L6207 datasheet(14/53 Pages) STMICROELECTRONICS

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AN1762 APPLICATION NOTE

14/53

3.11 Programmable off-time Monostable (L6207 ONLY)

The L6207 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 be-

tween the source of the two lower power MOS 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 com-

parator triggers the monostable switching the low-side MOS off. The low-side MOS remain off for the time set

by the monostable and the motor current recirculates in the upper path. When the monostable times out the

bridge will again turn on. Since the internal dead time, 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 dead time.

Figure 14. PWM Current Control Circuitry (L6207 ONLY).

Figure 15 shows the typical operating waveforms of the output current, the voltage drop across the sensing re-

sistor, the RC pin voltage and the status of the bridge. Immediately after the low-side Power MOS turns on, a

high peak current flows through the sensing resistor due to the reverse recovery of the freewheeling diodes. The

L6207 provides a 1µs Blanking Time tBLANK that inhibits the comparator output so that this current spike cannot

prematurely re-trigger the monostable.

DRIVERS

DEAD TIME

DRIVERS

DEAD TIME

OUT2A(or B)

SENSEA(or B)

RSENSE

D02IN1352

RCA(or B)

ROFF

COFF

VREFA(or B)

IOUT

OUT1A(or B)

µs

5mA

BLANKER

SENSE

COMPARATOR

OUTPUT

MONOSTABLE

RESET

2.5V

FROM THE

LOW-SIDE

GATE DRIVERS

LOADA

(or B)

BLANKING TIME

MONOSTABLE

VSA (or B)

TO GATE LOGIC

(0)

(1)

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