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DL0165R

Green Mode Fairchild Power Switch (FPS)

Fairchild Semiconductor 

FSDL0165RN

Vcc

8V

OLP

6V

FPS switching

3V

t1

Delay current (5uA) charges the Cfb

t2

t3

Following Vcc

t4

t

t1 =

−

1

RC fb

In (1 − V (t1) ); V (t1)

R

=

3V , R

=

2.8 K Ω , C fb

=

C fb _

fig .2

t2

=

C fb

(V

(t1+ t2) −

I delay

V

(t1))

;

I

delay

= 5uA,V(t1+ t2) −V(t1)

= 3V

Figure 6. Over load protection

4.2 Thermal Shutdown (TSD) : The Sense FET and the con-

trol IC are integrated, making it easier for the control IC to

detect the temperature of the Sense FET. When the tempera-

ture exceeds approximately 140°C, thermal shutdown is acti-

vated.

enabled and monitors the current through the sensing resis-

tor. The voltage across the resistor is then compared with a

preset AOCP level. If the sensing resistor voltage is greater

than the AOCP level, pulse by pulse AOCP is triggered

regardless of uncontrollable LEB time. Here, pulse by pulse

AOCP stops Sense FET within 350nS after it is activated.

4.4 Over Voltage Protection (OVP) : In case of malfunc-

tion in the secondary side feedback circuit, or feedback loop

open caused by a defect of solder, the current through the

opto-coupler transistor becomes almost zero. Then, Vfb

climbs up in a similar manner to the over load situation, forc-

ing the preset maximum current to be supplied to the SMPS

until the over load protection is activated. Because excess

energy is provided to the output, the output voltage may

exceed the rated voltage before the over load protection is

activated, resulting in the breakdown of the devices in the

secondary side. In order to prevent this situation, an over

voltage protection (OVP) circuit is employed. In general,

Vcc is proportional to the output voltage and the FPSTM uses

Vcc instead of directly monitoring the output voltage. If

VCC exceeds 19V, OVP circuit is activated resulting in ter-

mination of the switching operation. In order to avoid undes-

ired activation of OVP during normal operation, Vcc should

be properly designed to be below 19V.

4.3 Abnormal Over Current Protection (AOCP) :

PWM

COMPARATOR

Vfb

LEB

Vsense

AOCP

COMPARATOR

CLK

SQ

R

Out Driver

Drain

5. Soft Start : The FPSTM has an internal soft start circuit that

increases the feedback voltage together with the Sense FET

current slowly after it starts up. The typical soft start time is

15msec, as shown in figure 8, where progressive increments

of Sense FET current are allowed during the start-up phase.

The pulse width to the power switching device is progres-

sively increased to establish the correct working conditions

for transformers, inductors, and capacitors. The voltage on

the output capacitors is progressively increased with the

intention of smoothly establishing the required output volt-

age. It also helps to prevent transformer saturation and

reduce the stress on the secondary diode.

VAOCP

Rsense

Figure 7. AOCP Function & Block

Drain current

[A]

2.15A

1mS

15steps

Even though the FPSTM has OLP (Over Load Protection)

and current mode PWM feedback, these are not enough to

protect the FPSTM when a secondary side diode short or a

0.98A

Current limit

transformer pin short occurs. In addition to start-up, soft-

start is also activated at each restart attempt during auto-

t

restart and when restarting after latch mode is activated. The

FPSTM has an internal AOCP (Abnormal Over Current Pro-

tection) circuit as shown in figure 7. When the gate turn-on

signal is applied to the power Sense FET, the AOCP block is

11

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