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LT1508C Datasheet(PDF) 8 Page - Linear Technology |
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LT1508C Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1508 APPLICATIONS INFORMATION To avoid subharmonic oscillations, the amplified downslope of the inductor current must be less than the slope of the oscillator ramp. VCA(OUT) VRS (VOSC)(L)(fSW) (VOUT)(RS) (5V)(500 µH)(100k) (382V)(0.15 Ω) ≤ = = 4.4 If the current amplifier gain at 100kHz is less than 4.4, there will be no subharmonic oscillation. The open-loop gain of the current loop is given by: VRS VCA(OUT) (VOUT)(RS) (j)(2 πf)(L)(VOSC) 3648 (j)(f) (382V)(0.15 Ω) (j)(2 πf)(500µH)(5V) = == The current error amp, with R5 = 4k, R6 = 20k, C3 = 0.001 µF and C4 = 300pF, provides zero pole compensa- tion resulting in 16kHz loop crossover frequency. The current amp gain at 100kHz is 1.7. The resulting current loop gain and phase margin is shown in Figure 4. VIN RS 0.15 Ω L VIN IIN IM RREF 4k R5 4k VOUT D1 COUT 470 µF C4 300pF C3 0.001 µF R6 20k ISENSE MOUT CAOUT CA VIN RIAC 1M IAC 1508 • F02 LT1508 + – Figure 2 FREQUENCY (kHz) 0.1 –40 40 60 80 1 10 100 1000 1508 • F04 20 0 –20 –30 30 45 60 15 0 –15 Figure 4 FREQUENCY (Hz) 0.1 –40 40 60 80 1 10 100 1000 1508 • F03 20 0 –20 –30 30 45 60 15 0 –15 Figure 3 Current Amplifier (PFC Section) The current amplifier has a 110dB DC gain, 3MHz unity- gain frequency and a 2V/ µs slew rate. It is internally clamped at 8.5V. Note that in the current averaging opera- tion, high gain at twice the line frequency is necessary to minimize line current distortion. Because CAOUT may need to swing 5V over one line cycle at high line condition, 20mV AC will be needed at the inputs of the current amplifier for a gain of 260 at 120Hz. Especially at light load when the current loop reference signal is small, lower gain will distort the reference signal and line current. But, if signal gain at switching frequency is too high, the system behaves more like a current mode system and can cause subharmonic oscillation. Multiplier The multiplier has high noise immunity and superior linearity over its full operating range. The current gain is IM = (IACIEA2)/(200µA2) with IEA = (VAOUT – 2V)/ 25k. The error amplifier output voltage required at the input to the multiplier is: |
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