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NCP1236

Fixed Frequency Current Mode Controller for Flyback Converters

ON Semiconductor 

NCP1236

DETAILED DESCRIPTION

High−Voltage Current Source with Built−in Brown−out

Detection

The NCP1236 HV pin can be connected either to the

rectified bulk voltage, or to the ac line through a rectifier.

Start−up

HV

VCC

Control

Istart

+

+−

VCC(on)

−

++

VCC(min)

TSD

IC Start

R

Q

S

−

+

+

blanking

tUVLO(blank)

VCC(off )

UVLO

−

++

VCC(reset)

Reset

Figure 30. HV Start−up Current Source Functional Schematic

At start−up, the current source turns on when the voltage

on the HV pin is higher than VHV(min), and turns off when

VCC reaches VCC(on), then turns on again when VCC reaches

VCC(min), until the input voltage is high enough to ensure a

proper start−up, i.e. when VHV reaches VHV(start). The

controller actually starts the next time VCC reaches VCC(on).

Even though the DSS is able to maintain the VCC voltage

between VCC(on) and VCC(min) by turning the HV start−up

current source on and off, it can only be used in light load

condition, otherwise the power dissipation on the die would

be too much. As a result, an auxiliary voltage source is

needed to supply VCC during normal operation.

The DSS is useful to keep the controller alive when no

switching pulses are delivered, e.g. in brown−out condition,

or to prevent the controller from stopping during load

transients when the VCC might drop.

If the voltage increases above the overvoltage protection

threshold VCC(ovp), the controller is latched off.

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