21
LT1777
Optional Input/Output Filtering
When minimum
conducted
noise is required, it is often
advantageous to add an explicit input and/or output filter
to the topology. This can be a cost-effective way to reduce
conducted noise on the input or output node by an order
of magnitude or more. The exact details involved are a bit
lengthy, so the user is referred to the thorough treatments
in Application Notes AN19 and AN44. However, an ex-
ample will be given to illustrate the principles involved.
Figure 12 shows the previous “Basic 5V Output Applica-
tion” modified with an additional input inductor and an
output L/C combination. The dramatic improvement in
noise performance is seen in the accompanying oscillo-
scope photos shown in Figures 13 and 14. Operating
conditions are V
IN
= 24V, I
OUT
= 400mA. The pair of scope
photos in Figure 13 show the response at the input node,
before and after the additional 33µH inductor is added. The
upper waveform shows an AC-coupled version of the
output voltage at 50mV/DIV, and the lower waveform is a
DC-coupled representation of current into the node at
50mA/DIV. Input voltage ripple is seen to decrease from
100mV
P-P
to perhaps 10mV
P-P
. Ripple current is also seen
to decrease dramatically. (This improvement in AC ripple
current actually affects
radiated
magnetic noise.)
The next pair of scope photos in Figure 14 show an
AC-coupled version of the output node at 2mV/DIV.
Voltage ripple is seen to be originally about 12mV
P-P
, with
most of the energy in the lowest harmonics. After the
addition of a 4.7µH inductor and a second 100µF output
capacitor, ripple is about 200µV
P-P
.
These input and output inductor requirements are typically
not very difficult to achieve, and inexpensive open style
DO1608C types were used in this example. Once again,
more costly closed-construction style inductors may be
employed, but these are usually not necessary, as the AC
fields generated by these inductors are typically small.
TYPICAL APPLICATIONS
U
Figure 13. Input Node Ripple
+
V
IN
V
CC
V
SW
LT1777
V
D
SHDN
SYNC
4
10
7
6
D1
L4
4.7µH
L3
33µH
1777 F12
V
OUT
5
3
V
IN
12
14
13
V
C
FB
SGND
+
C8
100µF
10V
+
ADDITIONAL FILTER COMPONENTS
L3: COILCRAFT D01608C-333 OR SIMILAR
L4: COILCRAFT D01608C-472 OR SIMILAR
C8: AVX D CASE TPSD107M010R0080
Figure 12. Basic 5V Application with Optional Input/Output Filters
2µs/DIV
V
IN
NODE VOLTAGE
AC COUPLED
50mV/DIV
1777 F13b
(b) After Input Inductor
GND, CH2
V
IN
NODE CURRENT
DC COUPLED
50mA/DIV
2µs/DIV
V
IN
NODE VOLTAGE
AC COUPLED
50mV/DIV
V
IN
NODE CURRENT
DC COUPLED
50mA/DIV
GND, CH2
1777 F13a
(a) Before Input Inductor
