L6228 STMicroelectronics, L6228 Datasheet - Page 11

• Manufacturer Part Number: L6228
• Description: DMOS DRIVER FOR BIPOLAR STEPPER MOTOR
• Manufacturer: STMicroelectronics

PWM CURRENT CONTROL
The L6228 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
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
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 L6228 provides a 1 s Blanking Time t
comparator output so that this current spike cannot prematurely re-trigger the monostable.
C
OFF
5V
5mA
(0)
TO GATE LOGIC
R
RC A(or B)
OFF
(1)
2.5V
Q
+
-
R
S
MONOSTABLE
BLANKING TIME
MONOSTABLE
SET
COMPARATOR
OUTPUT
BLANKER
1 s
COMPARATOR
SENSE
DEAD TIME
DRIVERS
+
-
+
VREF A(or B)
GATE DRIVERS
FROM THE
LOW-SIDE
2H
2L
R SENSE
DEAD TIME
VS
DRIVERS
A
+
(or
SENSE A(or B)
B
)
1H
1L
OUT2 A(or B)
OUT1 A(or B)
A
D01IN1332
BLANK
or VREF
I OUT
that inhibits the
STEPPER MOTOR
B
) the sense
2 PHASE
L6228
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