L6208Q View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6208Q' PDF : 34 Pages View PDF
Circuit description
L6208Q
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
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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.
14/34
DocID018710 Rev 3
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