PFS7323 datasheet(5/30 Pages) POWERINT

5 / 30 page

Rev. B 06/13

PFS7323-7329

www.powerint.com

IS dt

VOFF

(VOUT-VIN)dt

Latch

RESET

Latch

SET

Gate

Drive (Q)

Maximum

ON-time

Minimum

OFF-time

Timing

Supervisor

Figure 4.

Idealized Converter Waveforms.

Figure 5.

Typical Normalized Output Voltage Characteristics as Function of

Normalized Peak Load Rating

Figure 6.

Normalized Minimum Power Limit as Function of Input Voltage.

0.2

0.4

0.6

1.2

1.4

0.8

Normalized to Peak Power Rating

1.2

0.8

0.6

0.4

0.2

The relationship of (4) demonstrates that by controlling a

constant amp-second on-time and constant volt-second

off-time, the input current I

IN is proportional to the input voltage

IN, therefore providing the fundamental requirement of power

factor correction.

This control produces a continuous mode power switch current

waveform that varies both in frequency and peak current value

across a line half-cycle to produce an input current proportional

to the input voltage.

Control Engine

The controller features a low bandwidth error-amplifier which

connects its non-inverting terminal to an internal voltage

reference of 6 V. The inverting terminal of the error-amplifier is

available on the external CONTROL pin which connects to the

loop compensation and voltage divider network to regulate the

output voltage. The FEEDBACK pin connects directly to the

divider network for fast transient load response.

The internal sense-FET switch current is integrated and scaled

by the input voltage peak detector current sense gain (M

ON) and

compared with the error-amplifier signal (V

E) to determine the

cycle on-time. Internally the difference between the input and

output voltage is derived and the resultant is scaled, integrated,

and compared to a voltage reference (V

OFF) to determine the

cycle off-time. Careful selection of the internal scaling factors

produce input current waveforms with very low distortion and

high power factor.

Line Feed-Forward Scaling Factor (M

ON)

The VOLTAGE MONITOR (V) pin current is used internally to

derive the peak of the input line voltage which is used to scale

the gain of the current sense signal through the M

ON variable.

This contribution is required to reduce the dynamic range of the

control feedback signal as well as maintain a constant loop gain

over the operating input line range. This line-sense feed-

forward gain adjustment is proportional to the square of the

peak rectified AC line voltage and is adjusted as a function of

VOLTAGE MONITOR pin current. The line-sense feed-forward

gain is also important in providing a switch power limit over the

input line range. Besides modifying brown- in/out thresholds,

the VOLTAGE MONITOR pin resistor also affects power limit of

the device.

This characteristic is optimized to maintain a relatively constant

internal error-voltage level at full load from an input line of 100 to

230 VAC input.

Beyond the specified peak power rating of the device, the

internal power limit feature will regulate the output voltage below

the set regulation threshold as a function of output overload

beyond the peak power rating. Figure 5 illustrates the typical

regulation characteristic as function of load.

Below the brown-in threshold (I

UV+) the power limit is reduced

when the device is operated in the ‘Full’ power mode as shown

in the figure below.

As the input line voltage is reduced toward the brown-out

threshold (I

UV-) and if the load exceeds the power limit derating

the boost output voltage will drop out of regulation in

accordance to Figure 6.

100

Input Voltage (VAC)

1.2

0.8

0.6

0.4

0.2

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