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AN1762 APPLICATION NOTE

3.11.2 Slow Decay Mode (L6207 ONLY)

Figure 17 shows the operation of the bridge in the Slow Decay mode. At the start of the off time, the lower power

MOS is switched off and the current recirculates around the upper half of the bridge. Since the voltage across

the coil is low, the current decays slowly. After the dead time the upper power MOS is operated in the synchro-

nous rectification mode. When the monostable times out, the lower power MOS is turned on again after some

delay set by the dead time to prevent cross conduction.

Figure 17. Slow Decay Mode Output Stage Configurations

In some conditions (short off-time, very low regulated current, high motor winding L / R) the system may need

an on-time shorter than 1.5µs. In these cases the PWM current controller can loose the regulation.

Figure 18 shows the operation of the circuit in this condition. When the current first reaches the threshold, the

bridge is turned off for a fixed time and the current decays. During the following on-time current increases above

the threshold, but the bridge cannot be turned off until the minimum 1.5µs on-time expires. Since current in-

creases more in each on-time than it decays during the off-time, it keeps growing during each cycle, with steady

state asymptotic value set by duty-cycle and load DC resistance: the resulting peak current will be

Ipk = VS · D / RLOAD,

where D = tON / (tON + tOFF) is the duty-cycle and RLOAD is the load DC resistance.

Figure 18. Minimum on-time can cause the PWM controller to loose the regulation (L6207 ONLY).

A) ON TIME

B) 1

µs DEAD TIME

C) SYNCHRONOUS

RECTIFICATION

D) 1

µs DEAD TIME

D01IN1336

needed tON is less

than 1.5 µs

minimum tON

is about 1.5 µs

Vref / RSENSE

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