NCP1200

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8

an auxiliary winding. It will automatically

disconnect the internal startup source and the IC

will be fully self−supplied from this winding.

Again, the total power drawn from the mains will

significantly decrease. Make sure the auxiliary

voltage never exceeds the 16 V limit.

Skipping Cycle Mode

The NCP1200 automatically skips switching cycles when

the output power demand drops below a given level. This is

accomplished by monitoring the FB pin. In normal

operation, pin 2 imposes a peak current accordingly to the

load value. If the load demand decreases, the internal loop

asks for less peak current. When this setpoint reaches a

determined level, the IC prevents the current from

decreasing further down and starts to blank the output

pulses: the IC enters the so−called skip cycle mode, also

named controlled burst operation. The power transfer now

depends upon the width of the pulse bunches (Figure 18 ).

Suppose we have the following component values:

Lp, primary inductance = 1 mH

Ip skip = 300 mA (or 350 mV / Rsense)

The theoretical power transfer is therefore:

1

2 @

If this IC enters skip cycle mode with a bunch length of

10 ms over a recurrent period of 100 ms, then the total power

transfer is: 2.2 . 0.1 = 220 mW.

To better understand how this skip cycle mode takes place,

a look at the operation mode versus the FB level

immediately gives the necessary insight:

1.4 V

4.8 V

FB

Normal Current Mode Operation

Skip Cycle Operation

Figure 17. Feedback Voltage Variations

3.8 V

When FB is above the skip cycle threshold (1.4 V by

default), the peak current cannot exceed 1 V/Rsense. When

the IC enters the skip cycle mode, the peak current cannot go

below Vpin1 / 4 (Figure 19). The user still has the flexibility

to alter this 1.4 V by either shunting pin 1 to ground through

a resistor or raising it through a resistor up to the desired

level.

Figure 18. Output pulses at various power levels

(X = 5 ms/div) P1<P2<P3

P1

P2

P3

Figure 19. The skip cycle takes place at low peak

currents which guarantees noise free operation

Max Peak

Current

Skip Cycle

Current Limit