Power-dissipation requirements of the MAX3120, IR
LED, and R
SET
must be met based on maximum duty
cycle and output current requirements.
MAX3120 Power Dissipation = I
SET
· V
LEDC
· Duty Cycle
IR LED Power Dissipation = I
SET
· V
LED
· Duty Cycle
R
SET
Power Dissipation = I
SET
2
· R
SET
· Duty Cycle
Power-Supply Noise Rejection
Because of the extremely sensitive nature of photodi-
ode amplifiers, it is important to maintain a quiet supply
voltage. Use a separate analog supply voltage where
possible. Place a 1µF ceramic bypass capacitor as
close to the V
CC
pin as possible. In especially noisy
systems, connect a small (10Ω) resistor in series with
V
CC
, in addition to the normal bypass capacitor.
Layout Considerations
The MAX3120 requires careful layout techniques to mini-
mize parasitic signal coupling to the PINC input. Keep
the lead length between the photodiode and PINC as
short as possible. Be sure to keep PC board traces to
the PIN diode separate from other noisy traces. To mini-
mize coupling, run the AGND trace adjacent to the PINC
trace on both sides. To prevent oscillation, avoid routing
the RXD signal near the PINC signal. Connect the anode
of the PIN diode, the GND pin, and the supply bypass
capacitor pin in a star-ground connection. Connect
PGND and GND together. Reduce the output trace
length from RXD as much as possible to minimize cou-
pling back to the input via parasitic capacitance.
Chip Information
TRANSISTOR COUNT: 256
MAX3120
Low-Profile, 3V, 120µA,
IrDA Infrared Transceiver
_______________________________________________________________________________________ 7
Package Information
