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NCV1397BDR2G

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27
NCP1397A/B, NCV1397A/B
www.onsemi.com
13
Figure 25. The Simplified VCO Architecture
Vref
V
DD
Rt sets
Fmin for V(FB) = 0
Cint
Imin
+
-
0 to IFmax
IDT
FBinternal
max
F
SW
max
+
-
+
Clk
D
S
Q
Q
R
AB
Vref
V
DD
RDT sets
the deadtime
DT
Imin
V
DD
Fmax
Fmax sets
the maximum F
SW
V
CC
FB
RFB
20 k
+
-
+
VFB < VFB(off)
Start fault timer
Vb(off)
Rt
The designer needs to program the maximum switching
frequency and the minimum switching frequency. In LLC
configurations, for circuits working above the resonant
frequency, a high precision is required on the minimum
frequency, hence the $3% specification. This minimum
switching frequency is actually reached when no feedback
closes the loop. It can happen during the startup sequence,
a strong output transient loading or in a short−circuit
condition. By installing a resistor from Pin 4 to GND, the
minimum frequency is set. Using the same philosophy,
wiring a resistor from Pin 2 to GND will set the maximum
frequency excursion. To improve the circuit protection
features, we have purposely created a dead zone, where the
feedback loop has no action. This is typically below 1.1 V.
Figure 26 details the arrangement where the internal voltage
(that drives the VCO) varies between 0 and 2.3 V. However,
to create this swing, the feedback pin (to which the
optocoupler emitter connects), will need to swing typically
between 1.1 V and 5.3 V.
Figure 26. The OPAMP Arrangement Limits the
VCO Modulation Signal between 0.5 and 2.3 V
V
CC
FB
R1
11.3 k
−
+
+
Vref
0.5 V
R2
8.7 k
R3
100 k
D1
2.3 V
RFmax
Fmax
NCP1397A/B, NCV1397A/B
www.onsemi.com
14
This techniques allows us to detect a fault on the converter
in case the FB pin cannot rise above 0.3 V (to actually close
the loop) in less than a duration imposed by the
programmable timer. Please refer to the fault section for
detailed operation of this mode.
As shown on Figure 26, the internal dynamics of the VCO
control voltage will be constrained between 0.5 V and 2.3 V,
whereas the feedback loop will drive Pin 6 (FB) between
1.1 V and 5.3 V. If we take the default FB pin excursion
numbers, 1.1 V = 50 kHz, 5.3 V = 500 kHz, then the VCO
maximum slope will be:
500 k * 50 k
4.2
+ 107 kHz/V
Figures 27 and 28 portray the frequency evolution
depending on the feedback pin voltage level in a different
frequency clamp combination.
Figure 27. Maximal Default Excursion,
Rt = 41 kW on Pin 4 and R
F(max)
= 1.9 kW on Pin 2
Figure 28. Here a Different Minimum Frequency was
Programmed as well as a Maximum Frequency
Excursion
Please note that the previous small−signal VCO slope has
now been reduced to 300k / 4.1 = 71 kHz / V on M
upper
and
M
lower
outputs. This offers a mean to magnify the feedback
excursion on systems where the load range does not generate
a wide switching frequency excursion. Due to this option,
we will see how it becomes possible to observe the feedback
level and implement skip cycle at light loads. It is important
to note that the frequency evolution does not have a real
linear relationship with the feedback voltage. This is due to
the deadtime presence which stays constant as the switching
period changes.
The selection of the three setting resistors (F
max
, F
min
and
deadtime) requires the usage of the selection charts
displayed below:
NCP1397A/B, NCV1397A/B
www.onsemi.com
15
50
150
250
350
450
550
1.9 11.9 21.9 31.9 41.9
Figure 29. Maximum Switching Frequency Resistor
Selection Depending on the Adopted Minimum
Switching Frequency
R
Fmax
(kW)
F
max
(kHz)
V
CC
= 15 V
V
FB
= 6.5 V
DT = 300 ns
F
min
= 200 kHz
F
min
= 50 kHz
100
150
200
250
300
350
400
450
500
2468101214161820
R
Fmin
(kW)
F
min
(kHz)
20
30
40
50
60
70
80
90
100
20 30 40 50 60 70 80 90 100 110
Figure 30. Minimum Switching Frequency Resistor
Selection (F
min
= 100 kHz to 500 kHz)
Figure 31. Minimum Switching Frequency Resistor
Selection (F
min
= 20 kHz to 100 kHz)
R
Fmin
(kW)
F
min
(kHz)
V
CC
= 15 V
V
FB
= 1 V
DT = 300 ns
V
CC
= 15 V
V
FB
= 1 V
DT = 300 ns
100
300
500
700
900
1100
1300
1500
1700
1900
3.5 13.5 23.5 33.5 43.5 53.5 63.5 73.5 83.5
R
DT
(kW)
DT (ns)
Figure 32. Deadtime Resistor Selection
ORing capability and optocoupler connection
configurations
If for any particular reason, there is a need for a frequency
variation linked to an event appearance (instead of abruptly
stopping pulses), then the FB pin lends itself very well to the
addition of other sweeping loops. Several diodes can easily
be used perform the job in case of reaction to a fault event
or to regulate on the output current (CC operation).
Figure 33 shows how to do it.
Figure 33. Thanks to the FB Configuration, Loop
ORing is Easy to Implement
V
CC
FB
In1
In2
20 k
VCO
The VCO configuration used in this IC also offers an easy
way to connect optocoupler (or pulldown bipolar) directly
to the Rt pin instead of FB pin (refer to Figures 34 and 35).
The optocoupler is then configured as “common emitter”
and the operating frequency is controlled by the current that
is taken out from the Rt pin – we have current controller
oscillator (CCO). If one uses this configuration it is needed
to maintain FB pin voltage between 0.3 V and 1 V otherwise
the FB fault will be detected. The FB pin can be still used for
open FB loop detection in some applications – to do so it is
needed to keep optcoupler emitter voltage higher then 0.3 V
for nominal load conditions. One needs to take R
FB
pulldown resistor into account when using this
configuration. It is possible to implement skip mode using
Skip/disable input and emitter resistors R
skip1
and R
skip2
.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27

NCV1397BDR2G

Mfr. #:
Buy NCV1397BDR2G
Manufacturer:
ON Semiconductor
Description:
Switching Controllers RESONANT MODE CONTRO
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