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NCP1607
http://onsemi.com
18
UVP and OVP protect the system from low bulk voltages
and rapid operating point changes respectively, while the
FPP protects the system against floating feedback pin
conditions. If FPP is not implemented and a manufacturing
error causes the feedback pin to float, then the feedback
voltage is dependent on the coupling within the system and
the surrounding environment. The coupled feedback
voltage may be within the regulation limits (i.e. above the
UVP threshold, but below V
REF
) and cause the controller
to deliver excessive power. The result is that the output
voltage rises until a component fails due to the voltage
stress.
The tradeoff for including FPP is that the value of R
FB
causes an error in the output voltage. The output voltage
including the error caused by R
FB
 (V
OUT
) is calculated
using Equation 16:
V
OUT
+ V
OUT
) R
OUT1
@
V
REF
R
FB
(eq. 16)
Using the values from the OVP calculation, the output
voltage including the error caused by R
FB
 is equal to:
V
OUT
+ 400 ) 4 M @
2.5
4.7 M
+ 402 V
The error caused by R
FB
 is compensated by adjusting
R
OUT2
. The parallel combination of R
FB
 and R
OUT2
 form
an equivalent resistor R
EQ
  that is calculated using
Equation 17.
R
EQ
+ R
OUT1
@
V
REF
V
OUT
* V
REF
(eq. 17)
R
EQ
+ 4 M @
2.5
400 * 2.5
+ 25.16 kW
R
EQ
 is used to calculate R
OUT2
.
R
OUT2
+
R
EQ
@ R
FB
R
FB
* R
EQ
(eq. 18)
R
OUT2
+
25.16 k @ 4.7 M
4.7 M * 25.16 k
+ 25.29 kW
The compensated output voltage is calculated using
Equation 19.
V
OUT
+ V
REF
@
R
OUT1
) R
OUT2
R
) R
OUT1
@
V
REF
R
(eq. 19)
V
OUT
+ 2.5 @
4 M) 25.29 k
25.29 k
) 4 M@
2.5
4.7 M
+ 400 V
Control
E/A
FB
UVP
Static OVP
Fault
Dynamic OVP
Enable
Condition 2
Condition 1
Condition 3
Figure 37. Open Feedback Loop Protection
+
-
+
+
-
+
R
FB
C
COMP
V
CONTROL
I
CONTROL
I
CONTROL
R
OUT2
R
OUT1
V
OUT
V
EAH
Clamp
V
DD
V
EAL
Clamp
Measure
Static OVP is triggered
when clamp is activated
(Enable EA)
I
CONTROL
 > I
ovp
V
REF
V
UVP
Overcurrent Protection (OCP)
A dedicated pin on the NCP1607 senses the peak current
and limits the driver on time if this current exceeds
V
CS(limit)
. This level is 0.5 V (typical). Therefore, the
maximum peak current can be adjusted by changing R
SENSE
according to:
I
peak
+
V
CS(limit)
R
S
(eq. 20)
An internal LEB filter (Figure 38) reduces the likelihood
of switching noise falsely triggering the OCP limit. This
filter blanks out the first 250 ns (typical) of the current
sense signal. If additional filtering is necessary, a small RC
filter can be added between R
SENSE
 and the CS pin.
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