NCP1601BDR2G【PDF 13/19 页】IC PFC CTRL CRM/TRANSITION 8SOIC

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Sheet目录45

9页 10页 11页 12页 [13页]14页 15页 16页 17页

NCP1601A, NCP1601B

http://onsemi.com

voltage is higher based on the regulation block

characteristic in Figure 31. On the other hand, the V

control

in the low V

condition is much higher than the high V

condition. In order to not over- -design the circuit in the

application, the V

control

in the low V

condition is usually

very closed to V

control(max)

. It makes the output voltage be

almost 96% of the nominal value of R

ref

in low V

condition while the output voltage is almost 100% of the

nominal value R

ref

in high V

condition.

The feedback resistor R

consists of two or three high

precisionresistorsinordertosetthenominalV

out

precisely

and safety purpose.

The regulation block output V

reg

is connected to control

voltage V

control

through an internal resistor R

control

(300 k?typical) for the low- -pass filter in Figure 30. The

control

and the time information of zero current are

collected in the V

control

processing circuit to generate V

ton

which is then compared to a ramp signal to generate the

MOSFET on time t

for power factor correction.

Overvoltage Protection (OVP)

WhenthefeedbackcurrentI

ishigherthan107%ofthe

reference current I

ref

(i.e., the output voltage V

out

ishigher

than 107% of its nominal value), the Drive Output pin

(Pin7)ofthe device goeslowforprotectionand the switch

of the V

control

processing circuit is kept off. The circuit

automatically resumes operation when the output voltage

is lower than 107%.

ThemaximumOVPthresholdislimitedto225mAwhich

corresponds to 225 mA ?1.95 M?+ 5 V = 443.75 V when

= 1.95 M?(1.8 M?+ 150 k? and V

FB1

= 5 V (for

theworstcasereferringtoFigure11). Hence,itisgenerally

recommendedtouse450Vratingoutputcapacitortoallow

some design margin.

Undervoltage Protection (UVP)

When the feedback current I

is lower than 8% of the

reference current I

ref

(i.e., the output voltage V

out

is lower

than 8% of its nominal value), the device is shut down and

consumes lower than 50 mA. In normal situation of boost

converter configuration, the output voltage V

out

is always

higher than the input voltage V

and the feedback current

is always higher than 8% of the reference current I

ref

It enables the NCP1601 to operate. Hence, UVP happens

when the output voltage is abnormally undervoltage, the

FB pin (Pin 1) is opened, or the FB pin (Pin 1) is manually

pulled low.

Current Sense

The device senses the inductor current I

by the current

senseschemeinFigure32.Thisschemehastheadvantages

of:(1)the inrushcurrentlimitationbythe resistorR

,and

(2) the overcurrent protection and zero current detection

implemented in the same pin.

Figure 32. Current Sensing

NCP1601

Gnd

- -

Inductor current I

passes through R

and creates a

negative voltage. This voltage is measured by a current I

flowing out of the CS pin (Pin 4). The CS pin has an offset

voltage V

. This offset voltage is studied in the setting of

zero inductor current I

L(ZCD)

and the maximum inductor

current I

L(OCP)

(i.e., overcurrent protection threshold). A

typical variation of offset voltage V

versus sense current

is shown in Figure 15. Higher the value of the offset

voltage atlowcurrentregioncreateslowerthe zerocurrent

threshold for better accuracy. Based on Figure 32, (eq.13)

is derived.

(eq.13)

= - -R

Zero Current Detection (ZCD)

ThedevicerecognizeszeroinductorcurrentwhentheCS

pin (Pin 4) sense current I

is lower than I

S(ZCD)

(14 mA

typical). The offset voltage of the CS pin in this condition

is V

S(ZCD)

(7.5 mV typical). It is illustrated in Figure 33.

The inductor current I

L(ZCD)

at the ZCD condition is

derived in (eq.14).

(eq.14)

L(ZCD)

S(ZCD)

It is obvious that the I

L(ZCD)

is not always zero. In order

to make it reasonably close to zero, the settings of R

and

are crucial.

Figure 33. CS Pin Characteristic when I

= 0

S(ZCD)

> R

S(ZCD)

= R

S(ZCD)

Operating ZCD point

Ideal ZCD point

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