Accessories

Filter & Ring Core Chokes FP, L and LP Series

REV. SEP 29, 2003

Page 4 of 5

Input Interference Reduction

Using L- or LP-series chokes together with an additional

external capacitor a similar attenuation can be achieved as

with filter blocks. The capacitor between the choke and the

converter input is necessary in order to avoid possible oscil-

lations caused by the negative input impedance of the regu-

lator. This phenomenon could cause the input voltage to

leave the specified regulator input range. The relatively

high ripple current flowing through the capacitor must be

considered for the design. Refer also to:

Technical Informa-

tion: Installation & Application.

The current compensated choke LP183 has a high perme-

ability ring core with two identical separate windings. The

normal operating current will only see the small stray in-

ductance between the windings. However common mode

interference will be blocked by the full inductance of the

choke.

Fig. 7

approx. 1000

µF) used as output filter

Reduction of Output Ripple

Even though switching regulators have an inherently low

output ripple, certain sensitive applications need even fur-

ther reduction. In such cases, the low-loss ring core chokes

designed to reduce disturbances at the input can also be

used for reducing the ripple on the output voltage. The

chokes in combination with an external capacitor can

achieve even better results than the Filter Blocks with re-

gard to the ripple and dynamic regulation.

The formula for the remaining output ripple at the load

calculated as follows:

Output ripple of the regulator

The impedance of the capacitor at the regu-

lator's switching frequency (150 kHz) corre-

sponds to the equivalent series resistance

(ESR) of the capacitor (please refer to the

corresponding data sheet).

150 kHz (regulator switching frequency)

Through the use of a common mode choke LP 183, the

common mode noise at the output can also be further re-

duced.

Consider that the filter not only affects the output ripple but

can also influence the voltage

event of load changes. The static regulation increases with

the ohmic resistance of the choke, i.e. 6 mV/A for the choke

L/LP20-7 and 20 mV/A for the LP34-3.

The dynamic regulation is dependent on the size of the ca-

pacitor. Generally, the bigger

namic, however, recovery will be slower.

Typical Application

A voltage drop

ground loop resistance

regulators output voltage

=

inductance

tively high AC component with a basic frequency

tor's switching frequency of approx. 150 kHz). This alternat-

ing current produces an AC voltage component across

which is superimposed upon

To prevent this phenomenon, an inductance

serted into the input circuit. This causes the AC component

of the input current to be supplied entirely from the input

capacitor

wired as close as possible to the regulator's input terminals

Vi+ and Gi–.

sients and reduce radio interference voltages.

External connection of Gi– and Go– or connection via a

common ground is not recommended. The internal voltage

drop

output voltage.

Vi+

Gi–

Vo+

Go–

PSR

U

12014

Fig. 5

L/LP type chokes and capacitors used as input filter

Vi+

Gi–

Vo+

Go–

PSR

U

LP34-3 or

L/LP20-7

LP183

12013

Vi+

Gi–

Vo+

Go–

PSR

U

12015

Fig. 6

Reduction of superimposed interference voltages in

grounded power supply systems, caused by ground loops