Datasheet

L6229Q Circuit description
Doc ID 15209 Rev 3 13/28
5.3 PWM current control
The L6229Q 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 9. 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 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 conduction 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 10 shows the typical operating waveforms of the output current, the voltage drop
across the sensing resistor, 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 reverse recovery of the freewheeling diodes. The L6229Q provides a 1 µs
blanking time t
BLANK
that inhibits the comparator output so that the current spike cannot
prematurely re trigger the monostable.
Figure 9. 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
(0) (1)