NCP1395A/B
http://onsemi.com
16
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 1 to AGND, the
minimum frequency is set. Using the same philosophy,
wiring a resistor from pin 2 to AGND 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 1.3 V.
Figure 34 details the arrangement where the internal
voltage (that drives the VCO) varies between 0 and 3.6 V.
However, to create this swing, the feedback pin (to which
the optocoupler emitter connects), will need to swing
typically between 1.3 V and 6.0 V.
V
CC
FB
Rfb
−
+
To VCO
0 to 3.6 V
+
1.3 V
VFB = 1.3−6 V
Figure 34. The OPAMP arrangement limits the VCO
internal modulation signal between 0 and 5.0 V.
This technique allows us to detect a fault on the converter
in case the FB pin cannot rise above 1.3 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 in Figure 34, the internal dynamics of the
VCO control voltage will be constrained between 0 V and
3.6 V, whereas the feedback loop will drive pin 5 (FB)
between 1.3 V and 6.0 V. If we take the external excursion
numbers, 1.3 V = 50 kHz, 6.0 V = 1.0 MHz, then the VCO
slope will then be
1Meg−50 k
4.7
+ 202 kHzńV.
Figures 35 and 36 portray the frequency evolution
depending on the feedback pin voltage level in a different
frequency clamp combination.
VFB
F
A&B
1.3 V
6 V
Fmin
Fmax
Fault
area
No variations
50 kHz
1 MHz
D
Fsw = 950 kHz
D
VFB = 4.7V
0.6 V
VFB
F
A&B
1.3 V
6 V
Fmin
Fmax
Fault
area
No variations
50 kHz
1 MHz
D
Fsw = 950 kHz
D
VFB = 4.7V
0.6 V
Figure 35. Maximal default excursion, Rt = 120 kW
on pin 1 and Rfmax = 35 kW on pin 2.
VFB
1.3 V
6 V
Fmin
Fmax
Fault
area
No variations
150 kHz
450 kHz
D
Fsw = 300 kHz
D
VFB = 4.7 V
F
A&B
0.6 V
VFB
1.3 V
6 V
Fmin
Fmax
Fault
area
No variations
150 kHz
450 kHz
D
Fsw = 300 kHz
D
VFB = 4.7 V
F
A&B
0.6 V
Figure 36. Here a different minimum frequency
was programmed as well as a different maximum
frequency excursion.
Please note that the previous small signal VCO slope has
now been reduced to 300 k/5.0 = 62.5 kHz/V. 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.
