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LT1248C Datasheet(PDF) 7 Page - Linear Technology |
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LT1248C Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LT1248 PI FU CTIO S Pin 12 (RSET): A resistor from RSET to GND sets the oscillator charging current and the maximum multiplier output current which is used to limit the maximum line current. IM(MAX) = 3.75V/RSET Pin 13 (SS): Soft-Start. When either VCC or EN/SYNC goes low, the SS pin will stay at 0V. With a capacitor from the pin to GND, the 12 µAchargingcurrentslowlybringsupthe SS to 8V; below 7.5V SS is the reference input to the voltage amplifier. At supply dropout or EN/SYNC low, the soft start capacitor will be quickly discharged. Pin 14 (CSET): The capacitor from this pin to GND, and RSET, determine oscillator frequency. The oscillator ramp is 5V, and the frequency = 1.5/(RSET • CSET). Pin 15 (VCC): This is the supply for the chip. The LT1248 has a very fast gate driver required to fast charge high power MOSFET gate capacitance. High current spikes occur during charging. For good supply bypass, a 0.1 µF ceramic capacitor in parallel with a low ESR electrolytic capacitor, 56 µF or higher is required in close proximity to IC GND. Pin 16 (GTDR): The MOSFET gate driver is a 1.5A fast totem pole output. It is clamped at 15V, but capacitive loads like MOSFET gates may cause overshoot. A gate series resistor of at least 5 Ω will prevent the overshoot. IAC (µA) 0 300 150 0 1248 G04 250 500 VAOUT = 7V VAOUT = 6.5V VAOUT = 6V VAOUT = 5.5V VAOUT = 5V VAOUT = 4.5V VAOUT = 4V VAOUT = 3.5V VAOUT = 3V VAOUT = 2.5V Figure 1. Multiplier Current IM vs IAC and VAOUT S APPLICATI I FOR ATIO Multiplier The multiplier is a current multiplier with high noise immunity in a high power switching environment. The current gain is: IM=(IAC• IEA2)/(200µA)2,with IEA=(VAOUT – 2V)/25k. With a square function, because of the lower gain at light power load, system stability is maintained and line current distortion caused by the line frequency AC Error Amplifier The error amplifier has a 100dB DC gain and 3MHz unity- gain frequency. The output is internally clamped at 13.5V. The noninverting input is tied to the 7.5V VREF through a diode and can be pulled down from the SS (soft-start) pin. Current Amplifier 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, 14mV AC will be needed at the inputs of the current amplifier for a gain of 350 at 120Hz. Especially at light load when the current loop reference signal is small, lower gain will distort the reference signal and line current. If signal gain at switching frequency is too high, the system be- haves more like a current mode system and can cause subharmonic oscillation. Therefore, the current amplifier should be compensated to have a gain of less than 15 at the switching frequency, but more than 250 at twice the line frequency. |
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