NCP1395APG 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1395APG

High Performance Resonant Mode Controller

ON Semiconductor 

NCP1395A/B

The designer needs to program the maximum switching

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 1 to AGND, the

minimum frequency is set. Using the same philosophy,

wiring a resistor from pin 2 to AGND 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 1.3 V.

Figure 34 details the arrangement where the internal

voltage (that drives the VCO) varies between 0 and 3.6 V.

However, to create this swing, the feedback pin (to which

the optocoupler emitter connects), will need to swing

typically between 1.3 V and 6.0 V.

VCC

Figures 35 and 36 portray the frequency evolution

depending on the feedback pin voltage level in a different

frequency clamp combination.

FA&B

Fmax

ÏÏÏÑÑÑÏÏÏÑÑÑNo vaÏÏÏÑÑÑriationsÏÏÏÑÑÑ1 MHz

DFsw = 950 kHz

ÎÌ Fmin

50 kHz

VFB

Fault

area

1.3 V DVFB = 4.7V 6 V

0.6 V

Figure 35. Maximal default excursion, Rt = 120 kW

on pin 1 and Rfmax = 35 kW on pin 2.

VFB = 1.3−6 V

FB

To VCO

+

0 to 3.6 V

−

FA&B

Fmax

ÔÔÒÒÔÔÒÒÔÔÒÒÔÔÒÒ No variations

450 kHz

Rfb

+

1.3 V

Figure 34. The OPAMP arrangement limits the VCO

internal modulation signal between 0 and 5.0 V.

This technique allows us to detect a fault on the converter

in case the FB pin cannot rise above 1.3 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 in Figure 34, the internal dynamics of the

VCO control voltage will be constrained between 0 V and

3.6 V, whereas the feedback loop will drive pin 5 (FB)

between 1.3 V and 6.0 V. If we take the external excursion

numbers, 1.3 V = 50 kHz, 6.0 V = 1.0 MHz, then the VCO

slope

will

then

be

1

Meg−50

4.7

k

+

202

kHzńV.

DFsw = 300 kHz

ÓÓÖÖ Fmin

Fault

area

1.3 V DVFB = 4.7 V 6 V

0.6 V

150 kHz

VFB

Figure 36. Here a different minimum frequency

was programmed as well as a different maximum

frequency excursion.

Please note that the previous small signal VCO slope has

now been reduced to 300 k/5.0 = 62.5 kHz/V. 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.

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