NCP1239_15 datasheet(15/25 Pages) ONSEMI

Part #	NCP1239

Description	Fixed Frequency Current?륪ode Controller for Flyback Converter

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Manufacturer	ONSEMI [ON Semiconductor]

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NCP1239

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15

If the VCC capacitor is first dimensioned to supply the

controller for the traditional 5 to 50 ms until the auxiliary

winding takes over, no-load standby requirements usually

cause it to be larger. The HV start-up current source is then

a key feature since it allows keeping short start-up times with

large VCC capacitors (the total start-up sequence duration is

often required to be less than 1 s).

BROWN-OUT CIRCUITRY

For the vast majority of controllers, input line sensing is

performed via a resistive network monitoring the bulk

voltage or the incoming ac signal. When in the quest of low

standby power, the external network adds a consumption

burden and deteriorates the power supply standby power

performance. Owing to its proprietary high-voltage

technology, ON Semiconductor now offers onboard line

sensing without using an external network. The system

includes a 90-M

W resistive network that brings a minimum

start-up

threshold

and

an

auto-recovery

brown-out

protection. Both levels are independent from the input

voltage ripple. The brown-out thresholds are fixed (see

levels in the electrical characteristics table), but they are

designed to fit most of standard ac-dc converter

applications. The simplified internal schematic appears in

Figure 33 while typical operating waveforms are drawn in

Figure 34 and Figure 35.

Figure 33. A Simplified View of the Brown-Out Circuitry

L1

N

EMI

Filter

Vbulk

Rbo_H

Rbo_L

HV

GND

BO_OK

VBO

When the HV pin voltage drops below the VBO(off)

threshold, the brown-out protection trips: the controller

stops generating DRV pulses once the 68-ms BO timer

elapses. VCC is discharged to VCC(min) by the controller

consumption itself. When this level is reached, the HV

current source is activated to lifts VCC up again. At new

VCC(on), BO signal is again sensed. If VHV >VBO(on), the

parts restarts. If the condition is not met, no drive pulse is

delivered and internal IC consumption brings VCC down

again. As a result, VCC operates in hiccup mode during a BO

event.

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