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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 VIN = 24V, IOUT = 400mA. The pair of scope

photos in Figure 13 show the response at the input node,

before and after the additional 33

µHinductorisadded.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

100mVP-P to perhaps 10mVP-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 12mVP-P, with

most of the energy in the lowest harmonics. After the

addition of a 4.7

µH inductor and a second 100µF output

capacitor, ripple is about 200

µVP-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

Figure 13. Input Node Ripple

VIN

VCC

VSW

LT1777

SHDN

SYNC

4.7

µH

1777 F12

VOUT

VIN

SGND

100

µF

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

µs/DIV

VIN NODE VOLTAGE

AC COUPLED

50mV/DIV

1777 F13b

(b) After Input Inductor

GND, CH2

VIN NODE CURRENT

DC COUPLED

50mA/DIV

µs/DIV

VIN NODE VOLTAGE

AC COUPLED

50mV/DIV

VIN NODE CURRENT

DC COUPLED

50mA/DIV

GND, CH2

1777 F13a

(a) Before Input Inductor

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