NCP1253

www.onsemi.com

13

F

sw

V

FB

V

CS

V

FB

65 kHz

26 kHz

350 mV

V

fold

3.4 V

V

fold

3.4 V

0.8 V

[0.36 V

FB

V

freeze

[0.25 V

1.05 V

1.5 V

1.5 V

max

min

max

min

V

fold,end

Frequency

Peak current setpoint

F

sw

V

FB

V

CS

V

FB

65 kHz

26 kHz

350 mV

V

fold

3.4 V

V

fold

3.4 V

0.8 V

[0.36 V

FB

V

freeze

[0.25 V

1.05 V

1.5 V

1.5 V

max

min

max

min

V

fold,end

Frequency

Peak current setpoint

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

down to a few

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.

500u

1.50m

2.50m

3.50m

4.50m

time in seconds

445m

1.41

2.38

3.35

4.32

−8.13

−2.12

3.89

9.90

15.9

−11.5

−2.72

6.05

14.8

23.6

2

1

3

()

cc

Vt

()

DRV

Vt

()

p

L

It

SS

500u

1.50m

2.50m

3.50m

4.50m

time in seconds

445m

1.41

2.38

3.35

4.32

−8.13

−2.12

3.89

9.90

15.9

−11.5

−2.72

6.05

14.8

23.6

2

1

3

()

cc

Vt

()

DRV

Vt

()

p

L

It

SS

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.