NCP1651DR2G【PDF 19/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

15页 16页 17页 18页 [19页]20页 21页 22页 23页

NCP1651

http://onsemi.com

The reference multiplier contains an internal loading

resistor, with a nominal value of 25 k? This is because the

resistor that converts the A input voltage into a current is

internal. Making both of these resistors internal, allows for

good accuracy and good temperature performance. Only a

capacitorneedstoaddedexternallytoproperlycompensate

this multiplier. It is not recommended that an external

parallel resistor be used at the Ref Gain pin, due to

tolerance variations of the internal resistor.

There is an offset in the compensation (A- -input) to the

referencemultiplier.ItisduetotheV- -Iconverterthatfeeds

the input.

The FB/SD signal is buffered by a voltage- -to- -current

converter for the appropriate signal into the multiplier. The

schematic for that converter follows.

Figure 33. Multiplier V- -I Converter

1.5 V

CURRENT

MIRROR

20 k

3.8 k

FB/SD

Reference Multiplier

6 X i

mult

The output current for this stage is:

mult

6 (V

1.5 V)

20 k

Figure 34. Reference Multiplier Clamp Circuit

Multiplier

25 k

AC Ref

4.5 V

AC Error

Amplifier

1 k

Thereisa1kresistorbetweentheACRefpinandtheAC

ErrorAmplifierforESDprotection.Duetothisresistor,the

voltageonpin10(ACRef)willexceed4.5voltsundersome

conditions, but the maximum voltage at the non- -inverting

AC Error Amplifier input will be clamped at 4.5 volts.

Feedback/Shutdown

The FB/SD pin is a multiple function pin. Its primary

function isto port the error signal to the voltage- -to- -current

converter that feeds the reference multiplier. The operating

rangeforthefeedbacksignalisfrom1.0to4.0volts.Below

an input level of 1.5 volts, the PWM duty cycle is reduced

to zero. At 4.0 volts the PWM is operating at its maximum

duty cycle.

The signal at this pin is also sensed by an internal

comparator that will shutdown the unit if the voltage falls

below 0.60 volts. Under normal operating conditions the

signal at this input will be 1.5 volts or greater, and the

shutdown circuit will be inactive. This circuit is designed

such that a 680 Ohm resistor in series with the optocoupler

willassurethattheconverterwillgotozerodutycyclewhen

theoptoisonfull,butwillnotgolowenoughtoputtheunit

into its shutdown mode.

Theshutdownfunctioncanbeusedformultiplepurposes

including overvoltage, undervoltage or hot- -swap control.

An external transistor, open collector or open drain gate,

connected to this pin can be used to pull it low, which will

inhibit the operation of the chip, and change the operating

state to a low power standby mode. An example of a

shutdown circuit is shown in Figure 23.

Ramp Compensation

The Ramp Compensation pin allows the amount of ramp

compensation to be adjusted for optimum performance.

Ramp compensation is necessary in a current mode

convertertostabilizetheunitsoperationwhenthedutycycle

is greater than 50%.

The amount of compensation required is dependant on

severalvariables,includingtheboostinductorvalue,andthe

desires of the designer. The value should be based on the

fallingdi/dtoftheinductorcurrent.Foraboostinductorwith

a variable input voltage, this will vary over the AC input

cycle, and with changes in the input line. A di/dt chart is

included in the design spreadsheet that is available for the

NCP1651.

For optimum load transient performance, the ramp

compensation should equal the falling di/dt at 100% duty

cycle. For optimum line transient response, it should equal

one half of the falling di/dt at 100% duty cycle.

This pin is a buffered output of the oscillator, which

providesavoltageequaltotherampontheoscillatorC

pin.

A resistor from this pin to ground, programs a current that

is transformed via a current mirror to the non- -inverting

input of the PWM comparator.

The ramp voltage due to the inductor di/dt at the input to

the PWM comparator is the current shunt voltage at pin 5

multiplied by 10, which is the gain of the current amplifier

output that feeds the PWM.

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