© 2007 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FSQ211 Rev. 1.0.0

9

4.1 Overload Protection (OLP):

Overload is defined as

the load current exceeding a pre-set level due to an

unexpected event. In this situation, the protection circuit

should be activated to protect the SMPS. However,

even when the SMPS is operating normally, the over-

load protection (OLP) circuit can be activated during the

load transition. To avoid this undesired operation, the

OLP circuit is designed to be activated after a specified

time to determine whether it is a transient situation or an

overload situation. If the output consumes more than the

maximum power determined by ILIM, the output voltage

(VO) decreases below its rating voltage. This reduces

the current through the opto-coupler LED, which also

reduces the opto-coupler transistor current, thus

increasing the feedback voltage (VFB). If VFB exceeds

3V, the feedback input diode is blocked and the 5µA

current source (IDELAY) starts to charge CFB slowly up to

VCC. In this condition, VFB increases until it reaches

4.5V, when the switching operation is terminated, as

shown in Figure 18. Shutdown delay is the time required

to charge CFB from 3V to 4.5V with 5µA current source.

VFB

t

3V

4.5V

Overload Protection

t12=CFB×(V(t2)-V(t1)) / IDELAY

t1

t2

V

t

V

V

t

V

A

I

I

t

V

t

V

C

t

DELAY

DELAY

FB

5

.

4

)

(

,

3

)

(

,

5

;

)

(

)

(

2

1

1

2

12

=

=

=

•

=m

Figure 18. Overload Protection (OLP)

4.2 Thermal Shutdown (TSD):

The SenseFET and the

control IC are integrated, making it easier for the control

IC to detect the temperature of the SenseFET. When

the temperature exceeds approximately 145

°C, thermal

shutdown is activated.

5. Soft-Start:

The FPS has an internal soft-start circuit

that slowly increases the feedback voltage, together with

the SenseFET current, right after it starts up. The typical

soft-start time is 15ms, as shown in Figure 19, where

progressive increment of the SenseFET current is

allowed during the start-up phase. Soft-start circuit

progressively increases current limits to establish proper

working

conditions

for

transformers,

inductors,

capacitors, and switching device. It also helps prevent

transformer saturation and reduces the stress on the

secondary diode.

2.14ms

7steps

ILIM

t

Drain current

Figure 19. Internal Soft-Start

6. Burst Operation:

To minimize the power dissipation

in standby mode, the FSQ211 enters burst mode

operation. As the load decreases, the feedback voltage

decreases. The device automatically enters burst mode

when the feedback voltage drops below VBURL (0.55V).

At this point, switching stops and the output voltages

start to drop. This causes the feedback voltage to rise.

Once is passes VBURH (0.70V), switching starts again.

The feedback voltage falls and the process repeats.

Burst mode operation alternately enables and disables

switching of the power MOSFET to reduce the switching

loss in the standby mode.

OSC

4

Vfb

S

R

Q

GATE

DR IVE R

5µA

400µA

0.70V

/0.55V

on/off

Burst Operation Block

Figure 20. Burst Operation Block

V

FB

Vds

0.55V

0.7V

Ids

Vo

Vo

set

t

Figure 21. Burst Operation Function