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LT1976 数据表(PDF) 10 Page - Linear Technology |
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LT1976 数据表(HTML) 10 Page - Linear Technology |
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10 / 24 page  LT1976 10 1976f APPLICATIO S I FOR ATIO and Soft-Start Current graphs in Typical Performance Characteristics). Frequency foldback is done to control power dissipation in both the IC and in the external diode and inductor during short-circuit conditions. A shorted output requires the switching regulator to operate at very low duty cycles. As a result the average current through the diode and induc- tor is equal to the short-circuit current limit of the switch (typically 2A for the LT1976). Minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 200kHz, so frequency is reduced by about 4:1 when the FB pin voltage drops below 0.4V (see Frequency Foldback graph). In addition, if the current in the switch exceeds 1.5 times the current limitations speci- fied by the VC pin, due to minimum switch on time, the LT1976 will skip the next switch cycle. As the feedback voltage rises, the switching frequency increases to 200kHz with 0.95V on the FB pin. During frequency foldback, external syncronization is disabled to prevent interference with foldback operation. Frequency foldback does not affect operation during normal load conditions. In addition to lowering switching frequency the soft-start ramp rate is also affected by the feedback voltage. Large capacitive loads or high input voltages can cause a high input current surge during start-up. The soft-start func- tion reduces input current surge by regulating switch current via the VC pin to maintain a constant voltage ramp rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from the CSS pin to the output determines the maximum output dV/dt. When the feedback voltage is below 0.4V, the VC pin will rise, resulting in an increase in switch current and output voltage. If the dV/dt of the output causes the current through the CSS capacitor to exceed ICSS the VC voltage is reduced resulting in a constant dV/dt at the output. As the feedback voltage increases ICSS increases, resulting in an increased dV/dt until the soft-start function is defeated with 0.9V present at the FB pin. The soft-start function does not affect operation during normal load conditions. However, if a momentary short (brown out condition) is present at the output which causes the FB voltage to drop below 0.9V, the soft-start circuitry will become active. INPUT CAPACITOR Step-down regulators draw current from the input supply in pulses. The rise and fall times of these pulses are very fast. The input capacitor is required to reduce the voltage ripple this causes at the input of LT1976 and force the switching current into a tight local loop, thereby minimiz- ing EMI. The RMS ripple current can be calculated from: I I V VV V RIPPLE RMS OUT IN OUT IN OUT () – = () Ceramic capacitors are ideal for input bypassing. At 200kHz switching frequency input capacitor values in the range of 4.7 µF to 20µF are suitable for most applications. If opera- tion is required close to the minimum input required by the LT1976 a larger value may be required. This is to prevent excessive ripple causing dips below the minimum operat- ing voltage resulting in erratic operation. Input voltage transients caused by input voltage steps or by hot plugging the LT1976 to a pre-powered source such as a wall adapter can exceed maximum VIN ratings. The sudden application of input voltage will cause a large surge of current in the input leads that will store energy in the parasitic inductance of the leads. This energy will cause the input voltage to swing above the DC level of input power source and it may exceed the maximum voltage rating of the input capacitor and LT1976. All input voltage transient sequences should be observed at the VIN pin of the LT1976 to ensure that absolute maximum voltage ratings are not violated. The easiest way to suppress input voltage transients is to add a small aluminum electrolytic capacitor in parallel with the low ESR input capacitor. The selected capacitor needs to have the right amount of ESR to critically damp the resonant circuit formed by the input lead inductance and the input capacitor. The typical values of ESR will fall in the range of 0.5 Ω to 2Ω and capacitance will fall in the range of 5 µF to 50µF. If tantalum capacitors are used, values in the 22 µF to 470 µF range are generally needed to minimize ESR and meet ripple current and surge ratings. Care should be |
类似零件编号 - LT1976 |
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类似说明 - LT1976 |
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