LT1175CS8 datasheet(13/18 Pages) LINER

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LT1175

1175fe

To estimate regulator output ripple under different

conditions, 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

frequency ripple may be reduced by using larger output

capacitors.

2. Afeedforwardcapacitoracrosstheresistordividerused

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 1Ω

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

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

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:

ESR

FIL

( )

(

)

10 20

ESR = Effective series resistance of filter capacitor. This

assumesthatthecapacitivereactanceissmallcom-

pared to ESR, a reasonable assumption for solid

tantalum capacitors above 2.2µF and 50kHz.

= Ripple frequency

rr = Ripple rejection ratio of filter in dB

Example: ESR = 1.2Ω, f = 100kHz, rr = –25dB.

FIL =

















−

1 2

6 3 10

25 20

Solid tantalum capacitors are suggested for the filter to

keep filter Q fairly low. This prevents unwanted ringing at

theresonantfrequencyofthefilterandoscillationproblems

with the filter/regulator combination.

applicaTions inForMaTion

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