NCP1253 データシートの表示（PDF） - ON Semiconductor

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NCP1253

Current-Mode PWM Controller for Off-line Power Supplies

ON Semiconductor 

FB

Frequency

Fsw

65 kHz

NCP1253

Peak current setpoint

VCS

max

0.8 V

max

26 kHz min

350 mV 1.5 V

Vfold,end Vfold

VFB

3.4 V

[0.36 V

[0.25 V min

Vfreeze Vfold

1.05 V 1.5 V

VFB

3.4 V

Figure 27. By Observing the Voltage on the Feedback Pin, the Controller Reduces its Switching Frequency for an

Improved Performance at Light Load

Auto−recovery Short−Circuit Protection

In case of output short−circuit or if the power supply

experiences a severe overloading situation, an internal error

flag is raised and starts a countdown timer. If the flag is

asserted longer than 100 ms, the driving pulses are stopped

and VCC falls down as the auxiliary pulses are missing.

When it crosses VCC(min), the controller consumption is

down to a few mA and the VCC slowly builds up again thanks

to the resistive starting network. When VCC reaches

VCCON, the controller attempts to re−start, checking for the

absence of the fault. If the fault is still there, the supply enters

another cycle of so−called hiccup. If the fault has

disappeared, the power supply resumes operations. Please

note that the soft−start is activated during each of the re−start

sequence.

1 vcc 2 vdrv 3 ilprim

23.6 15.9 4.32

1

Vcc (t)

14.8 9.90 3.35

6.05 3.89 2.38

VDRV (t)

−2.72 −2.12 1.41

−11.5 −8.13 445m

2

SS

ILp (t)

500u

1.50m

2.50m

time in seconds

3.50m

3

4.50m

Figure 28. An Auto−Recovery Hiccup Mode is Entered in Case a Faulty Event Longer Than 100 ms is

Acknowledged by the Controller

Ramp compensation

The NCP1253 includes an internal ramp compensation

signal. This is the buffered oscillator clock delivered during

the on time only. Its amplitude is around 2.5 V at the

maximum duty−cycle. Ramp compensation is a known

means used to cure sub harmonic oscillations in

CCM−operated current−mode converters. These

oscillations take place at half the switching frequency and

occur only during Continuous Conduction Mode (CCM)

with a duty−cycle greater than 50%. To lower the current

loop gain, one usually injects between 50% and 100% of the

inductor downslope.

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