LT1509 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1509' PDF : 16 Pages View PDF
LT1509
APPLICATIONS INFORMATION
The total equivalent 120Hz ripple in the output capacitor
can be calculated by:
( ) IRMS =
I120HZ2 +
IHF
1.43
2
IHF = 100kHz Ripple Current.
For ILOAD(DC) = 0.88A, 1120Hz = 0.62A and the equivalent
120Hz ripple current is:
( ) IRMS =
0.622 +
1.79 2
1.43
= 1.4ARMS
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 470µF 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
10
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
Table 1. PFC Capacitor RMS Ripple Current
VINRMS
100
120
230
100W
I120HZ
IHF
0.2
0.6
0.2
0.5
0.2 0.53
200W
I120HZ
IHF
0.41 1.18
0.41 0.97
0.41 0.87
300W
I120HZ
IHF
0.62 1.79
0.62 1.45
0.62 1.26
In our example, LO = 2000 hours assuming ∆TK = 5°C at
rated 1.72A. ∆TO can then be calculated from:
( ) ( ) ∆TO = ∆TK
IRMS
1.72A
2
= 5°C
1.4A
1.72A
2
= 3.3°C
Assuming the operating ambient temperature is 60°C, the
approximate lifetime is:
(105°C + 5°C) – (60 + 3.3°C)
L = (2000)(2)
10
= 50,870 Hours
For longer life a capacitor with a higher ripple current rating
or parallel capacitors should be used.
PWM Comparators
The LT1509 includes two comparators in the PWM section
which implement peak current mode control. The primary
current sense voltage is fed into the RAMP pin. The VC or
Control Voltage pin sets the primary peak current level. An
additional current limit comparator turns GTDR2 off in the
event the RAMP pin voltage exceeds 1V. Referring to the
Block Diagram, there is a 1.2V offset between the RAMP
and VC pin. This feature simplifies the connection to an
optocoupler because the VC pin no longer has to be pulled
all the way to ground to inhibit switching. 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 LT1509’s 50% maximum
duty cycle.
12
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