FSDH321, FSDL321

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9

Figure 16. Over Load Protection (OLP)

Over Load Protection

VFB

6 V

3 V

t1

t2

t

t

12 + CFB

V(t

2) * V(t1)

I

DELAY

t

12 + CFB

V(t

2) * V(t1)

I

DELAY

;I

DELAY + 5 mA, V(t1) + 3V, V(t2) + 6V

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.

Abnormal Over Current Protection (AOCP)

Even though the power switch has OLP (Over Load

Protection) and current mode PWM feedback, these are not

enough to protect the power switch when a secondary side

diode short or a transformer pin short occurs. In addition to

start−up, soft−start is also activated at each restart attempt

during auto−restart and when restarting after latch mode is

activated. The power switch has an internal AOCP

(Abnormal Over Current Protection) circuit, as shown in

Figure 17. When the gate turn−on signal is applied to the

power SENSEFET, the AOCP block is enabled and

monitors the current through the sensing resistor. 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 the SENSEFET within 350 ns after it is activated.

Figure 17. Abnormal Over Current Protection

(AOCP)

Vsense

V

FB,in

Gate Driver

Rsense

CLK

Drain

V

AOCP

LEB

R

S

PWM

COMPARATOR

Q

AOCP

COMPARATOR

Over Voltage Protection (OVP)

In the event of a malfunction in the secondary side

feedback circuit, or an open feedback loop caused by a

soldering defect, the current through the opto−coupler

transistor becomes almost zero (refer to Figure 15). Then,

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

forcing 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 power

switch uses VCC instead of directly monitoring the output

voltage. If VCC exceeds 19 V, OVP circuit is activated

resulting in termination of the switching operation. In order

to avoid undesired activation of OVP during normal

operation, Vcc should be properly designed to be below

19 V.

Soft Start

The power switch has an internal soft start circuit that

slowly increases the feedback voltage together with the

SENSEFET current after it starts up. The typical soft start

time is 15 ms, as shown in Figure 18, where progressive

increments of the SENSEFET current are allowed during the

start−up phase. The pulse width to the power switching

device is progressively 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 voltage. It also helps to

prevent transformer saturation and reduce the stress on the

secondary diode.

Figure 18. Soft Start Function

0.7 A

0.4 A

Drain Current

1ms

15 steps

Current limit

t

Burst Operation

In order to minimize power dissipation in standby mode,

the power switch enters burst mode operation. As the load

decreases, the feedback voltage decreases. As shown in

Figure 19, the device automatically enters burst mode when

the feedback voltage drops below VBURH (500 mV).

Switching still continues but the current limit is set to a fixed

limit internally to minimize flux density in the transformer.

The fixed current limit is larger than that defined by VFB =

VBURH and therefore, VFB is driven down further.

Switching continues until the feedback voltage drops below

VBURL (350 mV). At this point switching stops and the