*6), then after the last rising edge the Smoothdrive commutation is

synchronized with the motor rotor position and it is ready to be programmed in closed loop commutation .

At least two ZCs must be observed before transitioning to closed loop spinup (CLSIX or CLSIN) (*7a or *7b) .

This ensures that the Smoothdrive circuitry is synchronized to the spindle motor.

Set Load Coarse Phase

Write Reg.07H

LoadCP = 1

(*1)

Set ADVANCE

Write Reg.07H

Advance = 1

Nadv=Nalign

Inc Nadv

START

Open Loop Commutation

Nadv=0 , i=0

Note2: Nalign is received from

the Inductive Sense routine

Indicating the rotor position

alignement

Accelerate in

Sine or Six

Set Open Loop SINE

Write Reg.03H

Spstate[3:0] =0011

Set Open Loop SIX

Write Reg.03H

Spstate[3:0] =0010

Set ADVANCE

Write Reg.07H

Advance = 1

Wait the End of

RAMP_DELAY[ i ]

Inc i

i = RAMP_Steps

Set OLCOAST

Write Reg.03H

Spstate[3:0]=0001

EXIT

Open Loop

Commutation

(*2)

(*3)

(*4)

(*5)

SINE

SIX

Note1: Spstate[3:0] condition has

been set in OLCOAST

by the Inductive Sense

Routine

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