User`s guide

LONWORKS LPT-10 Transceiver User’s Guide 4-3
IR LED driver's power supply may need to be isolated from Vcc with an L-C filter.
Typical R-C and L-C filters for isolating loads are shown in figure 4.1
NET_A
NET_B
V+
INDUCTOR
Vcc
GND
CLK
CLKSEL0
L1
C2
C1
+5V
C3
Ro
Lo
Co1
Co2
TO GENERAL
CIRCUITRY
TO SENSITIVE
LOW-POWER
ANALOG CIRCUITS
TO HIGHER-POWER
RESONANT LOADS
LPT-10
TO
NETWORK
Figure 4.1 Post-Filtering of Vcc
Use the following guidelines to design the resistor and capacitor values for the R-C
filter:
1. Set the resistance value R
o
as high as possible without causing an excessive
voltage drop in the Vcc supplied to the filtered load;
2. Choose the capacitor value C
o1
to set the cutoff frequency of the R-C filter
at least 10x below the frequency (f
n
) of noise sensitivity in the circuit being
powered. That is, choose C
o1
≥ 10/(2πRf
n
);
3. R
o
must be large enough to keep the "effective capacitance" of C
o1
as seen
by the LPT-10 transceiver to less than 5% of the transceiver’s 22µF output
capacitor C
2
. This can be accomplished by ensuring that
R
o
≥ 100Ω(C
o1
/1µF).
The L-C filter should be used for higher-power isolation (when the voltage drop
through R would be too high for the application circuit to tolerate). Use the following
steps to design the inductor and capacitor values for the L-C filter: