L6699DTR STMicroelectronics, L6699DTR Datasheet - Page 25

• Manufacturer Part Number: L6699DTR
• Description: AC/DC Switching Converters Enhanced High Volt Res CTRL 600V
• Manufacturer: STMicroelectronics

Specifications of L6699DTR

• Product Category: AC/DC Switching Converters

L6699
The ISEN pin, which is also able to withstand negative voltages, is internally connected to
the input of a first comparator, referenced to
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
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
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
value can be determined using the equation:
Equation 7
where I
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
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
conditions. Overloads or short-circuits lasting less than T
Crpkx
is the maximum expected peak current flowing through the resonant capacitor
Figure 19
Figure 18
Doc ID 022835 Rev 2
operates as a capacitive current divider; Cs is typically
SH
Rs
, Rs is placed directly in series to the resonant tank. Its
that the converter is allowed to run under these
=
SS
P
. 0
I
Crpkx
(see
R s
Rs
77
≈
=
Section 6.3: Safe-start procedure
V
·
0
ISENx
I
. 0
(
Crpkx
4 .
76
1
I ·
+
(0.8 V typ. 0.76 V min.), and to that of a
C rp k x
Cs
C
r
SH
)
Current sensing, OCP and OLP
Figure 18
do not cause any other action,
and
19
exceeds 0.8 V
). This quickly
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