NCP1217AP100 datasheet(8/18 Pages) ONSEMI

Part #	NCP1217AP100

Description	Enhanced PWM Current?묺ode Controller for High?뭁ower Universal Off?묹ine Supplies

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Manufacturer	ONSEMI [ON Semiconductor]

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NCP1217, NCP1217A

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8

APPLICATION INFORMATION

Introduction

The NCP1217 implements a standard current mode

architecture where the switch−off event is dictated by the

peak current setpoint. This component represents the ideal

candidate where low part−count is the key parameter,

particularly in low−cost AC/DC adapters, TV power

supplies, etc. Due to its high−performance High−Voltage

technology, the NCP1217 incorporates all the necessary

components normally needed in UC384X based supplies:

timing components, feedback devices, low−pass filter and

startup device but also enhances the original component

by

offering:

1)

an

externally

triggerable

latchoff

2) ramp

compensation

and

finally, 3) short−circuit

protection. Due to its high−voltage current source,

ON Semiconductor’s NCP1217 does not need an external

startup resistance but supplies the startup current directly

from the high−voltage rail. On the other hand, more and

more applications are requiring low no−load standby power,

e.g. for AC/DC adapters, VCRs, etc. UC384X series have a

lot of difficulty to reduce the switching losses at low power

levels. NCP1217 elegantly solves this problem by skipping

unwanted switching cycles at a user−adjustable power level.

By ensuring that skip cycles take place at low peak current,

the device ensures quiet, noise−free operation:

Current−Mode Operation: As the UC384X series, the

NCP1217 features a well−known current mode control

architecture

which

provides

superior

input

audio−

susceptibility compared to traditional voltage−mode

controllers. Primary current pulse−by−pulse checking

together with a fast over current comparator offers greater

security in the event of a difficult fault condition, e.g. a

saturating transformer.

Ramp Compensation: By inserting a resistor between the

current−sense (CS) pin and the actual sense resistor, it

becomes possible to inject a given amount of ramp

compensation since the internal saw tooth clock is routed to

the CS pin. Subharmonic oscillations in Continuous

Conduction Mode (CCM) can thus be compensated via a

single resistor.

Adjustable Skip Cycle Level: By offering the ability to

tailor the level at which the skip cycle takes place, the

designer can make sure that the skip operation only occurs

at low peak current. This point guarantees a noise−free

operation with cheap transformers. Skip cycle offers a

proven mean to reduce the standby power in no or light loads

situations.

Wide Switching−Frequency Offer: Three different options

are available: 65 kHz – 100 kHz–133 kHz. Depending on

the application, the designer can pick up the right device to

help reducing magnetics or improve the EMI signature

before reaching the 150 kHz starting point.

Overcurrent

Protection

(OCP):

By

continuously

monitoring the VCC auxiliary winding voltage, NCP1217

enters burst mode as soon as the power supply undergoes an

overload: when the VCC voltage goes down until it crosses

the undervoltage lockout level (VCCmin). When the

NCP1217 reaches this level (typically 7.6 V), it stops the

switching pulses until the VCC pin voltage reaches VCClatch

(5.6 V). At VCClatch, the NCP1217 attempts to restart. As

soon as the default disappears, the power supply resumes

operation.

Overvoltage Protection (OVP): If pin1 is brought to a level

higher than the internal 3.2 V reference voltage, the

controller is permanently shut down until the user cycles the

VCC OFF and ON again. This allows the building of

efficient and low−cost over voltage protection circuits.

Wide Duty−Cycle Operation: Wide mains operation

requires a large duty−cycle excursion. The NCP1217 can go

up to 74% typically.

Low Standby−Power: If SMPS naturally exhibit a good

efficiency at nominal load, they begin to be less efficient

when the output power demand diminishes. By skipping

unneeded switching cycles, the NCP1217 drastically

reduces the power wasted during light load conditions. In

no−load conditions, the NPC1217 allows the total standby

power to easily reach next International Energy Agency

(IEA) recommendations.

No Acoustic Noise While Operating: Instead of skipping

cycles at high peak currents, the NCP1217 waits until the

peak current demand falls below a user−adjustable 1/3 of the

maximum limit. As a result, cycle skipping can take place

without having a singing transformer

… You can thus select

cheap magnetic components free of noise problems.

External MOSFET Connection: By leaving the external

MOSFET external to the IC, you can select avalanche proof

devices, which in certain cases (e.g. low output powers), let

you work without an active clamping network. Also, by

controlling the MOSFET gate signal flow, you have an

option to slow down the device commutation, therefore

reducing the amount of ElectroMagnetic Interference

(EMI).

SPICE Model: A dedicated model to run transient

cycle−by−cycle simulations is available but also an

averaged version to help you closing the loop. Ready−to−use

templates can be downloaded in OrCAD’s Pspice and

INTUSOFT’s IsSpice from ON Semiconductor web site,

NCP1217 related section.

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