NCP1397A データシートの表示（PDF） - ON Semiconductor

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NCP1397A

High Performance Resonant Mode Controller with Integrated High-Voltage Drivers

ON Semiconductor 

NCP1397A, NCP1397B

This techniques allows us to detect a fault on the converter

in case the FB pin cannot rise above 0.3 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 on Figure 26, the internal dynamics of the VCO

control voltage will be constrained between 0.5 V and 2.3 V,

whereas the feedback loop will drive Pin 6 (FB) between

1.1 V and 5.3 V. If we take the default FB pin excursion

numbers, 1.1 V = 50 kHz, 5.3 V = 500 kHz, then the VCO

maximum slope will be:

500k * 50k + 107 kHz/V

4.2

Figures 27 and 28 portray the frequency evolution

depending on the feedback pin voltage level in a different

frequency clamp combination.

Figure 27. Maximal Default Excursion,

Rt = 34 kW on Pin 4 and RF(max) = 1.9 kW on Pin 2

Figure 28. Here a different minimum frequency was

programmed as well as a maximum frequency

excursion

Please note that the previous small−signal VCO slope has

now been reduced to 300k / 4.1 = 71 kHz / V on Mupper and

Mlower outputs. 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.

The selection of the three setting resistors (Fmax, Fmin and

deadtime) requires the usage of the selection charts

displayed below:

VCC = 15 V

550

VFB = 6.5 V

DT = 300 ns

450

350

250

Fmin = 200 kHz

150

Fmax = 50 kHz

50

1.9

11.9

21.9

31.9

41.9

RFmax (kW)

Figure 29. Maximum Switching Frequency Resistor

Selection Depending on the Adopted Minimum

Switching Frequency

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14

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