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1977EFE Datasheet(PDF) 11 Page - Linear Technology |
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1977EFE Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT1977 11 1977f APPLICATIO S I FOR ATIO 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 LT1977 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 500kHz switching frequency input capacitor values in the range of 4.7µF to 20µF are suitable for most applications. If opera- tion is required close to the minimum input required by the LT1977 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 LT1977 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 LT1977. All input voltage transient sequences should be observed at the VIN pin of the LT1977 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 0.5Ω to 2Ω and capacitance will fall in the range of 5µF to 50µF. If tantalum capacitors are used, values in the 22µF to 470µF range are generally needed to minimize ESR and meet ripple current and surge ratings. Care should be taken to ensure the ripple and surge ratings are not exceeded. The AVX TPS and Kemet T495 series are surge rated. AVX recommends derating capacitor operating voltage by 2:1 for high surge applications. OUTPUT CAPACITOR The output capacitor is normally chosen by its effective series resistance (ESR) because this is what determines output ripple voltage. To get low ESR takes volume, so physically smaller capacitors have higher ESR. The ESR range for typical LT1977 applications is 0.05Ω to 0.2Ω. A typical output capacitor is an AVX type TPS, 100µF at 10V, with a guaranteed ESR less than 0.1Ω. This is a “D” size surface mount solid tantalum capacitor. TPS capacitors are specially constructed and tested for low ESR, so they give the lowest ESR for a given volume. The value in microfarads is not particularly critical and values from 22µF to greater than 500µF work well, but you cannot cheat Mother Nature on ESR. If you find a tiny 22µF solid tantalum capacitor, it will have high ESR and output ripple voltage could be unacceptable. Table 3 shows some typical solid tantalum surface mount capacitors. Table 3. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E CASE SIZE ESR MAX (Ω) RIPPLE CURRENT (A) AVX TPS 0.1 to 0.3 0.7 to 1.1 D CASE SIZE AVX TPS 0.1 to 0.3 0.7 to 1.1 C CASE SIZE AVX TPS 0.2 0.5 Many engineers have heard that solid tantalum capacitors are prone to failure if they undergo high surge currents. This is historically true and type TPS capacitors are specially tested for surge capability but surge ruggedness is not a critical issue with the output capacitor. Solid tantalum capacitors fail during very high turn-on surges which do not occur at the output of regulators. High discharge surges, such as when the regulator output is dead shorted, do not harm the capacitors. Unlike the input capacitor RMS, ripple current in the output capacitor is normally low enough that ripple cur- rent rating is not an issue. The current waveform is |
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