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NCP1238

Fixed Frequency Current Mode Controller for Flyback Converters

ON Semiconductor 

NCP1238

Fixed Frequency Current

Mode Controller for Flyback

Converters

The NCP1238 is a new generation of the NCP12xx fixed−frequency

current−mode controllers featuring Dynamic Self−Supply (DSS),

pin−to−pin compatible with the previous generation.

The DSS function greatly simplifies the design of the auxiliary supply

and the VCC capacitor by activating the internal startup current source to

supply the controller during transients.

Due to its proprietary Soft−Skip™ mode combined with frequency

foldback, the controller exhibits excellent efficiency in light load

condition while still achieving very low standby power consumption.

This Soft−Skip feature also dramatically reduces the risk of acoustic

noise, which enables the use of inexpensive transformers and

capacitors in the clamping network.

Internal frequency jittering, ramp compensation, and a versatile

latch input make this controller an excellent candidate for converters

where ruggedness and components cost are the key constraints.

In addition, the controller includes a new high voltage circuitry that

combines a startup current source and a brown−out / line OVP detector

able to sense the input voltage either from the rectified ac line or the dc

filtered bulk voltage.

Finally, due to a careful design, the precision of critical parameters

is well controlled over the entire temperature range (−40°C to

+125°C), enabling easier design and increased safety (e.g. ±5% for the

peak current limit, ±7% for the oscillator).

http://onsemi.com

MARKING

DIAGRAM

SOIC−7

CASE 751U

8

38Xff

ALYWX

G

1

38Xff = Specific Device Code

X = A or B

ff = 65, 00, or 33

A = Assembly Location

L = Wafer Lot

Y = Year

W = Work Week

G = Pb−Free Package

ORDERING INFORMATION

See detailed ordering and shipping information in the package

dimensions section on page 29 of this data sheet.

Features

• Timer−Based Overload Protections with Auto−

Recovery (Option B) or Latched (Option A) Operation

• High−Voltage Current Source with DSS with Built−in

Brown−out and Line Overvoltage Protections

• Fixed−Frequency Current−Mode Operation with

Built−in Ramp Compensation

• Frequency Jittering for a Reduced EMI Signature

• Adjustable Overpower Compensation

• Latch−off Input for Severe Fault Conditions, with

Direct Connection of an NTC for Overtemperature

Protection (OTP)

• Protection Against Winding Short−Circuit

• Frequency Foldback transitioning into Soft−Skip for

Improved Performance in Standby

• 65 kHz Oscillator (100 kHz and 133 kHz Versions

Available Upon Request)

• VCC Operation up to 28 V

• Increased Precision on Critical Parameters

• ±1.0 A Peak Drive Capability

• 4.0 ms Soft−Start

• Internal Thermal Shutdown with Hysteresis

• These Devices are Pb−Free, Halogen Free/BFR Free

and are RoHS Compliant*

Typical Applications

• ac−dc Adapters for Notebooks, LCD, and Printers

• Offline Battery Chargers

• Consumer Electronic Power Supplies

• Auxiliary/Housekeeping Power Supplies

*For additional information on our Pb−Free strategy and soldering details, please

download the ON Semiconductor Soldering and Mounting Techniques

Reference Manual, SOLDERRM/D.

© Semiconductor Components Industries, LLC, 2010

1

February, 2010 − Rev. 1

Publication Order Number:

NCP1238/D

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