L6238S datasheet(20/31 Pages) STMICROELECTRONICS

20 / 31 page

given application is too slow. Figure 4-9 is an os-

cillograph taken on a device that had a fairly large

value for Rslew and failed to spin up and phase

lock a motor.

The problem manifests itself as the motor begins

to spin up. At lower RPMs, the Bemf of the motor

is relatively small resulting in higher amounts of

commutation current. In figure 4-9, the upper

waveform is the voltage appearing at OUTPUT

relative to the CENTER TAP input. The lower

waveform is the actual output of the Bemf ampli-

fier available on special engineering prototypes.

The oscillograph was taken just as the problem

occured. The period between zero crossings was

~800

µs resulting in a mask time period of 200µs.

As can be seen, the excessively long slew rate

actually exceeded the mask period and was de-

tected as a zero crossing.

This resulted in improper sequencing of the out-

puts relative to the proper phases and caused the

motor to spin down.

4.7 Ext PFET Driver

The power handling capabilities of the 3 phase

output stage can be extended with the addition of

a single P-Channel FET.

Figure 4-10 shows the Ext FET connection and

demonstrates how the L6238S automatically

senses the FETs presence. When the voltage at

the Gate Drive pin is

≥ 0.7V, the output of com-

parator A3 goes high, removing the variable drive

A1 from the internal FETs and connects them in-

stead to Vanalog via the commutation switches to

facilitate full conduction.

The upper FETs drive paths are not shown for

clarity. A3 also closes SW2 allowing A1 to linearly

drive the external P-Channel FET Q1 via inverter

A2.

4.8 Bemf Ampolifier

Since no Hall Effect Sensors are required, the

commutation information is derived from the Bemf

voltage zero-crossings of the undriven phase with

respect to the center tap. The Bemf comparator

and associated signal levels are depicted in figure

4-11. For reliable operation, the Bemf signal am-

plitude should be a minimum of

± 60 mV to be

properly detected. In order to provide for noise

immunity, internal hysteresis is incorporated in

the detection circuitry to prevent false zero cross-

ing detection.

For laboratory evaluation purposes, a simple re-

Figure 4-10: External P-Fet.

L6238S

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