ncp1397a ON Semiconductor, ncp1397a Datasheet - Page 13

• Manufacturer Part Number: ncp1397a
• Description: High Performance Resonant Mode Controller With Integrated High-voltage Drivers
• Manufacturer: ON Semiconductor

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.
The designer needs to program the maximum switching
V
CC
Rt sets
Fmin for V(FB) = 0
RDT sets
the deadtime
Fmax sets
the maximum F
Fmax
DT
Rt
FB
SW
RFB
20 k
Figure 25. The Simplified VCO Architecture
Vref
Vref
V
V
V
DD
DD
DD
http://onsemi.com
Imin
Cint
Imin
0 to IFmax
IDT
13
Figure 26. The OPAMP Arrangement Limits the
+
VCO Modulation Signal between 0.5 and 2.3 V
V
FBinternal
-
+
+
-
CC
FB
max
max
F
SW
Vb(off)
R1
11.3 k
R2
8.7 k
100 k
R3
+
+
-
D
D1
2.3 V
Clk
S
R
Q
Q
VFB < VFB(off)
Start fault timer
Vref
0.5 V
+
+
−
A
RFmax
Fmax
B