NCP1252 View Datasheet(PDF) - ON Semiconductor

Part Name

Description

Manufacturer

NCP1252

Current Mode PWM Controller for Forward and Flyback Applications

ON Semiconductor 

FB level

VFBOL = 6.0 V

NCP1252

Normal Operation:

DCmin < DC < DCmaxA/B/C

Vf = 0.75 V

Vskip = 0.3 V

Operation @ Ton_min

DC = DCmin

Skip: DC = 0%

Time

Figure 29. Mode of Operation versus the FB Pin Level

Startup Sequence:

The startup sequence is activated when Vcc pin reaches

VCC(on) level. Once the startup sequence has been activated

the internal delay timer (SSdelay) runs. Only when the

internal delay elapses the soft start can be allowed if the BO

pin level is above VBO level. If the BO pin threshold is

reached or as soon as this level will be reached the soft start

is allowed. When the soft start is allowed the SS pin is

released from the ground and the current source connected

to this pin sources its current to the external capacitor

connected on SS pin. The voltage variation of the SS pin

divided by 4 gives the same peak current variation on the CS

pin.

The following figures illustrate the different startup cases.

VCC pin

VCC(on)

VCC pin

VCC(on)

BO pin

Time

BO pin

Time

VBO

SS pin

DRV pin

120 ms: Internal

delay

Time

VBO

SS pin

Soft start

Time

DRV pin

120 ms: Internal

delay

Time

Soft start Time

CASE #1

Time

Figure 30. Different Startup Sequence Case #1 & #2

No

pulse

CASE #2

Time

With the Case #1, when the VCC pin reaches the VCC(on)

level, the internal timer starts. As the BO pin level is above

the VBO threshold at the end of the internal delay, a soft start

sequence is started.

With the Case #2, at the end of the internal delay, the BO

pin level is below the VBO threshold thus the soft start

sequence can not start. A new soft start sequence will start

only when the BO pin reaches the VBO threshold.

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