LT1249 Ver la hoja de datos (PDF) - Linear Technology
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LT1249 Datasheet PDF : 12 Pages
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LT1249
APPLICATIONS INFORMATION
VP-P = (2)(0.78A)(7.4Ω) = 11.5V. If less ripple is desired,
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:
I1RMS ≈ (0.71)(ILOADDC)
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.
I2RMS = 0.82A at 120VAC, 200W
The third component is the switching ripple from the load,
if the load is a switching regulator.
I3RMS ≈ ILOADDC
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:
( ) IRMS =
I1RMS
2
+
I2RMS
1.43
2
+
I3RMS 2
1.43
For a typical system that runs at an average load of 200W
and 385V output:
ILOADDC = 0.52A
I1RMS ≈ (0.71)(0.52A) = 0.37A
I2RMS ≈ 0.82A at 120VAC
I3RMS ≈ ILOADDC = 0.52A
( ) IRMS =
0.37A
2
+
0.82A 2
1.43
+
0.52A 2
1.43
=
0.77A
The 120Hz ripple current rating at 105°C ambient is 0.95A
for the 180µF KMH 400V capacitor. The expected life of the
output capacitor may be calculated from the thermal
stress analysis:
(105°C+∆TK )–(TAMB +∆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 condi-
tion. ∆TK = (I2R)/(KA), where I is the rated current, R is
capacitor ESR, and KA is a volume constant.
TAMB = operating ambient temperature
∆TO = capacitor internal temperature rise at operating
condition
In our example, LO = 2000 hours and ∆TK = 10°C at rated
0.95A. ∆TO can then be calculated from:
∆TO
=
IRMS
0.95A
2
(∆TK)
=
0.77A
0.95A
2
(10°C
)
=
6.6°C
Assuming the operating ambient temperature is 60°C, the
approximate life time is:
(105°C+10°C)–(60°C+6.6°C)
LO ≈ (2000)(2)
10
≈ 57,000 Hrs.
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
MOUT pin connected to this resistor must not exceed – 6V
while the chip is running and –12V under any conditions.
The LT1249 gate drive will malfunction if the MOUT pin
voltage exceeds – 6V while VCC is powered, destroying the
power FET. The 12V absolute limit is imposed by ESD
clamps on the MOUT pin. Large currents will flow at
9
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