STSR2P/STSR2PM
6/12
EXAMPLE OF COMPONENTS SELECTION FOR A FORWARD CONVERTER
Forward Specification:
V
IN
=36-72V
V
OUT
=3.3V
n=Np/Ns=4.5
R
3
and R
4
are calculated assuring a minimum voltage of 2.8V at CK pin. At 36V input, the voltage on the
secondary winding is 36/4.5=8V. Choosing R
3
=1.5KΩ,R
4
results to be:
R
4
=1kΩ is chosen. At 72V input the current at CK pin is calculated as:
This value is below the maximum allowable current flowing into the CK pin (10mA). If the 10mA value is
exceeded an external diode connected to V
CC
must be added (D3).
R
1
and R
2
values set the anticipation time for OUT
GATE2
.ForR
1
=∞and R
2
=0, t
ANT2
=75ns; for
R
1
=R
2
=10kΩ,t
ANT2
=150ns; for R
1
=0 and R
2
=∞,t
ANT2
=225ns.
The RC group composed by R
5
and the parasitic capacitance of Inhibit pin (typically 5pF) delays the
signal on Inhibit comparator. This delay must be lower than 200ns. This condition imposes a maximum
value for R
5
of about 20kΩ.
In general a suggested value for R
5
is 10kΩ. At 72V input, the secondary voltage is 16V, so the maximum
current flowing into Inhibit pin is 16V/10kΩ=1.6mA which is below the maximum allowable current for the
pin (10mA). If the 10mA value is exceeded an external diode (D2) connected to V
CC
must be added.
The maximum negative voltage of –0.6V must be guaranteed for the Inhibit pin. If this negative voltage is
exceeded the current must be limited to 50mA. If necessary, a diode (D1) connected to SGLGND can be
added to satisfy this specification.
INHIBIT OPERATION OF OUT
GATE2
IN DISCONTINUOUS CONDUCTION MODE
R
4
V
CK
R
3
×
V
IN
I
CK 2.8()
– R
3
V
CK
–×
----------------------------------------------------------------
≥ 1kΩ
2.8V 1.5kΩ()×
8V 220µA– 1.5kΩ 2.8V–×
--------------------------------------------------------------------------
× 862Ω==
I
CK
V
IN max()
V
CC
– 0.3–
R
3
-----------------------------------------------------
16 5– 0.3–
1.5kΩ
------------------------------
7.13mA===
