AN2839 datasheet(14/40 Pages) STMICROELECTRONICS

14 / 40 page

Designing an application with L6228

AN2839

14/40

Doc ID 15104 Rev 1

1.11

Programmable off time monostable

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 between the source of the two lower power MOSFET

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 (VrefA or VrefB),

the sense comparator triggers the monostable, switching the bridge off. The power

MOSFETs 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 MOSFETs, the effective off time is

the sum of the monostable time plus the dead time.

Figure 12.

Simplified schematic of the PWM current controller

Figure 13 on page 15 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 MOSFET 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 µs

blanking time, tBLANK, that inhibits the comparator output so that this current spike cannot

prematurely re-trigger the monostable.

AN2839 Datasheet(PDF) 14 Page - STMicroelectronics