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LT1961 Datasheet(PDF) 7 Page - Linear Technology |
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LT1961 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LT1961 1961fa APPLICATIONS INFORMATION FB RESISTOR NETWORK The suggested resistance (R2) from FB to ground is 10k 1%. This reduces the contribution of FB input bias current to output voltage to less than 0.2%. The formula for the resistor (R1) from VOUT to FB is: R RV RA OUT 1 21 2 12 2 0 2 = − () −μ . .( . ) defines the pole frequency of the output stage, an X7R or X5R type ceramic, which have good temperature stability, is recommended. Tantalum capacitors are usually chosen for their bulk capacitance properties, useful in high transient load appli- cations. ESR rather than absolute value defines output ripple at 1.25MHz. Values in the 22 μF to 100μF range are generally needed to minimize ESR and meet ripple current ratings. Care should be taken to ensure the ripple ratings are not exceeded. Table 1. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E Case Size ESR (Max, Ω) Ripple Current (A) AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 AVX TAJ 0.7 to 0.9 0.4 D Case Size AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 C Case Size AVX TPS 0.2 (typ) 0.5 (typ) INPUT CAPACITOR Unlike the output capacitor, RMS ripple current in the input capacitor is normally low enough that ripple current rating is not an issue. The current waveform is triangular, with an RMS value given by: I VV V Lf V RIPPLE RMS IN OUT IN OUT () = () − () ()( )( ) 029 . At higher switching frequency, the energy storage require- ment of the input capacitor is reduced so values in the range of 1 μF to 4.7μF are suitable for most applications. Y5V or similar type ceramics can be used since the absolute value of capacitance is less important and has no significant effect on loop stability. If operation is required close to the minimum input voltage required by either the output or the LT1961, a larger value may be necessary. This is to prevent excessive ripple causing dips below the minimum operating voltage resulting in erratic operation. Figure 2. Feedback Network OUTPUT CAPACITOR Step-up regulators supply current to the output in pulses. The rise and fall times of these pulses are very fast. The output capacitor is required to reduce the voltage ripple this causes. The RMS ripple current can be calculated from: II V V V RIPPLE RMS OUT OUT IN IN () = − ()/ The LT1961 will operate with both ceramic and tantalum output capacitors. Ceramic capacitors are generally cho- sen for their small size, very low ESR (effective series resistance), and good high frequency operation, reducing output ripple voltage. Their low ESR removes a useful zero in the loop frequency response, common to tantalum capacitors. To compensate for this, the VC loop compen- sation pole frequency must typically be reduced by a factor of 10. Typical ceramic output capacitors are in the 1 μF to 10 μF range. Since the absolute value of capacitance – + 1.2V VSW VC GND 1961 F02 R1 R2 10k OUTPUT ERROR AMPLIFIER FB LT1961 + |
Similar Part No. - LT1961_15 |
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Similar Description - LT1961_15 |
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