Low-Power, Slew-Rate-Limited
RS-485/RS-422 Transceivers
10dB/div
10dB/div
0Hz
5MHz
0Hz
5MHz
500kHz/div
500kHz/div
Figure 12. Driver Output Waveform and FFT Plot of MAX481/
MAX485/MAX490/MAX491/MAX1487 Transmitting a 150kHz
Signal
Figure 13. Driver Output Waveform and FFT Plot of MAX483/
MAX487–MAX489 Transmitting a 150kHz Signal
Low-Power Shutdown Mode
(MAX481/MAX483/MAX487)
A low-power shutdown mode is initiated by bringing
both RE high and DE low. The devices will not shut
down unless both the driver and receiver are disabled.
In shutdown, the devices typically draw only 0.1µA of
supply current.
Driver Output Protection
Excessive output current and power dissipation caused
by faults or by bus contention are prevented by two
mechanisms. A foldback current limit on the output
stage provides immediate protection against short cir-
cuits over the whole common-mode voltage range (see
Typical Operating Characteristics). In addition, a ther-
mal shutdown circuit forces the driver outputs into a
high-impedance state if the die temperature rises
excessively.
RE and DE may be driven simultaneously; the parts are
guaranteed not to enter shutdown if RE is high and DE
is low for less than 50ns. If the inputs are in this state
for at least 600ns, the parts are guaranteed to enter
shutdown.
Propagation Delay
Many digital encoding schemes depend on the differ-
ence between the driver and receiver propagation
delay times. Typical propagation delays are shown in
Figures 15–18 using Figure 14’s test circuit.
For the MAX481, MAX483, and MAX487, the t
and
ZH
t
enable times assume the part was not in the low-
ZL
power shutdown state (the MAX485/MAX488–MAX491
and MAX1487 can not be shut down). The t
ZH(SHDN)
The difference in receiver delay times, | t
- t
|, is
PLH PHL
and t
enable times assume the parts were shut
down (see Electrical Characteristics).
ZL(SHDN)
typically under 13ns for the MAX481, MAX485,
MAX490, MAX491, and MAX1487 and is typically less
than 100ns for the MAX483 and MAX487–MAX489.
It takes the drivers and receivers longer to become
enabled from the low-power shutdown state
The driver skew times are typically 5ns (10ns max) for
the MAX481, MAX485, MAX490, MAX491, and
MAX1487, and are typically 100ns (800ns max) for the
MAX483 and MAX487–MAX489.
(t
), t
) than from the operating mode
ZL(SHDN)
ZH(SHDN
–—–
(t , t ). (The parts are in operating mode if the RE,
ZH ZL
DE inputs equal a logical 0,1 or 1,1 or 0, 0.)
______________________________________________________________________________________ 11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
