L6911E View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6911E' PDF : 34 Pages View PDF
L6911E
Device description
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
IP
=
I--O----C----S-----⋅---R----O----C----S--
RDSON
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
IP
≥
IOUTMAX
+
-∆---l
2
=
IPEAK
where ∆I is the inductance ripple current and IOUTMAX is the maximum output current.
In case of output short circuit the soft start capacitor is discharged with constant current
(10µA 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.
15/34
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