Datasheet

NCP1351
http://onsemi.com
16
Figure 19. The Internal Fault Management Differs Depending on the Considered Version
-
+
-
+
C1
100n
V
CC
I
FB
C
timer
100nF
V
CC
R1
2.5k
+
+
+
V
timer
5
I
timer
10
20s
Filter
P
on
Reset
D
FB
I
FB
I
FB
Timer
L
aux
CV
CC
I
CC
V
CC
D
aux
I
FB
< 40 A ? = Low
Else = High
DRV Pulses
V
CC
==
VCC
(min)
? Reset
Q
Q
S
to DRV
Stage
Auto-Recovery - B Version
-
+
-
+
C1
100n
V
CC
I
FB
C
timer
100nF
V
CC
R1
2.5k
+
+
+
V
timer
5
I
timer
10
20s
Filter
P
on
Reset
D
FB
I
FB
I
FB
Timer
L
aux
CV
CC
I
CC
V
CC
D
aux
I
FB
< 40 A ? = Low
Else = High
SCR Delatches When
I
SCR
< ICC
latch
(Few A)
Latched - A Version
6V
FB
FB
R
Knowing both the ending voltage and the charge current,
we can easily calculate the timer capacitor value for a given
delay. Suppose we need 40 ms. In that case, the capacitor is
simply:
C
timer
+
I
timer
T
V
timer
+
11.7 40m
5
+ 94nF
(eq. 16)
Select a 100 nF value.
To let the designer understand the behavior behind the
four different options (A, B, C and D), we have graphed
important signals during a fault condition. In versions A and
B, an internal error flag is raised as soon the controller hits
the maximum operating frequency. At this moment, the
external timer capacitor charge begins. If the fault persists,
the timer capacitor hits the fault level and the circuit is either
latched (A) or enters auto-recovery burst mode (B). If the
fault disappears, the timer capacitor is simply reset to 0 V by
an internal switch.
On version C and D, the error flag is asserted as soon as
the current feedback imposes a switching frequency roughly
equal to half of the maximum limit. For instance, should the
designer select a 100 kHz maximum switching frequency,
then the error flag would raise and start the timer for an
operating frequency above ≈ 50 kHz. Below 50 kHz, the
timer pin remains grounded. If we consider a DCM
operation at full load, as the inductor peak current is kept
constant, these 50 kHz correspond to 50% of the maximum
delivered power. If the load stays between 50% and 100% of
its nominal value, the timer continues to charge until it
reaches the final level. In that case, the circuit latches off (C)
or enters auto-recovery (D). This behavior is particularly
well suited for applications where the converter delivers a
moderate average power but is subjected to sudden peak
loading conditions. For instance, a power supply is designed
to permanently deliver 20 W but is sized to deliver 80 W in
peak conditions. During these 80 W power excursions, the
timer will react but will not shut down the power supply. On
the contrary, if a short-circuit appends or if the transient
overload lasts too long, the timer will immediately start to
further shutdown the controller in order to protect both the
application and downstream load.