NCP1396B ON Semiconductor, NCP1396B Datasheet - Page 17

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

During the discharge time, the clock comparator is high
and un-validates the AND gates: both outputs are low.
When the comparator goes back to the low level, during
the timing capacitor Ct recharge time, A and B outputs
are validated. By connecting a resistor RDT to ground, it
creates a current whose image serves to discharge the Ct
capacitor: we control the dead-time. The typical range
evolves between 100 ns (RDT = 3.5 kΩ) and 2 us (RDT =
83.5 kΩ). Figure 42 shows the typical waveforms.
Soft-start sequence
avoid suddenly applying the full current into the
In resonant controllers, a soft-start is needed to
Figure 40. Soft-start behavior
RDT
DT
Vdd
Vref
Ct
Icharge :
Fsw min + Fsw max
Idis
Figure 39. Dead-time generation
NCP1396A, NCP1396B
-10.0
-20.0
20.0
10.0
177
175
173
171
169
0
http://onsemi.com
Figure 41. A typical start-up sequence on a LLC converter
SS
action
3V - 1V
+
-
200u
17
Target is
reached
resonating circuit. In this controller, a soft-start capacitor
connects to pin 1 and offers a smooth frequency variation
upon start-up: when the circuit starts to pulse, the VCO is
pushed to the maximum switching frequency imposed by
pin 2. Then, it linearly decreases its frequency toward the
minimum frequency selected by a resistor on pin 4. Of
course, practically, the feedback loop is suppose to take
over the VCO lead as soon as the output voltage has
reached the target. If not, then the minimum switching
frequency is reached and a fault is detected on the
feedback pin (typically below 600mV). Figure 40 depicts
a typical frequency evolution with soft-start.
600u
D
Clk
S
R
time in seconds
Q
1.00m
Q
A
1.40m
B
Ires
Vout
1.80m
1
2