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L6235PD

DMOS driver for 3-phase brushless DC motor

STMicroelectronics 

L6235

6

PWM current control

PWM current control

The L6235 device includes a constant off time PWM current controller. The current control

circuit senses the bridge current by sensing the voltage drop across an external sense

resistor connected between the source of the three lower power MOS transistors and

ground, as shown in Figure 9. As the current in the motor increases 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 pin VREF 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 around the upper half of the bridge

in slow decay mode as described in Section 7: Slow decay mode on page 17. 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 tOFF is the sum of the monostable time plus the deadtime.

Figure 10 shows the typical operating waveforms of the output current, the voltage drop

across the sensing resistor, the pin RC voltage and the status of the bridge. More details

regarding the synchronous rectification and the output stage configuration are included in

Section 7.

Immediately after the power MOS turns on, a high peak current flows through the sense

resistor due to the reverse recovery of the freewheeling diodes. The L6235 device provides

a 1 µs blanking time tBLANK that inhibits the comparator output so that the current spike

cannot prematurely retrigger the monostable.

Figure 9. PWM current controller simplified schematic

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