R6986 datasheet(29/62 Pages) STMICROELECTRONICS

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R6986

Functional description

5.7

Overcurrent protection

The current protection circuitry features a constant current protection, so the device limits

the maximum peak current (see Table 5: Electrical characteristics on page 8) in overcurrent

condition.

The R6986 device implements a pulse by pulse current sensing on both power elements

(high-side and low-side switches) for effective current protection over the duty cycle range.

The high-side current sensing is called “peak” the low-side sensing “valley”.

The internal noise generated during the switching activity makes the current sensing

circuitry ineffective for a minimum conduction time of the power element. This time is called

“masking time” because the information from the analog circuitry is masked by the logic to

prevent an erroneous detection of the overcurrent event. As a consequence, the peak

current protection is disabled for a masking time after the high-side switch is turned on, the

valley for a masking time after the low-side switch is turned on. In other words, the peak

current protection can be ineffective at extremely low duty cycles, the valley current

protection at extremely high duty cycles.

The R6986 device assures an effective overcurrent protection sensing the current flowing in

both power elements. In case one of the two current sensing circuitry is ineffective because

of the masking time, the device is protected sensing the current on the opposite switch.

Thus, the combination of the “peak” and “valley” current limits assure the effectiveness of

the overcurrent protection even in extreme duty cycle conditions.

The valley current threshold is designed higher than the peak to guarantee a proper

operation. In case the current diverges because of the high-side masking time, the low-side

power element is turned on until the switch current level drops below the valley current

sense threshold. The low-side operation is able to prevent the high-side turn on, so the

device can skip pulses decreasing the swathing frequency.

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