LT1175
applicaTions inForMaTion
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. 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 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 “V
R
,” and
maximum load current is I
LOAD
, R = V
R
/I
LOAD
. 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:
L
FIL
=
ESR
2
π
(
f
)
10
rr / 20
(
)
ESR = Effective series resistance of filter capacitor. This
assumes that the capacitive reactance is small com-
pared to ESR, a reasonable assumption for solid
tantalum capacitors above 2.2µF and 50kHz.
f
rr
= Ripple frequency
= Ripple rejection ratio of filter in dB
1.2
=
34
µ
H
5
−
25 / 20
6.3
10
10
Example: ESR = 1.2Ω, f = 100kHz, rr = – 25dB.
L
FIL
=
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 problems
with the filter/regulator combination.
1175fe
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