1207 データシートの表示（PDF） - ON Semiconductor

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1207

PWM Current-Mode Controller for Free Running Quasi-Resonant Operation

ON Semiconductor 

NCP1207

RESET

DRIVER

DRIVER = HIGH ? I = 0

DRIVER = LOW ? I = 200 mA

−

3

+

2

Rskip

Rsense

+

Figure 19. A patented method allows for skip level

selection via a series resistor inserted in series

with the current

The skip level selection is done through a simple resistor

inserted between the current sense input and the sense element.

Every time the NCP1207 output driver goes low, a 200 mA

source forces a current to flow through the sense pin

(Figure 19): 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 across Rskip. If this

voltage is below the feedback 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 voltage is below Rskip level, then the

current sense comparator permanently resets the latch and the

next clock cycle (given by the demagnetization detection) is

ignored: we are skipping cycles as shown by Figure 20. 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

NCP1207. 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, depending on the

Lprimary−Cparasitic network damping 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 NCP1207 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 20 depicts these two different situations:

Drain

Signal

Timeout

Signal

Drain

Signal

Demag Restart

Current Sense and Timeout Restart

Timeout

Signal

5 ms

5 ms

Figure 20. 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.

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