L6983I datasheet(23/56 Pages) STMICROELECTRONICS

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Device protections

7.1

Overvoltage protection

The overvoltage is a second level protection, and it should never be triggered in normal operating conditions if the

system is properly dimensioned. In other words, the selection of the external power components and the dynamic

performance determined by the compensation network should guarantee an output voltage regulation within the

overvoltage threshold even during the worst-case scenario in terms of load transitions.

7.2

Overcurrent protection

Primary side

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

current (see Table 6. Electrical characteristics) in an overcurrent condition.

The L6983I 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” (see section

Section 5.1.4 Minimum on-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 (typ. 390 ns) after the high-side switch is turned on. The masking time for the valley sensing

(TOFF_MIN, see Table 6. Electrical characteristics) 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 L6983I device assures an effective overcurrent protection sensing the current flowing in both power

elements. In case one of the two current sensing circuitries 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 (IPK and IVY respectively) assure the effectiveness of the overcurrent protection even in extreme duty cycle

conditions.

Figure 28. Effects of the peak and valley current protections on duty cycle and frequency

L6983I

Device protections

DS14040 - Rev 1

page 23/56

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