DM0565R データシートの表示（PDF） - Fairchild Semiconductor

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DM0565R

Green Mode Fairchild Power Switch (FPS)

Fairchild Semiconductor 

output voltage may exceed the rated voltage before the over

load protection is activated, resulting in the breakdown of the

Vo

devices in the secondary side. In order to prevent this

Voset

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, an OVP

VFB

circuit is activated resulting in the termination of the

switching operation. In order to avoid undesired activation of

0.7V

OVP during normal operation, Vcc should be designed to be

0.5V

below 19V.

Ids

3.4 Thermal Shutdown (TSD) : The Sense FET and the

control IC are built in one package. This makes it easy for

the control IC to detect the heat generation from the Sense

FET. When the temperature exceeds approximately 150°C,

Vds

the thermal shutdown is activated.

FSDM0565R

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

that increases PWM comparator inverting input voltage

together with the Sense FET current slowly after it starts up.

The typical soft start time is 10msec, 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 during startup.

Switching

Switching

disabled

T1

T2 T3 disabled

T4

Figure 9. Waveforms of burst operation

time

5. Burst operation : In order to minimize power dissipation

in standby mode, the FPSTM enters burst mode operation.

As the load decreases, the feedback voltage decreases. As

shown in figure 9, the device automatically enters burst

mode when the feedback voltage drops below

VBURL(500mV). At this point switching stops and the

output voltages start to drop at a rate dependent on standby

current load. This causes the feedback voltage to rise. Once

it passes VBURH(700mV) switching resumes. The feedback

voltage then falls and the process repeats. Burst mode

operation alternately enables and disables switching of the

power Sense FET thereby reducing switching loss in

Standby mode.

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