7M0680 データシートの表示（PDF） - Fairchild Semiconductor

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7M0680

Fairchild Power Switch

Fairchild Semiconductor 

to the SPS VCC voltage. If VCC exceeds 24 V, the SPS IC

starts the protection circuit. Therefore, VCC should be

appropriately kept below 24V during normal operation.

OCP (Over Current Protection)

OCP Operating

200ns

100ns delay

S Q Latch signal

R

OCP time

R

C

Minimum Turn-on Time

Fiqure 5. OCP Function & Block

Rsense

OCP Level

The FPS has various built-in, basic protection features. They

are the UVLO (Under Voltage Lock Out), OLP (Over Load

Protection) and OCP (Over Current Protection). However, if

a secondary side diode short or load short occur due to a

worst case condition, such as a maximum input voltage

putting a large strain on the device, another external

component may need to be added. By adding these

requirements in the FPS, superior reliability and

advantageous cost can be achieved.

When gate on signal of the SenseFet is received,

simultaneously the OCP block senses Ipeak through the

sense resistor for 1us. After the OCP block has turned on,

the voltage across the resistor is compared to the pre-set

voltage in the comparator, and, if it takes longer than 200ns

within the allowed comparison time of 1us, then the

comparator produces a high signal, which latches the OCP.

fig 4. shows the OCP latch waveform. When there is a diode

short/load short, the SPS turns on for the minimum turn-on

time. If the instantaneous current is of the form shown in fig

4., the OCP block opens a 1us window to compare the

voltage proportional to the current across the resistor with

the reference voltage and latches. Here, the 100ns delay

after the 200ns is the delay time to SenseFET gate off and is

generated from the comparison of the voltage across the

sense resistor.

Soft start operation

Normally, the SMPS output voltage increases from start up

with a fixed time constant. This is due to the capacitive

component of the load. At start up, therefore, the feedback

signal applied to the PWM comparator's inverting input

reaches its maximum value (1V), This is because the

feedback loop is effectively open. Also at this time, the

drain current is at its peak value (Ipeak) and maximum

allowable power is being delivered to the secondary load.

With that said, note that when the SMPS pushes maximum

power to the secondary side for this initial fixed time, the

FS7M0880

entire

circuit is seriously stressed. Use of a soft start function

avoids such stresses. Figure 6 shows how to implement a

soft start for a Fairchild Power Switch(FPS). At turn on, the

soft start capacitor on pin 5 of the Fairchild Power

Switch(FPS) starts to charge through the 1mA current

source. When the voltage across CS reaches 3V, diode DS

turns off. No more current flows to it from the 1mA current

source. Cs then continues to charge to 5V through the 20kΩ

resistor.

10V

4uA

0.9mA

Fairchild Power

Switch(FPS)

PWM

Comparator

5V

20K

#4

#5

CS

Figure 6. Soft Start Circuit.

Note that when the voltage across CS exceeds 3V, The voltage

at the comparator’s inverting terminal no longer follows the

voltage across CS. Instead, it follows the output voltage

feedback signal. In shutdown or protection circuit operation,

capacitor CS is discharged, to enable it to charge from 0V at

restart.

7

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