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L6228

PWM CURRENT CONTROL

The L6228 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

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 L6228 provides a 1

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

OUT2A(or B)

SENSEA(or B)

RSENSE

D01IN1332

RCA(or B)

VREFA(or B)

IOUT

OUT1A(or B)

+

+

-

-

1

µs

5mA

BLANKER

SENSE

COMPARATOR

COMPARATOR

OUTPUT

MONOSTABLE

SET

2.5V

5V

FROM THE

LOW-SIDE

GATE DRIVERS

2 PHASE

STEPPER MOTOR

BLANKING TIME

MONOSTABLE

TO GATE LOGIC

(0)

(1)