MAX3086EEPD 查看數據表（PDF） - Maxim Integrated

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MAX3086EEPD

±15kV ESD-Protected, Fail-Safe, High-Speed (10Mbps), Slew-Rate-Limited RS-485/RS-422 Transceivers

Maxim Integrated 

±15kV ESD-Protected, Fail-Safe, High-Speed (10Mbps),

Slew-Rate-Limited RS-485/RS-422 Transceivers

RO

R

RE

DE

DI

D

MAX3080E/MAX3081E/MAX3083E/

MAX3084E/MAX3086E/MAX3087E/

MAX3089E (FULL DUPLEX)

A

120Ω

B

DATA IN

Z

120Ω

Y

DATA OUT

DI

VA - VB

RO

MAX3080/3089 FIG-18

5V/div

1V/div

5V/div

NOTE: RE AND DE ON MAX3080E/MAX3083E/MAX3086E/MAX3089E ONLY.

Figure 18. Line Repeater for MAX3080E/MAX3081E/MAX3083E/

MAX3084E/MAX3086E/MAX3087E, and MAX3089E in Full-Duplex

Mode

during test and assembly. All pins require this protec-

tion, not just RS-485 inputs and outputs.

Applications Information

256 Transceivers on the Bus

The standard RS-485 receiver input impedance is 12kΩ

(one-unit load), and the standard driver can drive up to

32 unit loads. The MAX3080E family of transceivers

have a 1/8-unit-load receiver input impedance (96kΩ),

allowing up to 256 transceivers to be connected in par-

allel on one communication line. Any combination of

these devices and/or other RS-485 transceivers with a

total of 32 unit loads or less can be connected to the

line.

Reduced EMI and Reflections

The MAX3080E–MAX3085E, and MAX3089E with SRL =

VCC or unconnected, are slew-rate limited, minimizing

EMI and reducing reflections caused by improperly ter-

minated cables. Figure 15 shows the driver output

waveform and its Fourier analysis of a 20kHz signal

transmitted by a MAX3086E/MAX3087E/MAX3088E,

and MAX3089E with SRL = GND. High-frequency har-

monic components with large amplitudes are evident.

Figure 16 shows the same signal displayed for a

MAX3083E/MAX3084E/MAX3085E, and MAX3089E with

SRL = VCC), transmitting under the same conditions.

Figure 16’s high-frequency harmonic components are

much lower in amplitude, compared with Figure 15’s,

and the potential for EMI is significantly reduced. Figure

17 shows the same signal displayed for a MAX3080E/

MAX3081E/MAX3082E, and MAX3089E with SRL =

unconnected, transmitting under the same conditions.

5µs/div

Figure 19. MAX3080E/MAX3081E/MAX3082E, and MAX3089E

with SRL = Unconnected, System Differential Voltage at 50kHz

Driving 4000ft of Cable

Figure 17’s high-frequency harmonic components are

even lower.

In general, a transmitter’s rise time relates directly to the

length of an unterminated stub, which can be driven with

only minor waveform reflections. The following equation

expresses this relationship conservatively:

Length = tRISE / (10 x 1.5ns/ft)

where tRISE is the transmitter’s rise time.

For example, the MAX3080E’s rise time is typically

1320ns, which results in excellent waveforms with a stub

length up to 90 feet. A system can work well with longer

unterminated stubs, even with severe reflections, if the

waveform settles out before the UART samples them.

Low-Power Shutdown Mode

(Except MAX3082E/MAX3085E/MAX3088E)

Low-power shutdown mode is initiated by bringing both

RE high and DE low. In shutdown, the devices typically

draw only 1nA of supply current.

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.

Enable times tZH and tZL in the Switching Char-

acteristics tables assume the part was not in a low-

power shutdown state. Enable times tZH(SHDN) and

tZL(SHDN) assume the parts were shut down. It takes

drivers and receivers longer to become enabled from

low-power shutdown mode (tZH(SHDN), tZH(SHDN)) than

from driver/receiver-disable mode (tZH, tZL).

18 ______________________________________________________________________________________

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