DDA002C
PFC_OK
CIG
CIG_End = 5V
SS
SS_End = 1.9V
SS_OK
Fop
Fop = Fstart
Fop = Fmin
Fop = Fmax
Fop = Fmin
FB
VFB > 6 V
0 < VFB < 1.2 V
1.2 V < VFB < 5.1 V
FB_Fault
PFC delay
Preheat
Soft − Start Transition Regulation
to regulation
less than 50 ms
Light load
operation
skip mode
Overload
50 ms
Figure 44. The IC Behavior under Various Operating Conditions
Operation
Interrupted
by FB_Fault
Timer
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.
VBULK
BO
Rupper
Rlower
VrefBO
20 ms
Filter
BO_ OK
to AND gates
SW
IBO
50 ms one shoot from
PFC timer. Comes
after any IC restart.
Figure 45. The Internal Brown−out
Configuration with an Offset Current Sink
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
ǒ Ǔ VrefBO +
Vbulk1
@
Rlower
Rlower ) Rupper
*
IBO
@
Rlower @ Rupper
Rlower ) Rupper
(eq. 2)
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
@
IBO
Vbulk1 *
@ (Vbulk2
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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