AN2836 datasheet(20/57 Pages) STMICROELECTRONICS

20 / 57 page

Designing an application with L6225, L6226, L6227

AN2836

20/57

Doc ID 15084 Rev 1

4.11

Programmable off-time monostable (L6227 only)

The L6227 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 14. 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. Since 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 14.

PWM current control circuitry (L6227 only)

Figure 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. 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 L6227 provides a 1 µs

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

prematurely re-trigger the monostable.

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