L6208PD datasheet(14/35 Pages) STMICROELECTRONICS

14 / 35 page

PWM current control

L6208

14/35

DocID7514 Rev 2

PWM current control

The L6208 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

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 (VREFA

or VREFB) the sense comparator triggers the monostable switching the bridge off. The

power MOS remains off for the time set by the monostable and the motor current

recirculates as defined by the selected decay mode, described in Section 7: Decay modes

on page 18. When the monostable times out the bridge will again turn on. Since the internal

deadtime, 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 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 7: Decay modes on page 18.

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 L6208 device provides

a 1

s blanking time t

BLANK that inhibits the comparator output so that this current spike

cannot prematurely retrigger the monostable.

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