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NCP1606
http://onsemi.com
16
Figure 34. OVP and UVP Circuit Blocks
FB
Control
+

E/A
+

Measure
300 mV
2.5 V
UVP
Fault
Dynamic OVP
V
EAH
Clamp
Static OVP is triggered
when clamp is activated.
V
EAL
Clamp
Static OVP
Enable
(Enable EA)
R
OUT2
R
OUT1
C
COMP
V
DD
I
CONTROL
 > I
ovp
V
OUT
I
CONTROL
V
CONTROL
I
CONTROL
When the output voltage is in steady state, R
OUT1
 and
R
OUT2
 regulate the FB voltage to 2.5 V. Also, during this
equilibrium   state,   no   current   flows   through   the
compensation capacitor (C
COMP
 of Figure 1). Therefore:
" The R
OUT1
 current is:
I
R
OUT1
+
(V
OUT
)
nom
* 2.5 V
R
OUT1
(eq. 6)
where (V
OUT
)
nom
 is the nominal output voltage.
" The R
OUT2
 current is:
I
R
OUT2
+
2.5 V
R
OUT2
(eq. 7)
" And since no current flows through C
COMP
,
I
R
OUT1
+I
R
OUT2
?/DIV>
(V
OUT
)
nom
* 2.5 V
R
OUT1
+
2.5 V
R
OUT2
(eq. 8)
Under stable conditions, these equations are true.
Conversely when V
OUT
 is not at its nominal level, the
output of the error amplifier sinks or sources the current
necessary to maintain 2.5 V on pin 1. In particular, in the
case of an overvoltage condition:
" The error amplifier maintains 2.5 V on pin 1, and the
R
OUT2
 current remains:
I
R
OUT2
+
2.5 V
R
OUT2
(eq. 9)
" The R
OUT1
 current is:
I
R
OUT1
+
V
OUT
2.5 V
R
OUT1
+
(V
OUT
)
nom
) DV
OUT
2.5 V
R
OUT1
(eq. 10)
where DV
OUT
 is the output voltage excess.
" Therefore, the error amplifier sinks:
I
R
OUT1
I
R
OUT2
+
(V
OUT
)
nom
) DV
OUT
2.5 V
R
OUT1

2.5 V
R
OUT2
(eq. 11)
The combination of Equations 2 and 11 leads to a very
simple expression of the current sunk by the error
amplifier:
I
CONTROL
+ I
R
OUT1
* I
R
OUT2
+
DV
OUT
R
OUT1
(eq. 12)
Hence, the current absorbed by pin 2 (I
CONTROL
) is
proportional to the output voltage excess. The circuit
senses this current and disables the drive (pin 7) when
I
CONTROL
 exceeds I
OVP
 (typically 40 mA in NCP1606A,
10.4 mA in NCP1606B). This gives the OVP threshold as:
(V
OUT
)
OVP
+ (V
OUT
)
nom
) (R
OUT1
@ I
OVP
)
By simply adjusting R
OUT1
, the OVP limit can be easily
set. Therefore, one can compute the R
OUT1
 and R
OUT2
resistances using the following procedure:
1. Select R
OUT1
 to set the desired overvoltage level:
R
OUT1
+
(V
OUT
)
OVP
* (V
OUT
)
nom
I
OVP
For instance if implementing the NCP1606B, and
420 V is the maximum output level and 400 V is the
nominal output level, then
R
OUT1
+
420 * 400
10.4 mA
+ 1.9 MW
2. Select R
OUT2
 to adjust the regulation level:
R
OUT2
+
2.5 V @ R
OUT1
V
OUT(nom)
* 2.5 V
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