LT1508CSW datasheet(12/16 Pages) LINER

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LT1508

The total equivalent 120Hz ripple in the output capacitor

can be calculated by:

IHF

1.43

IRMS =I120HZ2 +

()2

IHF = 100kHz Ripple Current.

For ILOAD(DC) = 0.88A, 1120Hz = 0.62A and the equivalent

120Hz ripple current is:

1.79

1.43

IRMS =

= 1.4ARMS

0.622 +

()2

Table 1 lists the ripple current components from lab

measurements for various output powers and line volt-

ages. The 120Hz ripple current rating at 105

°C ambient is

1.72A for the 470

µF KMH 35mm × 50mm capacitor. The

expected life of the output capacitor may be calculated

from thermal stress analysis:

(105

°C + ∆TK) – (TA + ∆TO)

L = (LO)2

where

L = Expected life time

LO = Hours of load life at rated ripple current and rated

ambient temperature

∆TK = Capacitor internal temperature rise at rated

condition.

∆TK = (I2R)/(KA), where I is the rated cur-

rent, R is capacitor ESR and KA is a volume constant.

TA = Operating ambient temperature

∆TO = Capacitor internal temperature rise at operating

condition

APPLICATIONS INFORMATION

Table 1. PFC Capacitor RMS Ripple Current

100W

200W

300W

VINRMS

I120HZ

IHF

I120HZ

IHF

I120HZ

IHF

100

0.2

0.6

0.41

1.18

0.62

1.79

120

0.2

0.5

0.41

0.97

0.62

1.45

230

0.2

0.53

0.41

0.87

0.62

1.26

In our example, LO = 2000 hours assuming ∆TK = 5°C at

rated 1.72A.

∆TO can then be calculated from:

IRMS

1.72A

∆TO = ∆TK

= 5

°C

()2 1.4A =3.3°C

1.72A

()2

Assuming the operating ambient temperature is 60

°C, the

approximate lifetime is:

(105

°C + 5°C) – (60 + 3.3°C)

L = (2000)(2)

= 50,870 Hours

For longer life a capacitor with a higher ripple current rating

or parallel capacitors should be used.

PWM Comparators

The LT1508 includes two comparators in the PWM section

which implement voltage mode PWM control. The VC or

control voltage pin sets duty factor. An additional current

limit comparator turns GTDR2 off in the event the ILIM pin

voltage exceeds 1.1V. On-chip blanking avoids reset due

to leading edge noise.

Typical Application

Figure 9 shows a 24VDC, 300W power factor corrected,

universal input supply. The 2-transistor forward converter

offers many benefits including low peak currents,

nondissipative snubber, 500VDC switches and automatic

core reset guaranteed by the LT1508’s 50% maximum

duty cycle.

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