SCY99080CDWR2G datasheet(20/27 Pages) ONSEMI

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DDA002C

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PFC_OK

CIG

Fop

FB_Fault

Fop = Fstart

Fop = Fmin

PFC delay

Preheat

Soft − Start Transition

to regulation

less than 50 ms

Regulation

Light load

skip mode

Overload

Fop = Fmax

Fop = Fmin

50 ms

Figure 44. The IC Behavior under Various Operating Conditions

Operation

Interrupted

by FB_Fault

Timer

operation

SS_End = 1.9V

CIG_End = 5V

0 < VFB < 1.2 V

VFB > 6 V

1.2 V < VFB < 5.1 V

SS_OK

Brown−Out Protection

The Brown−Out circuitry (BO) offers a way to protect the

application from low DC input voltages. The controller

disables the output pulses if below a given level, if above it,

it authorizes them. The internal circuitry, shown in

Figure 45, offers a way to observe the high−voltage (HV) rail.

IBO

BO_ OK

to AND gates

50 ms one shoot from

PFC timer. Comes

after any IC restart.

Figure 45. The Internal Brown−out

Configuration with an Offset Current Sink

VBULK

Rlower

Rupper

Filter

20 ms

VrefBO

A resistive divider made of Rupper and Rlower, brings a

portion of the HV rail on pin 1. The 20

mA current sink (IBO)

is on if below the BO turn−on level. Therefore, the turn−on

level is higher than the level given by the division ratio

brought by the resistive divider. To the contrary, when the

internal BO_OK signal is high, the IBO sink is deactivated.

As a result, it becomes possible to select the turn−on and

turn−off levels using the below equations:

IBO is On

Vbulk1 @

Rlower

Rlower ) Rupper

* IBO @

Rlower @ Rupper

Rlower ) Rupper

(eq. 2)

VrefBO +

IBO is Off

VrefBO + Vbulk2 @

Rlower

Rlower ) Rupper

(eq. 3)

We can extract Rlower from Equation 2 and plug it into

Equation 1, then solve for Rupper:

Rlower + VrefBO @

Vbulk1 * Vbulk2

IBO @ (Vbulk2 * VrefBO)

(eq. 4)

Rupper + Rlower @

Vbulk2 * VrefBO

VrefBO

(eq. 5)

If we decide to turn−on our converter for Vbulk1 = 350 V

and turn it off for Vbulk2 = 250 V, then for IBO = 20 mA and

VrefBO = 1 V we obtain:

Rupper = 5 MΩ

Rlower = 20 kΩ

The bridge power dissipation is 4002 / 5.02 * 1E06 =

32 mW when front−end PFC stage delivers 400 V. Figure 46

simulation result confirms our calculations.

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