NCP1237 datasheet(11/34 Pages) ONSEMI

11 / 34 page

NCP1237

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At startup, 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). It turns on again when VCC reaches

VCC(min). This sequence repeats until the input voltage is

high enough to ensure a proper startup, i.e. when VHV

reaches VHV(start). The switching actually starts the next

time VCC reaches VCC(on), as shown in Figure 5.

Even though the DSS is able to maintain the VCC voltage

between VCC(on) and VCC(min) by turning the HV startup

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

conditions, otherwise the power dissipation on the die would

be too high. 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 a brown−out

condition, or to prevent the controller from stopping during

load transients when the VCC might drop below VCC(off).

Figure 5. Startup Timing Diagram

time

VHV

time

VCC

time

DRV

VHV(start)

VHV(min)

VCC(on)

VCC(min)

VCC(inhibit)

current

source =

Istart1

current

source =

Istart2

Waits

VCC(on)

before

starting

NCP1237 Datasheet(PDF) 11 Page - ON Semiconductor