L6985F datasheet(29/65 Pages) STMICROELECTRONICS

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L6985F

Functional description

4.7

Overcurrent protection

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

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

condition.

The L6985F 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 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 L6985F 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 switching frequency.

L6985F 数据表(PDF) 29 Page - STMicroelectronics