Datasheet

MAX14821
IO-Link Device Transceiver
27Maxim Integrated
Applications Information
UART Interfacing
The logic levels of the microcontroller interface I/Os
(TXC, TXQ, TXEN, and RX) are defined by V
L
.
The device can be interfaced to microcontrollers where
the on-board UART TX output cannot be programmed as
a logic output (GPO). In this case, connect the TX output
of the UART to the TXC input for IO-Link communication
and connect a separate GPO output on the microcon-
troller to TXQ for standard IO (SIO) mode operation
(Figure 13). As the TXQ and TXC inputs are internally
logically ANDed, the unused input (TXC or TXQ) must be
held high while the other is in operation.
Transient Protection
Inductive load switching, surges, and bursts create high
transient voltages. C/Q, DO, and DI should be protected
against high overvoltage and undervoltage transients.
Positive voltage transients on C/Q, DO, and DI must be
limited to +55V relative to GND and negative voltage
transients must be limited to -55V (relative to V
CC
) on DO
and C/Q and to -55V (relative to GND) on DI. Figure 14
shows suitable protection using TVS diodes to meet both
the IEC 61000-4-2 ESD and IEC 61000-4-5 surge testing.
Other protection schemes may also be suitable.
The device has to be protected against transients that
occur during hot-plugging of the L+ sensor supply (VCC
input). This is achieved by placing a 10W resistor with
1µF capacitor before LDOIN and connecting an RC
between the sensor supply input and the VCC pin, as
shown in Figure 14. The RC time constant of the filter on
VCC should be larger or equal to 0.8µs. In case that VL is
supplied by V5 and the bypass capacitor on V5 is 100nF,
the 10W resistor in series with LDOIN is not needed.
Optional External Powering
The MAX14821 is powered by V
CC
and V
5
. V
L
is a
reference voltage input to set the logic levels of the
microcontroller interface. The logic and SPI interface are
operational when V
5
and V
L
are present even if V
CC
is
not present.
The V
P
output provides a reverse-polarity-protected
voltage one diode drop below V
CC
and can be used
for supplying external circuitry, like power supplies. The
current drawn from V
P
cannot exceed 50mA. Be aware
that capacitance on V
P
can cause transient currents at
power-up equal to C x dV
CC
/dt.
In order to reduce power dissipated in the device, an
reverse protection diode can be used to power the exter-
nal circuitry, instead of using VP, as shown in Figure 15.
V
5
is typically powered by the internal 5V regulator, but
can alternatively be powered by an external 5V regula-
tor. When powering V
5
externally, connect LDOIN to V
5
.
(Figure 15). This configuration disables operation of the
internal 5V regulator and reduces the on-chip power
consumption.
When an external 5V regulator is used to power V5, the
V5 bypass capacitance is determined by that regulator,
and is not limited to 100nF.
Figure 13. UART Interface
Figure 14. MAX14821 Operating Circuit with TVS Protection
MICROCONTROLLER
TXQGPO
TXCTX
TXENRTS
RXRX
MAX14821
MAX14821
0.8Ω
1μF
1μF
V
CC
LDOIN
DO
1/2
SDC36C
1/2
SDC36C
GND
C/Q
1/2
SDC36C
SDC36C
DI
V
P
10Ω