NCP1607
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16
Figure 34. OVP and UVP Circuit Blocks
FB
Control
+
E/A
+
Measure
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
I
CONTROL
V
OUT
I
CONTROL
V
CONTROL
R
FB
I
ROUT1
I
ROUT2
V
REF
V
UVP
I
RFB
When the output voltage is in steady state equilibrium,
R
OUT1
and R
OUT2
regulate the FB voltage to V
REF
. During
this equilibrium state, no current flows through the
compensation capacitor (C
COMP
shown in Figure 34).
These facts allow the following equations to be derived:
" The R
OUT1
current is:
I
ROUT1
+
V
OUT
* V
REF
R
OUT1
(eq. 7)
" The R
EQ
current is:
I
EQ
+
V
REF
R
EQ
+ I
ROUT2
) I
FB
(eq. 8)
" And since no current flows through C
COMP
,
I
ROUT1
+
V
OUT
* V
REF
R
OUT1
*
V
REF
R
EQ
(eq. 9)
Under stable conditions, Equations 7 through 9 are true.
Conversely, when V
OUT
is not at the target voltage, the
output of the error amplifier sinks or sources the current
necessary to maintain V
REF
on pin 1.
In the case of an overvoltage condition:
" The error amplifier maintains V
REF
on pin 1, and the
R
EQ
current remains the same as the steady state
value:
I
EQ
+
V
REF
R
EQ
(eq. 10)
" The R
OUT1
current is increased and is calculated using
Equation 11:
I
ROUT1
+
V
OUT(OVP)
* V
REF
R
OUT1
+
V
OUT
) DV
OUT
* V
REF
R
OUT1
(eq. 11)
where DV
OUT
is the output voltage excess.
" The error amplifier sinks:
I
Control
+ I
ROUT1
* I
EQ
+
V
OUT
) DV
OUT
* V
REF
R
OUT1
*
V
REF
R
EQ
(eq. 12)
The combination of Equations 2 and 12 yield a simple
expression of the current sunk by the error amplifier:
I
CONTROL
+
DV
OUT
R
OUT1
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
(10.4 mA typical). The OVP threshold is calculated using
Equation 13.
V
OUT(OVP)
+ V
OUT
) R
OUT1
@ I
OVP
(eq. 13)
The OVP limit is set by adjusting R
OUT1
. R
OUT1
is
calculated using Equation 14.
R
OUT1
+
V
OUT(OVP)
* V
OUT
I
OVP
(eq. 14)
For example, if 440 V is the maximum output voltage
and 400 V is the target output voltage, then R
OUT1
is
calculated using Equation 14.
R
OUT1
+
440 * 400
10.4m
+ 3.846 MW
If R
OUT1
is selected as 4 MW,, then V
OUT(OVP)
= 442 V.
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