IR2157 データシートの表示（PDF） - International Rectifier

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IR2157

FULLY INTEGRATED BALLAST CONTROL IC

International Rectifier 

ADVANCED INFORMATION

IR2157

10

tDEAD

(usec)

1

CT = 220 pF

CT = 470 pF

CT = 1 nF

0.1

1

10

100

RDT (Kohms)

Figure 5: Deadtime versus RDT

During the Startup Mode, the operating frequency is

determined by the parallel combination of RPH,

RSTART , and RT , combined with the values of

CSTART, CT and RDT , as shown in Figure 6. This

frequency is normally chosen to ensure that the

instantaneous voltage across the lamp during the

first few cycles of operation does not exceed the

strike potential of the lamp. As the voltage across

CSTART charges up to the RT pin voltage, the output

frequency exponentially decays to the preheat

frequency.

During the Preheat Mode, the operating frequency

is determined by the parallel combination of RPH

and RT , combined with the value of CT and RDT .

This frequency, along with the Preheat Time, is

normally chosen to ensure that adequate heating of

the lamp filaments occurs. Typically, a 4.5:1 ratio of

CPH

CPH

2

7.6V

1.0uA

5.1V

4.0V

RPH

4.0V

3

2.0V

C IGN

RPH

RT

R RUN

RT

4

RUN

IRT

2.0V

5

CSTART RSTART

CT

ICT = IRT

6

CT

RDT

DT

7

SQ

R1

R2 Q

UNDER-

VOLTAGE

DETECT

Figure 6: Oscillator section block diagram with external component connection

11

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