L6228Q datasheet(13/32 Pages) STMICROELECTRONICS

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L6228Q

Circuit description

Doc ID 14321 Rev 4

13/32

4.3

PWM current control

The L6228Q 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 Figure 12. As the current in the motor builds up 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 (VREF

A or

VREF

B) 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 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 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 shows the typical operating waveforms of the output current, the voltage drop

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 power MOS turns on, a high peak current flows through the sensing

resistor due to the reverse recovery of the freewheeling diodes. The L6228Q provides a 1

μs

blanking time t

BLANK that inhibits the comparator output so that this current spike cannot

prematurely re-trigger the monostable.

DRIVERS

DEAD TIME

DRIVERS

DEAD TIME

OUT2A(or B)

SENSEA(or B)

RSENSE

D01IN1332

RCA(or B)

ROFF

COFF

VREFA(or B)

IOUT

OUT1A(or B)

μs

5mA

BLANKER

SENSE

COMPARATOR

OUTPUT

MONOSTABLE

SET

2.5V

FROM THE

LOW-SIDE

GATE DRIVERS

2 PHASE

STEPPER MOTOR

BLANKING TIME

MONOSTABLE

VSA (or B)

TO GATE LOGIC

(0)

(1)

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