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LT1175IT-5 Datasheet(PDF) 11 Page - Linear Technology |
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LT1175IT-5 Datasheet(HTML) 11 Page - Linear Technology |
11 / 12 page 11 LT1175 APPLICATIONS INFORMATION To estimate regulator output ripple under different condi- tions, the following general comments should be helpful: 1. Output ripple at high frequency is only weakly affected by load current or output capacitor size for medium to heavy loads. At very light loads (<10mA), higher fre- quency ripple may be reduced by using larger output capacitors. 2. A feedforward capacitor across the resistor divider used with the adjustable part is effective in reducing ripple only for output voltages greater than 5V and only for frequencies less than 100kHz. 3. Input-to-output voltage differential has little effect on ripple rejection until the regulator actually enters a dropout condition of 0.2V to 0.6V. If ripple rejection needs to be improved, an input filter can be added. This filter can be a simple RC filter using a 1 Ω to 10 Ω resistor. A 3.3Ω resistor for instance, combined with a 0.3 Ω ESR solid tantalum capacitor, will give an additional 20dB ripple rejection. The size of the resistor will be dictated by maximum load current. If the maximum voltage drop allowable across the resistor is “VR,” and maximum load current is ILOAD, R = VR/ILOAD. At light loads, larger resistors and smaller capacitors can be used Dimensions in inches (millimeters) unless otherwise noted. PACKAGE DESCRIPTION to save space. At heavier loads an inductor may have to be used in place of the resistor. The value of the inductor can be calculated from: L ESR f FIL rr = () () 2 10 20 π / ESR = Effective series resistance of filter capacitor. This assumes that the capacitive reactance is small compared to ESR, a reasonable assumption for solid tantalum capacitors above 2.2 µF and 50kHz. f = Ripple frequency rr = Ripple rejection ratio of filter in dB Example: ESR = 1.2 Ω, f = 100kHz, rr = –25dB. LH FIL = = − 12 63 10 10 34 5 25 20 . . / µ Solid tantalum capacitors are suggested for the filter to keep filter Q fairly low. This prevents unwanted ringing at the resonant frequency of the filter and oscillation prob- lems with the filter/regulator combination. Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. Q Package 5-Lead Plastic DD Pak (LTC DWG # 05-08-1461) Q(DD5) 0396 0.028 – 0.038 (0.711 – 0.965) 0.143 +0.012 –0.020 () 3.632 +0.305 –0.508 0.057 – 0.077 (1.447 – 1.955) 0.013 – 0.023 (0.330 – 0.584) 0.095 – 0.115 (2.413 – 2.921) 0.004 +0.008 –0.004 () 0.102 +0.203 –0.102 0.050 ± 0.012 (1.270 ± 0.305) 0.059 (1.499) TYP 0.045 – 0.055 (1.143 – 1.397) 0.165 – 0.180 (4.191 – 4.572) 0.330 – 0.370 (8.382 – 9.398) 0.060 (1.524) TYP 0.390 – 0.415 (9.906 – 10.541) 15 ° TYP 0.300 (7.620) 0.075 (1.905) 0.183 (4.648) 0.060 (1.524) 0.060 (1.524) 0.256 (6.502) BOTTOM VIEW OF DD PAK HATCHED AREA IS SOLDER PLATED COPPER HEAT SINK |
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