NCP1308

http://onsemi.com

9

The skip level selection is done through a simple resistor

inserted between the current sense input and the sense

element. Every time the NCP1308 output driver goes low,

a 200

mA source forces a current to flow through the sense

pin (Figure 15): when the driver is high, the current source

is off and the current sense information is normally

processed. As soon as the driver goes low, the current

source delivers 200

mA and develops a ground−referenced

voltage, the current sense comparator stays in the high state

and the internal latch can be triggered by the next clock

cycle. Now, if because of a low load mode the feedback

comparator permanently resets the latch and the next clock

cycle (given by the demagnetization detection) is ignored:

we are skipping cycles as shown in Figure 15. As soon as

the feedback voltage goes up again, there can be two

situations: the recurrent period is small and a new

demagnetization detection (next wave) signal triggers the

NCP1308. To the opposite, in low output power conditions,

no more ringing waves are present on the drain and the

toggling of the current sense comparator together with the

internal 5

ms timeout initiates a new cycle start. In normal

operating conditions, e.g. when the drain oscillations are

generous, the demagnetization comparator can detect the

50 mV crossing and gives the “green light”, alone, to

re−active the power switch. However, when skip cycle

takes place (e.g. at low output power demands), the restart

event slides along the drain ringing waveforms (actually

the valley locations) which decays more or less quickly,

factor. The situation can thus quickly occur where the

ringing becomes too weak to be detected by the

demagnetization comparator: it then permanently stays

locked in a given position and can no longer deliver the

“green light” to the controller. To help in this situation, the

NCP1308 implements a 5

ms timeout generator: each time

the 50 mV crossing occurs, the timeout is reset. So, as long

as the ringing becomes too low, the timeout generator starts

to count and after 5

ms, it delivers its “green light”. If the

skip signal is already present then the controller restarts;

otherwise the logic waits for it to set the drive output high.

Figure 16 depicts these two different situations:

Figure 16. When the primary natural ringing becomes too low, the internal Timeout

together with the sense comparator initiates a new cycle when FB passes the skip level.

Dmg Restart

Current Sense and Timeout Restart

5

ms

5

ms

Drain

Signal

Timeout

Signal

Drain

Signal

Timeout

Signal