L6208Q datasheet(14/34 Pages) STMICROELECTRONICS

14 / 34 page

Circuit description

L6208Q

14/34

DocID018710 Rev 3

5.3

PWM current control

The L6208Q device 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 load

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 low-side

MOSFET off. The low-side MOSFET remains off for the time set by the monostable and the

motor current recirculates in the upper path. When the monostable times out, the bridge

again turns on. As the internal deadtime, used to prevent cross conduction in the bridge,

delays the turn-on of the power MOSFET, the effective OFF time is the sum of the

monostable time plus the deadtime.

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

Section 5.4.

Immediately after the low-side Power MOSFET turns on, a high peak current flows through

the sensing resistor due to the reverse recovery of the freewheeling diodes. The L6208Q

device provides a 1

s blanking time tBLANK that inhibits the comparator output so that this

current spike cannot prematurely re-trigger the monostable.

L6208Q датащи(PDF) 14 Page - STMicroelectronics