Figure 3b. ENDEC Operation, Transmitting Infrared
LapLink is a trademark of Traveling Software.
1µs and three BAUD16 clock cycles) into a full baud
period (Figure 3a). Signals applied to TXD are inverted
and compressed to three BAUD16 clock cycles by the
ENDEC before being transmitted (Figure 3b). The
ENDEC is disabled by connecting the BAUD16 input to
V
CC
or GND.
Dual Charge-Pump Voltage Converter
The MAX3130/MAX3131’s internal power supply con-
sists of a regulated dual charge pump that provides
output voltages of +5.5V (doubling charge pump) and
-5.5V (inverting charge pump) for supply voltages from
+3.0V to +5.5V. The charge pump operates in a dis-
continuous mode: if the output voltages are less than
5.5V, the charge pumps are enabled; if the output volt-
ages exceed 5.5V, the charge pumps stop switching.
Each charge pump requires a flying capacitor (C1, C2)
and a reservoir capacitor (C3, C4) to generate the V+
and V- supplies (Figures 1 and 2). If RSSD (or IRMODE
for MAX3130) is low, both charge pumps shut down.
RS-232 Transmitters
The RS-232 transmitters are inverting level translators
that convert CMOS-logic levels to ±5.0V EIA/TIA-232
levels. The MAX3130/MAX3131 transmitters are guar-
anteed for data rates of 120kbps, providing compatibili-
ty with PC-to-PC communication software, such as
LapLink™. These RS-232 transmitters typically operate
at data rates of 235kbps. The RS-232 transmitter out-
puts are high impedance when either IRMODE or RSSD
are low.
The MAX3130/MAX3131 RS-232 receivers translate RS-
232 signal levels to CMOS-level logic. The RS-232
receivers also perform a logic inversion from input to
output. The receivers are always active and are not
affected by the RS-232 shutdown input (RSSD).
__________ Applications Information
Shutdown
The MAX3130/MAX3131 have split analog and digital
supplies (V
CC
and AV
CC
) with separate shutdown
modes. When IRSD is pulled low, the IR receiver is dis-
abled and AV
CC
current reduces to <1µA. When RSSD
or IRMODE is pulled low, the RS-232 charge pumps
are disabled and the RS-232 transmitter outputs
become high impedance. In this mode, the V
CC
current
reduces to <10µA.
IR LED Selection
The IrDA specification calls for an IR transmitter with a
peak wavelength between 850nm and 900nm. Within a
±15° half-cone angle, the output intensity of the IR LED
must be between 40mW/sr and 500mW/sr. Outside a
±30° half-cone angle, the output intensity of the IR LED
must fall below 40mW/sr. Within these cases, the opti-
cal rise and fall times of the IR LED must be less than
600ns. Based on these system requirements the HP
HSDL-4220, the Temic TSHF5400, or equivalent IR
LEDs are appropriate choices.
Powering the IR LED
Set the current in the IR LED with an external resistor.
Using the IR LED manufacturer’s data sheet, select a
forward current that meets the IrDA specifications dis-
cussed in the
IR LED Selection
section. Determine the
forward bias voltage of the IR LED (V
IRLED
) and the
voltage drop across the MAX3130/MAX3131 LED driver
(see LEDC Voltage vs. LEDC Current graph in the
Typical Operating Characteristics
) and choose the cur-
rent-setting resistor based on the following equation:
R
SET
= (V
CC
- V
IRLED
- V
LEDC
) / I
SET
Using the HP HSDL-4220 IR LED as an example:
V
CC
= 5V, I
SET
= 100mA, V
IRLED
= 1.67V
V
LEDC
= 90mV
R
SET
= (5V - 1.67V - 90mV) / 0.1A = 32.4Ω
