L6983 datasheet(22/63 Pages) STMICROELECTRONICS

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6.8

Overcurrent protection

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

current (please refer to Table 6. Electrical characteristics) in overcurrent condition.

The L6983 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. Therefore,

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

time for the valley sensing is activated 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 L6983 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.

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.

Figure 19. Over current protection behavior

In worst case scenario, reported in Figure 19. Over current protection behavior of the overcurrent protection the

switch current is limited to:

IMAX=IVY+VIN−VOUT

L ∙TMASKHS

(7)

Where IVY is the current threshold of the valley sensing circuitry (please refer to Table 6. Electrical

characteristics) and TMASKHS is the masking time of the high-side switch (75 ns typ.).

L6983

Overcurrent protection

DS13116 - Rev 1

page 22/63

L6983 数据表(PDF) 22 Page - STMicroelectronics