EVAL6208N Datasheet, PDF (11/27 Pages) STMicroelectronics – DMOS DRIVER FOR BIPOLAR STEPPER MOTOR

L6208
PWM CURRENT CONTROL
The L6208 includes a constant off time PWM current controller for each of the two bridges. The current control
circuit senses the bridge current by sensing the voltage drop across an external sense resistor connected be-
tween the source of the two lower power MOS transistors and ground, as shown in Figure 10. As the current in
the motor builds up the voltage across the sense resistor increases proportionally. When the voltage drop
across the sense resistor becomes greater than the voltage at the reference input (VREF_A or VREF_B) the sense
comparator triggers the monostable switching the bridge off. The power MOS remain off for the time set by the
monostable and the motor current recirculates as defined by the selected decay mode, described in the next
section. When the monostable times out the bridge will again turn on. Since the internal dead time, used to pre-
vent cross conduction in the bridge, delays the turn on of the power MOS, the effective off time is the sum of the
monostable time plus the dead time.
Figure 10. PWM Current Controller Simplified Schematic
TO GATE LOGIC
5mA
(0)
(1)
S
Q
R
-
5V
+
2.5V
C_OFF
RCA(or _B)
R_OFF
BLANKING TIME
MONOSTABLE
1µs
VS_A (or _B)
FROM THE
LOW-SIDE
GATE DRIVERS
MONOSTABLE
SET
BLANKER
DRIVERS
+
DEAD TIME
2H
DRIVERS
+
DEAD TIME
1H
IOUT
2 PHASE
STEPPER MOTOR
OUT2A(or _B)
OUT1A(or _B)
SENSE
COMPARATOR
+
COMPARATOR
-
OUTPUT
VREFA(or _B)
2L
RSENSE
1L
SENSEA(or _B)
D01IN1332
Figure 11 shows the typical operating waveforms of the output current, the voltage drop across the sensing re-
sistor, the RC pin voltage and the status of the bridge. More details regarding the Synchronous Rectification and
the output stage configuration are included in the next section.
Immediately after the Power MOS turns on, a high peak current flows through the sensing resistor due to the
reverse recovery of the freewheeling diodes. The L6208 provides a 1µs Blanking Time t_BLANK that inhibits the
comparator output so that this current spike cannot prematurely re-trigger the monostable.
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