START MODE

During startup, all 3 back EMF phases are used.

The output going to the BEMF_Det line is the ex-

clusive or of all 3 phases, and follows the polarity

of whichever phase is currently tristated (relative

to the center tap voltage). If the processor is driv-

ing the motor directly from the CPR,

the back

EMF circuitry is still active. Tripolar mode by na-

ture prevents EMF detection. EMF transitions are

partially blanked by the controller chip in all

modes.

Run Phased Mode

The Run Phased Mode is now the default (and

only) state in the DOLPHIN during run mode.

The chop waveform is injected into the back emf

waveform of the unused winding due to trans-

former coupling and the action of the center tap.

Most of the transient is removed by using a high

CMRR amplifier before the signal gets to the

sample/hold circuit, and by holding during the off

time of the chop. However, a small residual is left

on the sample/hold waveform.

This small tran-

sient must be kept away from the back EMF

crossing point or it will cause an unstable loop.

This can only be done if the PWM on edge is at

the proper timing relative to the back EMF point.

A comparator is provided internally that generates

the proper delay point by comparing the back

EMF waveform against a small offset voltage

(settable externally). The programmer must then

vary the commutation timing until the PH_DET bit

goes high, indicating that the chop and EMF

crossing waveforms are properly phased. This

timing is produced by modifying the phase delay

in the controller chip until PH_DET is high.

Note that the PH_DET bit is updated at the rising

edge of the chop blanking comparator.

If the

On_Del signal (the blanking signal that frames the

EMF on transient) is high during the edge, then

the transient is in the proper position.

The timing of the proper position is set by several

external variables; the PWM frequency, the run-

ning RPM speed, and the external reference volt-

age.

The relationships of the chop blanking compara-

tor, the On_Del signal, and the Ph_Det bit in the

serial port are illustrated in fig. 13.

Back Emf Detection

Back EMF detection has 2 different modes of operation: Start mode (detemined by the manufacturer),

and Run Phased mode. These are determined by the state of the Start bit.

BACK EMF RUN MODE SPECIFICATIONS

Parameter

Test Conditions

Min.

Typ.

Max.

Units

Rshold Output Impedance

25°C

150

250

Ω

Chop Blank Pulse width tchb

(1)

23.5

25

27.5

µs

On Delay width

(2)

20

22

24

% of ramp

rise time

BEMF start mode detect offset - Vebias

±40mV

BEMF detect hysteresis

10mV nominal

(1) At 6000 rpm, 8 pole motor, sinusoidal EMF from center tap to the A phase of 8 volts p-p, Rref = 62.5K 1%, Rref2 = 120K 1%.

(2) on delay width is from the end of off time to the end of On_Del. Measured at max duty cycle (no off time due to feedforward compensa-

tion). Specification allows for toff min, to set nominally 20% width of total cycle time.

Figure 13. Back EMF Chop Blanking Mode

Waveforms

L6256

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