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LT1371IT7 Datasheet(PDF) 10 Page - Linear Technology |
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LT1371IT7 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT1371 APPLICATIO S I FOR ATIO generates a loop “zero” at 5kHz to 50kHz that is instrumen- tal in giving acceptable loop phase margin. Ceramic ca- pacitors remain capacitive to beyond 300kHz and usually resonate with their ESL before ESR becomes effective. They are appropriate for input bypassing because of their high ripple current ratings and tolerance of turn-on surges. Output Diode The suggested output diode (D1) is a 1N5821 Schottky or its Motorola equivalent MBR330. It is rated at 3A average forward current and 30V reverse voltage. Typical forward voltage is 0.6V at 3A. The diode conducts current only during switch OFF time. Peak reverse voltage for boost converters is equal to regulator output voltage. Average forward current in normal operation is equal to output current. Frequency Compensation Loop frequency compensation is performed on the output of the error amplifier (VC pin) with a series RC network. The main pole is formed by the series capacitor and the output impedance ( ≈500kΩ) of the error amplifier. The pole falls in the range of 2Hz to 20Hz. The series resistor creates a “zero” at 1kHz to 5kHz, which improves loop stability and transient response. A second capacitor, typi- cally one-tenth the size of the main compensation capaci- tor, is sometimes used to reduce the switching frequency ripple on the VC pin. VC pin ripple is caused by output voltage ripple attenuated by the output divider and multi- plied by the error amplifier. Without the second capacitor, VC pin ripple is: VC Pin Ripple = VRIPPLE = Output ripple (VP–P) gm = Error amplifier transconductance ( ≈1500µmho) RC = Series resistor on VC pin VOUT = DC output voltage 1.245(VRIPPLE)(gm)(RC) (VOUT) To prevent irregular switching, VC pin ripple should be kept below 50mVP–P. Worst-case VC pin ripple occurs at Output Capacitor Ripple Current (RMS) IRIPPLE (RMS) = IOUT = IOUT VOUT – VIN VIN DC 1 – DC DC = Switch Duty Cycle Input Capacitors The input capacitor of a boost converter is less critical due to the fact that the input current waveform is triangular and does not contain large squarewave currents as is found in the output capacitor. Capacitors in the range of 10 µF to 100 µF, with an ESR of 0.2Ω or less, work well up to full 3A switch current. Higher ESR capacitors may be acceptable at low switch currents. Input capacitor ripple current for a boost converter is : IRIPPLE = f = 500kHz Switching Frequency 0.3(VIN)(VOUT – VIN) (f)(L)(VOUT) The input capacitor can see a very high surge current when a battery or high capacitance source is connected “live” and solid tantalum capacitors can fail under this condition. Several manufacturers have developed tantalum capaci- tors specially tested for surge capability (AVX TPS series, for instance) but even these units may fail if the input voltage approaches the maximum voltage rating of the capacitor during a high surge. AVX recommends derating capacitor voltage by 2:1 for high surge applications. Ceramic, OS-CON and aluminum electrolytic capacitors may also be used and have a high tolerance to turn-on surges. Ceramic Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempt- ing for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems. Solid tantalum capacitor ESR |
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