Datasheet

L6699 Current sensing, OCP and OLP
Doc ID 022835 Rev 2 25/38
The ISEN pin, which is also able to withstand negative voltages, is internally connected to
the input of a first comparator, referenced to
V
ISENx
(0.8 V typ. 0.76 V min.), and to that of a
second comparator referenced to 1.5 V. The operation of the second comparator is
described later.
If the voltage externally applied to the pin by either circuit in
Figure 18 and 19 exceeds 0.8 V
the first comparator is tripped and this causes an internal switch to be turned on for 5 µs and
discharges the soft-start capacitor C
SS
(see Section 6.3: Safe-start procedure ). This quickly
increases the oscillator frequency and thereby limits energy transfer. Under output short-
circuit conditions, this operation results in a peak primary current that periodically oscillates
below the maximum value allowed by the sense resistor Rs.
In the circuit shown in
Figure 18 , Rs is placed directly in series to the resonant tank. Its
value can be determined using the equation:
Equation 7
where I
Crpkx
is the maximum expected peak current flowing through the resonant capacitor
and the primary winding of the transformer, which is related to the maximum load and the
minimum input voltage. The power dissipation in Rs can be approximately found with:
Equation 8
The circuit shown in
Figure 19 operates as a capacitive current divider; Cs is typically
selected equal to Cr/100 or less and is a low-loss type, and the sense resistor Rs is selected
as:
Equation 9
and the associated power dissipation is reduced by a factor (1+Cr/Cs). This circuit is then
recommended when the efficiency target is very high.
This OCP is effective in limiting primary-to-secondary energy flow in case of an overload or
an output short-circuit, but the output current through the secondary winding and rectifiers
under these conditions may be so high that it endangers converter safety if continuously
flowing. To prevent any damage it is customary to force the converter to work intermittently,
which brings the average output current to values such that the thermal stress for the
transformer and the rectifiers can be easily handled.
If intermittent operation upon overload or short-circuit is desired, the designer can program
externally the maximum time T
SH
that the converter is allowed to run under these
conditions. Overloads or short-circuits lasting less than T
SH
do not cause any other action,
Crpkx
I
76.0
=Rs
CrpkxRs
I ·4.0P ≈
)(
Cs
C
+1 ·
I
77.0
=Rs
r
Crpkx