MAX3130/MAX3131
3V to 5.5V, IrDA Infrared Transceiver with
Integrated RS-232 Interface
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Power dissipation of the MAX3130/MAX3131, IR LED,
and R
SET
are based on the maximum LED current and
duty cycle.
Use the following equations to calculate the power dis-
sipation in each component:
MAX3130 power dissipation = I
SET
· V
DRV
· duty cycle
IR LED power dissipation = I
SET
· V
IRLED
· duty cycle
R
SET
power dissipation = I
SET
2
· R
SET
· duty cycle
For reliable operation, do not exceed maximum power
dissipation of the components.
PIN Photodiode Selection
PIN photodiode selection is extremely important to sys-
tem performance. The PIN diode must generate at least
200nA (minimum sensitivity of the MAX3130/MAX3131)
of current when aimed ±15° off-axis with an incident
irradiance of 4µW/cm
2
. The following equation deter-
mines if the Temic BPV22NF meets these requirements:
I
PIN
= (4µW/cm
2
) (0.075cm
2
) (0.95) (0.95) (1.8) (0.6A/W)
= 292nA
The first term (4mW/cm
2
) is the minimum guaranteed
irradiance in the ±15° angular range. The second term
(0.075cm
2
) is the sensitive area of the PIN diode. The
first 0.95 factor normalizes the sensitivity to the 875nm
wavelength and the second 0.95 factor adjusts for the
decreased receiver efficiency at ±15° off-axis. The 1.8
factor accounts for the round lens which increases the
effective PIN diode area. The last term (0.6A/W) is the
sensitivity of the PIN diode. Based on this example, the
Temic BPV22NF is an appropriate selection.
The final important factor in selecting a PIN diode is the
effective diode capacitance. It is important to keep this
capacitance below 70pF at 1.2V reverse bias. Higher
input capacitance compromises the noise performance
of the system by increasing the noise gain of the input
transimpedance amplifier.
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; either polarized or nonpolarized
capacitors are good choices. The charge pump
requires 0.1µF capacitors for 3.3V operation. For other
supply voltages, refer to Table 2 for suggested capaci-
tor values. Do not use values smaller than those listed
in Table 2. Increasing the capacitor values (e.g., by a
factor of 2) reduces ripple on the transmitter outputs
and slightly reduces power consumption. C2, C3, and
C4 can be increased without changing C1’s value.
However, do not increase C1 without also increas-
ing the values of C2, C3, and C4.
When using the minimum required capacitor values,
make sure the capacitor value does not degrade
excessively with temperature. If in doubt, use capaci-
tors with a larger nominal value. The capacitor’s equiv-
alent series resistance (ESR) usually rises at low
temperatures and increases the amount of ripple on V+
and V-.
Power-Supply Noise Rejection
Because of the extremely sensitive nature of photodi-
ode amplifiers, it is important to maintain a low-noise
supply voltage. Use a separate analog supply voltage
where possible. Place a 1µF ceramic bypass capacitor
as close as possible to the AV
CC
and V
CC
pins. In
especially noisy systems, connect a small (10Ω) resis-
tor in series with V
CC
, in addition to the normal bypass
capacitors.
IrDA or RS-232 Application Circuit
Figure 4 shows how the MAX3130 is used to multiplex
between RS-232 and IrDA communication while using
only one UART. By using the IRMODE input, the type of
communication (infrared or RS-232) is controlled by the
I/O of a µP. The internal MAX3130 ENDEC is used to
translate between UART-type and IrDA-type bit-
streams. If the UART has this capability, connect
BAUD16 of the MAX3130 to GND.
Figure 5 shows the MAX3131 used with two UARTs to
perform simultaneous IrDA and RS-232 communication.
UART1 is a software UART used to perform infrared
IrDA communication. The internal ENDEC on the
MAX3131 translates between UART-type and IrDA-type
bit-streams. The MAX3100 is implemented as UART2
and communicates via the RS-232 interface. The
MAX3100 interfaces to the µP using a SPI interface.
Layout Considerations
The MAX3130/MAX3131 require careful layout tech-
niques to minimize parasitic signals coupling to the
PINC input. Keep the lead length between the photodi-
ode and PINC as short as possible. Keep PC board
traces to the PIN diode away from other noisy traces.
To minimize coupling, run the AGND trace adjacent to
the PINC trace on both sides. To prevent oscillation,
avoid routing the RXD trace near the PINC trace.
Connect the anode of the PIN diode, GND, and the
ground lead of the AV
CC
bypass capacitor in a star-
connection. Keep the output pins RXD and TXD as
short as possible to minimize coupling back to the input
via parasitic capacitance.
