LT1175
applicaTions inForMaTion
To estimate regulator output ripple under different
conditions, the following general comments should be
helpful:
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:
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.
ESR
L
=
FIL
rr / 20
10
2π f
( )
ESR = Effective series resistance of filter capacitor. This
assumesthatthecapacitivereactanceissmallcom-
pared to ESR, a reasonable assumption for solid
tantalum capacitors above 2.2µF and 50kHz.
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.
f
= Ripple frequency
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.
rr = Ripple rejection ratio of filter in dB
Example: ESR = 1.2Ω, f = 100kHz, rr = –25dB.
1.2
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
L
=
= 34µH
FIL
5
−25/ 20
6.3 10
10
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.
voltage drop allowable across the resistor is “V ,” and
R
maximum load current is I
, R = V /I
. At light
LOAD
R LOAD
1175fe
ꢀꢂ
Linear [ Linear ]
