L6238 datasheet(25/35 Pages) STMICROELECTRONICS

25 / 35 page

actually exceeded the mask period and was de-

tected as a zero crossing. This resulted in im-

proper sequencing of the outputs relative to the

proper phases and caused the motor to spin

down.

If the application requires a slow rate of slew at

the output, an external network can be connected

as shown in figure 25. A resistor, Rl is selected to

achieve the desired slew rate when the system is

in phase lock. A second resistor, R2, in series

with a diode, Dl, is connected between the SLEW

RATE pin, and the LOCK output. At start up, the

LOCK output is low, and R2 is in parallel with Rl

resulting is a faster slew rate. When lock is

achieve, the LOCK output is high, and R2 is es-

sentially disconnected from the circuit.

5.7 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 26 shows the Ext FET connection and

demonstrates how

the

L6238

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

Al 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 Al to linearly drive the external P-

Channel FET Ql via inverter A2.

5.8 Bemf Sensing

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

27. For reliable operation, the Bemf signal ampli-

tude should be a minimum of

± 60 mV to be prop-

erly detected. In order to provide for noise immu-

nity, internal hysteresis is incorporated in the

detection circuitry to prevent false zero crossing

detection.

Figure 25: Dual Slew Rate.

Figure 26: External P-Fet.

L6238

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