400EAFAB1 datasheet(19/47 Pages) ONSEMI

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NCP12400

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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).

The controller then delivers pulses, starting with a soft−start

period tSSTART during which the peak current linearly

increases before the current−mode control takes over.

Even though the Dynamic Self−Supply 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 Dynamic Self−Supply 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.

The NCP12400 accepts a supply voltage as high as 28 V,

with an overvoltage threshold VCC(ovp) that latches the

controller off.

Figure 46. VCC Start−up Timing Diagram

time

VHV

time

VCC

time

DRV

VHV(start)

V HV(min)

VCC(on)

V CC(min)

HV current

source = Istart1

HV current

source = Istart2

Waits next

VCC(on)

before starting

VCC(inhibit)

For safety reasons, the start−up current is lowered when

VCC is below VCC(inhibit), to reduce the power dissipation in

case the VCC pin is shorted to GND (in case of VCC capacitor

failure, or external pull−down on VCC to disable the

controller). There is only one condition for which the current

source doesn’t turn on when VCC reaches VCC(inhibit): the

voltage on HV pin is too low (below VHV(min)).

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