L6911E datasheet(15/34 Pages) STMICROELECTRONICS

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L6911E

Device description

15/34

5.5

Monitor and protection

The output voltage is monitored by means of pin 1 (VSEN). If it is not within ±10% (typ.) of

the programmed value, the powergood output is forced low.

The device provides overvoltage protection, when the output voltage reaches a value 17%

(typ.) greater than the nominal one. If the output voltage exceed this threshold, the OVP pin

is forced high (5V) and the lower driver is turned on as long as the over-voltage is detected.

The OVP pin is capable to deliver up to 60mA (min) in order to trigger an external SCR

connected to burn the input fuse. The low-side mosfet turn-on implement this function when

the SCR is not used and helps in keeping the ouput low.

To perform the overcurrent protection the device compares the drop across the high side

MOS, due to its

RDSON, with the voltage across the external resistor (ROCS) connected between the

OCSET pin and drain of the upper MOS. Thus the overcurrent threshold (IP) can be

calculated with the following relationship:

Equation 4

where the typical value of IOCS is 200µA.

To calculate the ROCS value it must be considered the maximum RDSON (also the variation

with temperature) and the minimum value of IOCS. To avoid undesirable trigger of

overcurrent protection this relationship must be satisfied:

Equation 5

where

∆I is the inductance ripple current and I

OUTMAX is the maximum output current.

In case of output short circuit the soft start capacitor is discharged with constant current

(10

µA typ.) and when the SS pin reaches 0.5V the soft start phase is restarted. During the

soft start the over-current protection is always active and if such kind of event occours, the

device turns off both mosfets, and the SS capacitor is dicharged again after reaching the

upper threshold of about 4V. The system is now working in HICCUP mode, as shown in

Figure 7 on page 16 a. After removing the cause of the over-current, the device restart

working normally without power supplies turn off and on.

OCS

⋅

DSON

--------------------------------

OUTMAX

∆l

-----

≥

PEAK

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