AN1625 STMicroelectronics, AN1625 Datasheet - Page 12

• Manufacturer Part Number: AN1625
• Description: L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER
• Manufacturer: STMicroelectronics

AN1625 APPLICATION NOTE
2.11 Programmable off-time Monostable
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 12. As the current in the motor increases
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 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 mo-
tor 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 conduction in the bridge, delays the turn on of the power MOS, the effective Off Time t
the monostable time plus the dead time.
Figure 13 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
comparator output so that the current spike cannot prematurely retrigger the monostable.
Figure 12. PWM Current Controller Simplified Schematic
12/39
C
OFF
5V
5mA
(0)
RCOFF
R
TO GATE
OFF
(1)
LOGIC
2.5V
Q
+
-
R
S
MONOSTABLE
BLANKING TIME
MONOSTABLE
SET
1 s
COMPARATOR
SENSE
BLANKER
DEAD TIME
DRIVERS
GATE DRIVERS
+
-
FROM THE
LOW-SIDE
+
R SENSE
VREF
VS
B
SENSE B
DEAD TIME
DRIVERS
+
SENSE A
DEAD TIME
DRIVERS
BLANK
+
D02IN1380
VS
OFF
that inhibits the
A
is the sum of
VS
OUT 2
OUT 3
OUT 1