Datasheet
11/27
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
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 1
µ
s Blanking Time t
BLANK
that inhibits the
comparator output so that this current spike cannot prematurely re-trigger the monostable.
DRIVERS
+
DEAD TIME
S
Q
R
DRIVERS
+
DEAD TIME
2H 1H
2L 1L
OUT2
A(or B)
SENSE
A(or B)
R
SENSE
D01IN1332
RC
A(or B)
R
OFF
C
OFF
VREF
A(or B)
I
OUT
OUT1
A(or B)
+
+
-
-
1µs
5mA
BLANKER
SENSE
COMPARATOR
COMPARATOR
OUTPUT
MONOSTABLE
SET
2.5V
5V
FROM THE
LOW-SIDE
GATE DRIVERS
2 PHASE
STEPPER MOTOR
BLANKING TIME
MONOSTABLE
VS
A
(or
B
)
TO GATE LOGIC
(0) (1)