Datasheet LT1175 (Analog Devices) - 14

LT1175
APPLICATIONS INFORMATION
To estimate regulator output ripple under different loads, larger resistors and smaller capacitors can be used conditions, the following general comments should be to save space. At heavier loads an inductor may have to helpful: be used in place of the resistor. The value of the inductor can be calculated from: 1. Output ripple at high frequency is only weakly affected by load current or output capacitor size for medium ESR L = to heavy loads. At very light loads (<10mA), higher FIL 2 f rr ( )(10 20 π / ) frequency ripple may be reduced by using larger output capacitors. ESR = Effective series resistance of fi lter capacitor. This 2. A feedforward capacitor across the resistor divider used assumes that the capacitive reactance is small com- with the adjustable part is effective in reducing ripple pared to ESR, a reasonable assumption for solid only for output voltages greater than 5V and only for tantalum capacitors above 2.2μF and 50kHz. frequencies less than 100kHz. f = Ripple frequency 3. Input-to-output voltage differential has little effect on rr = Ripple rejection ratio of fi lter in dB ripple rejection until the regulator actually enters a dropout condition of 0.2V to 0.6V. Example: ESR = 1.2Ω, f = 100kHz, rr = – 25dB. If ripple rejection needs to be improved, an input fi lter can 1 2 . L = 34 H μ be added. This fi lter can be a simple RC fi lter using a 1Ω FIL = ⎛ 5⎞ ⎛ −25/ 20⎞ 6 3 . 10 10 to 10Ω resistor. A 3.3Ω resistor for instance, combined ⎝ ⎠ ⎝ ⎠ with a 0.3Ω ESR solid tantalum capacitor, will give an ad- ditional 20dB ripple rejection. The size of the resistor will Solid tantalum capacitors are suggested for the fi lter to be dictated by maximum load current. If the maximum keep fi lter Q fairly low. This prevents unwanted ringing at voltage drop allowable across the resistor is “V the resonant frequency of the fi lter and oscillation problems R,” and maximum load current is I with the fi lter/regulator combination. LOAD, R = VR/ILOAD. At light 1175ff 14