NCP1397A/B, NCV1397A/B
www.onsemi.com
17
Figure 36. Dead−time Generation
V
DD
I
charge
:
F
SW(min)
+ F
SW(max)
I
dis
Ct
RDT
DT
Vref
+
3 V−1 V
−
+
Clk
D
S
Q
Q
R
AB
Soft−Start Sequence
In resonant controllers, a soft−start is needed to avoid
suddenly applying the full current into the resonating circuit.
With this controller the soft−start duration is fully adjustable
using eternal components. The purpose of the Soft−Start pin
is to discharge Soft−Start capacitor before IC restart and in
case of fault conditions detected by Fault input.
Once the controller starts operation, the Soft−Start
capacitor (refer to Figure 37) is fully discharged and thus it
starts charging from the Rt pin. The charging current
increases operating frequency of the controller above F
min
.
As the soft−start capacitor charges, the frequency smoothly
decreases down to F
min
. Of course, practically, the feedback
loop is supposed 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 300 mV).
Figure 38 depicts a typical LLC startup using NCP1397A/B
controller.
Figure 37. Soft−Start Components Arrangement
SS
Fmax
Rt
GND
NCP1397
RF(start)
RFmin
RFmax
CSS
Fstart(adj) − RFstart/RFmin
Fmin(adj) − RFmin
Fmax(adj) − RFmax
Figure 38. A Typical Startup Sequence on a LLC
Converter Using NCP1397
SS
Action
Target is
Reached
Please note that the soft−start capacitor is discharged in the
following conditions:
− A startup sequence
− During auto−recovery burst mode
− A brown−out recovery
− A temperature shutdown recovery
The skip/disable input undergoes a special treatment.
Since we want to implement skip cycle using this input, we
cannot activate the soft−start every time the feedback pin
stops the operations in low power mode. Therefore, when
the skip/enable pin is released, no soft−start occurs to offer
the best skip cycle behavior. However, it is very possible to
combine skip cycle and true disable, e.g. via ORing diodes
driving Pin 8. In that case, if a signal maintains the
skip/disable input high long enough to bring the feedback
level down (below 0.3 V) since the output voltage starts to
fall down, then the soft−start discharge switch is activated.
