NCP1396B ON Semiconductor, NCP1396B Datasheet - Page 15

• Manufacturer Part Number: NCP1396B
• Description: High Performance Resonant Mode Controller featuring High-voltage Drivers
• Manufacturer: ON Semiconductor

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
1.2 V. Figure 32 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 1.2 V and 5.3 V.
converter in case the FB pin cannot rise above 0.6 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.
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 1.2V and 5.3 V. If we take the default FB
pin excursion numbers, 1.2 V = 50 kHz, 5.3 V = 500 kHz,
then the VCO maximum slope will be
109.7kHz / V.
Figure 32. The OPAMP Arrangement limits the VCO
Vcc
modulation signal between 0.5 and 2.3V
This techniques allows us to detect a fault on the
As shown on figure 32, the internal dynamics of
FB
R1
11.3 k
R2
8.7 k
R3
100 k
D1
2.3V
Vref
0.5 V
+
-
500
k −
NCP1396A, NCP1396B
4
Fmax
1 .
50
Rfmax
http://onsemi.com
k
=
15
Figure 33 and 34 portray the frequency evolution
depending on the feedback pin voltage level in a different
frequency clamp combination.
Please note that the previous small-signal VCO slope has
now been reduced to 300k / 4.1 = 73 kHz / V on Mupper
and Mlower outputs. This offers a mean to magnify the
feedback excursion on systems where the load range does
not generate a wide switching frequency excursion.
Thanks 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.
Figure 33. Maximal default excursion, Rt = 22 kΩ on pin
programmed as well as a maximum frequency excursion.
Figure 34. Here a different minimum frequency was
4 and Rfmax = 1.3 kΩ on pin 2.