STSR3 datasheet(6/12 Pages) STMICROELECTRONICS

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STSR3

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EXAMPLE OF COMPONENTS SELECTION FOR A FLYBACK CONVERTER

Flyback Specification:

VIN=36-72V

VOUT=3.3V

n=Np/Ns=4.5

R3 and R4 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 R3=1.5KΩ,R4 results to be:

R4=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 VCC must be added (D3).

R1 and R2 values set the anticipation time for OUTGATE.For R1=∞ and R2=0, tANT=75ns; for

R1=R2=10kΩ,tANT=150ns; for R1=0 and R2=∞,tANT=225ns.

The RC group composed by R5 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 R5 of about 20kΩ.

In general a suggested value for R5 is 10kΩ. At 72V input, the secondary voltage is 16V, so the maximum

current flowing into Inhibit pin is 16V/10k

Ω=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 VCC 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.

CK 2.8

()

–

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

≥

Ω

2.8V

1.5 k

Ω

220

µA

–

1.5 k

Ω 2.8V

–

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

862

Ω

IN m ax

()

–

0.3

–

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

–

0.3

–

1.5k

Ω

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

7.13m A

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