Datasheet

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L6235
PWM CURRENT CONTROL
The L6235 includes a constant off time PWM Current Controller. The current control circuit senses the bridge
current by sensing the voltage drop across an external sense resistor connected between the source of the
three lower power MOS transistors and ground, as shown in Figure 7. As the current in the motor increases the
voltage across the sense resistor increases proportionally. When the voltage drop across the sense resistor be-
comes greater than the voltage at the reference input pin VREF 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 around the upper half of the bridge in Slow Decay Mode as described in the next section. When the
monostable times out, the bridge will again turn on. Since the internal dead time, used to prevent cross conduc-
tion in the bridge, delays the turn on of the power MOS, the effective Off Time t
OFF
is the sum of the monostable
time plus the dead time.
Figure 8 shows the typical operating waveforms of the output current, the voltage drop across the sensing re-
sistor, the pin RC 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 turn on, a high peak current flows through the sense resistor due to the re-
verse recovery of the freewheeling diodes. The L6235 provides a 1µs Blanking Time t
BLANK
that inhibits the
comparator output so that the current spike cannot prematurely retrigger the monostable.
Figure 7. PWM Current Controller Simplified Schematic
DRIVERS
+
DEAD TIME
S
Q
R
DRIVERS
+
DEAD TIME
DRIVERS
+
DEAD TIME
OUT
3
OUT
2
SENSE
B
SENSE
A
R
SENSE
D02IN1380
RCOFF
R
OFF
C
OFF
VREF
OUT
1
+
+
-
-
1µs
5mA
BLANKER
SENSE
COMPARATOR
MONOSTABLE
SET
2.5V
5V
FROM THE
LOW-SIDE
GATE DRIVERS
BLANKING TIME
MONOSTABLE
VS
B
VS
VS
A
TO GATE
LOGIC
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