L6238 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6238' PDF : 35 Pages View PDF
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
Figure 26: External P-Fet.
Figure 25: Dual Slew Rate.
L6238
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.
25/35
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