ncp1395 ON Semiconductor, ncp1395 Datasheet - Page 18

• Manufacturer Part Number: ncp1395
• Description: High Performance Resonant Mode Controller
• Manufacturer: ON Semiconductor

Deadtime Control
half−bridge configuration comes to play. The deadtime
technique consists of inserting a period during which both
high and low side switches are off. Of course, the deadtime
amount differs depending on the switching frequency,
and unvalidates the AND gates: both outputs are low. When
the comparator goes back to the high 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 deadtime. The typical range
evolves between 150 ns (RDT = 30 kW) and 1.0 ms (RDT
= 600 kW). Figure 44 shows the typical waveforms
obtained on the output.
Soft−Start Sequence
suddenly applying the full current into the resonating
Deadtime control is an absolute necessity when the
During the discharge time, the clock comparator is high
In resonant controllers, a soft−start is needed to avoid
RDT
Vdd
DT
Idis
Vref
Ct
Icharge:
Fsw min + Fsw max
Figure 41. Deadtime Generation
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http://onsemi.com
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NCP1395A/B
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hence the ability to adjust it on this controller. The option
ranges between 150 ns and 1.0 ms. The deadtime is actually
made by controlling the oscillator discharge current.
Figure 41 portrays a simplified VCO circuit based on
Figure 33.
circuit. In this controller, a soft−start capacitor connects to
pin 4 and offers a smooth frequency variation upon startup:
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 1. 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 600 mV). Figure 43 depicts a typical
frequency evolution with soft−start.
D
Clk
S
R
Q
Q
A
B