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 1Ω
to 10Ω resistor. A 3.3Ω resistor for instance, combined
with a 0.3Ω 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 “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:
U
W
U
U
L
FIL
=
ESR
2
π
(
f
)
10
rr / 20
(
)
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 2.2µF and 50kHz.
f
rr
= Ripple frequency
= Ripple rejection ratio of filter in dB
Example: ESR = 1.2Ω, f = 100kHz, rr = – 25dB.
L
FIL
=
1.2
=
34
µ
H
⎛
10
5
⎞ ⎛
10
−
25 / 20
⎞
6.3
⎝
⎠⎝
⎠
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.
1175fd
11
LINER [ LINEAR TECHNOLOGY ]
