L6208Q STMicroelectronics, L6208Q Datasheet - Page 12

• Manufacturer Part Number: L6208Q
• Description: Dmos Driver For Bipolar Stepper Motor
• Manufacturer: STMicroelectronics

Circuit description
4.3
12/33
PWM current control
The L6208Q 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 between the source of the two lower power MOS
transistors and ground, as shown in
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
VREF
low-side MOS remain off for the time set by the monostable and the motor current
recirculates in the upper path. 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 is the sum of the monostable time plus the dead
time.
Figure 12. PWM current controller simplified schematic
Figure 13
across the sensing resistor, 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 low-side Power MOS turns on, a high peak current flows through the
sensing resistor due to the reverse recovery of the freewheeling diodes. The L6208Q
provides a 1 μs blanking time t
spike cannot prematurely re-trigger the monostable.
C
OFF
5V
5mA
B
(0)
) the sense comparator triggers the monostable switching the low-side MOS off. The
TO GATE LOGIC
R
RC A(or B)
OFF
shows the typical operating waveforms of the output current, the voltage drop
(1)
2.5V
Q
+
-
R
S
MONOSTABLE
BLANKING TIME
MONOSTABLE
SET
Doc ID 018710 Rev 1
BLANK
COMPARATOR
OUTPUT
Figure 12.
BLANKER
that inhibits the comparator output so that this current
1μs
COMPARATOR
SENSE
DEAD TIME
DRIVERS
+
-
+
VREF A(or B)
As the current in the load builds up the
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)
D01IN1332
I OUT
STEPPER MOTOR
A
2 PHASE
L6208Q
or