Datasheet LT1249 (Linear Technology) - 9

LT1249
U U W U APPLICATIONS INFORMATION
VP-P = (2)(0.78A)(7.4Ω) = 11.5V. If less ripple is desired, The 120Hz ripple current rating at 105°C ambient is 0.95A higher capacitance should be used. for the 180µF KMH 400V capacitor. The expected life of the The selection of the output capacitor should also be based output capacitor may be calculated from the thermal on the operating ripple current through the capacitor. stress analysis: The ripple current can be divided into three major compo- (105 C ° + T ∆ K)–(TAMB+ T ∆ O) nents. The first is at 120Hz whose RMS value is related to L = L ( O)( ) 2 10 the DC load current as follows: where I1RMS ≈ (0.71)(ILOADDC) L = expected life time The second component contains the PF switching fre- LO = hours of load life at rated ripple current and rated quency ripple current and its harmonics. Analysis of this ambient temperature ripple is complicated because it is modulated with a 120Hz ∆T signal. However, computer numerical integration and Fou- K = capacitor internal temperature rise at rated condi- tion. ∆T rier analysis approximate the RMS value reasonably close K = (I2R)/(KA), where I is the rated current, R is capacitor ESR, and KA is a volume constant. to the bench measurements. The RMS value is about 0.82A at a typical condition of 120VAC, 200W load. This TAMB = operating ambient temperature ripple is line voltage dependent, and the worst case is at ∆TO = capacitor internal temperature rise at operating low line. condition I2RMS = 0.82A at 120VAC, 200W In our example, LO = 2000 hours and ∆TK = 10°C at rated The third component is the switching ripple from the load, 0.95A. ∆TO can then be calculated from: if the load is a switching regulator. 2 2 I  0.7 A RMS 7  I ∆TO =  ∆ ( T ) 10 ( C) 6.6 C 3RMS ≈ ILOADDC  . A K  = 0.9 A 5  ° = ° 0 95 For United Chemicon KMH 400V capacitor series, ripple Assuming the operating ambient temperature is 60°C, the current multiplier for currents at 100kHz is 1.43. The approximate life time is: equivalent 120Hz ripple current can then be found: (105 C ° +10 C ° )–(60 C ° +6.6 C ° ) 2 2 LO ≈ ( ) 2000 ( ) 2 10 2 I  I  I I RMS 2 RMS = ( 3 ≈ 57,000 Hrs. 1 ) + RMS RMS  1 4 . 3  +  1 4 . 3  For longer life, capacitor with higher ripple current rating For a typical system that runs at an average load of 200W or parallel capacitors should be used. and 385V output:
Protection Against Abnormal Current Surge
ILOADDC = 0.52A
Conditions
I1RMS ≈ (0.71)(0.52A) = 0.37A The LT1249 has an upper limit on the allowed voltage I2RMS ≈ 0.82A at 120VAC across the current sense resistor. The voltage into the I3RMS ≈ ILOADDC = 0.52A MOUT pin connected to this resistor must not exceed – 6V while the chip is running and –12V under any conditions. 2 2 The LT1249 gate drive will malfunction if the MOUT pin 2  0 8 A 2   0.5 A 2  I = 0. 37A 0.77A ( ) + voltage exceeds – 6V while V RMS CC is powered, destroying the  1.43  +  1.43  = power FET. The 12V absolute limit is imposed by ESD clamps on the MOUT pin. Large currents will flow at 9