LT1249CS8 Linear Technology, LT1249CS8 Datasheet - Page 9

• Manufacturer Part Number: LT1249CS8
• Description: IC PFC CTRLR AVERAGE CURR 8SOIC
• Manufacturer: Linear Technology

Specifications of LT1249CS8

• Mode: Average Current
• Frequency - Switching: 100kHz
• Current - Startup: 250µA
• Voltage - Supply: 15.5 V ~ 27 V
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
V
higher capacitance should be used.
The selection of the output capacitor should also be based
on the operating ripple current through the capacitor.
The ripple current can be divided into three major compo-
nents. The first is at 120Hz whose RMS value is related to
the DC load current as follows:
The second component contains the PF switching fre-
quency ripple current and its harmonics. Analysis of this
ripple is complicated because it is modulated with a 120Hz
signal. However, computer numerical integration and Fou-
rier analysis approximate the RMS value reasonably close
to the bench measurements. The RMS value is about
0.82A at a typical condition of 120VAC, 200W load. This
ripple is line voltage dependent, and the worst case is at
low line.
The third component is the switching ripple from the load,
if the load is a switching regulator.
For United Chemicon KMH 400V capacitor series, ripple
current multiplier for currents at 100kHz is 1.43. The
equivalent 120Hz ripple current can then be found:
For a typical system that runs at an average load of 200W
and 385V output:
P-P
I
I
I
I
I
I
I
I
I
1RMS
2RMS
3RMS
LOAD
1RMS
2RMS
3RMS
RMS
RMS
= (2)(0.78A)(7.4 ) = 11.5V. If less ripple is desired,
DC = 0.52A
= 0.82A at 120VAC, 200W
(0.71)(I
I
(0.71)(0.52A) = 0.37A
0.82A at 120VAC
I
LOAD
LOAD
I
0 37
1
RMS
DC
DC = 0.52A
A
2
LOAD
U
2
DC)
I
1 43
2
INFORMATION
0 82
RMS
1 43
U
A
2
2
W
I
1 43
3
RMS
0 52
1 43
2
A
2
U
0 77
A
The 120Hz ripple current rating at 105 C ambient is 0.95A
for the 180 F KMH 400V capacitor. The expected life of the
output capacitor may be calculated from the thermal
stress analysis:
where
L = expected life time
L
ambient temperature
tion. T
capacitor ESR, and KA is a volume constant.
T
condition
In our example, L
0.95A. T
Assuming the operating ambient temperature is 60 C, the
approximate life time is:
For longer life, capacitor with higher ripple current rating
or parallel capacitors should be used.
Protection Against Abnormal Current Surge
Conditions
The LT1249 has an upper limit on the allowed voltage
across the current sense resistor. The voltage into the
M
while the chip is running and –12V under any conditions.
The LT1249 gate drive will malfunction if the M
voltage exceeds – 6V while V
power FET. The 12V absolute limit is imposed by ESD
clamps on the M
O
AMB
T
T
OUT
K
O
L
L
= hours of load life at rated ripple current and rated
O
= capacitor internal temperature rise at rated condi-
T
= capacitor internal temperature rise at operating
= operating ambient temperature
O
pin connected to this resistor must not exceed – 6V
( )( )
L
K
(
57,000 Hrs.
O
2000 2
= (I
O
0 95
can then be calculated from:
2
I
RMS
.
2
R)/(KA), where I is the rated current, R is
)( )
(
105
A
O
(
OUT
105
= 2000 hours and T
C
2
(
C
T
pin. Large currents will flow at
T K
K
10
)
)–(
10
CC
C
T AMB
)–(
10
is powered, destroying the
0 77
0 95
.
.
60
A
A
C
T O
6 6
2
.
)
(
K
10
= 10 C at rated
C
)
C
LT1249
)
OUT
6 6
.
9
pin
C