L6986I datasheet(19/60 Pages) STMICROELECTRONICS

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Figure 21. Input voltage effect on the load regulation of the isolated voltage

-20%

-15%

-10%

-5%

10%

15%

20%

100

IOUT [mA]

VIN = 12V

VIN = 18V

VIN = 24V

4.9

Overcurrent protection

Protection against over current conditions and/or short circuit at the output is ensured at both primary and

secondary side of the iso-buck converter.

Overcurrent protection at the primary side

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

current (see Table 5) in overcurrent condition.

The L6986I device implements a pulse by pulse current sensing on both power elements (high-side and low-side

switches), ensuring an 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.

Consequently, 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.

Summarizing, 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 assures 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. Under this condition the device can skip some cycle, leading

to a switching frequency reduction. The Figure 22 shows an example of valley current limit intervention in a short

circuit event (switching frequency set to 500 kHz, actual switching frequency 250 kHz).

L6986I

Overcurrent protection

DS13647 - Rev 1

page 19/60

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