© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FSFR-XS Series • Rev.1.0.1

10

It is typical to set the initial frequency of soft-start two to

network. The soft-start time is three to four times the RC

time constant. The RC time constant is:

SS

SS

C

R

•

=

τ

(4)

Figure 19. Frequency Sweeping of Soft-Start

4. Self Auto-Restart

: The FSFR-XS series can restart

automatically even though any built-in protections are

triggered with external supply voltage. As can be seen in

Figure 20 and Figure 21, once any protections are

triggered, the M1 switch turns on and the V-I converter is

off, and the V-I converter resumes at the same time. The

FSFR-XS starts switching again with soft-start. If the

discharged by M1.

Figure 20. Internal Block of AR Pin

After protections trigger, FSFR-XS is disabled during the

()

{}

Ω

+

•

=

k

R

R

C

t

MIN

SS

SS

STOP

5

||

(5)

(a )

(a )

(a )

(b )

(b )

(a ) P ro te ction s a re trigge re d, (b ) F SF R-U S restarts

(b )

Figure 21. Self Auto-Restart Operation

5. Protection Circuits

: The FSFR-XS series has several

self-protective functions, such as Over-Current Protection

(OCP), Abnormal Over-Current Protection (AOCP), Over-

Voltage Protection (OVP), and Thermal Shutdown (TSD).

These protections are auto-restart mode protections, as

shown in Figure 22.

Once a fault condition is detected, switching is terminated

stop voltage of 10V or AR signal is HIGH, the protection is

reset. The FSFR-XS resumes normal operation when

Figure 22. Protection Blocks

5.1

Over-Current

Protection

(OCP)

: When the

sensing pin voltage drops below -0.58V, OCP is

triggered and the MOSFETs remain off. This protection

has a shutdown time delay of 1.5µs to prevent

premature shutdown during startup.

5.2 Abnormal Over-Current Protection (AOCP)

: If

the secondary rectifier diodes are shorted, large

current with extremely high di/dt can flow through the

MOSFET before OCP is triggered. AOCP is triggered

without shutdown delay if the sensing pin voltage

drops below -0.9V.