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MAX3293/MAX3294/MAX3295

20Mbps, +3.3V, SOT23 RS-485/

RS-422 Transmitters

8 _______________________________________________________________________________________

Detailed Description

The MAX3293/MAX3294/MAX3295 are low-power

transmitters for RS-485/RS-422 communication. The

MAX3295 operates at data rates up to 20Mbps, the

MAX3294 up to 2.5Mbps (slew-rate limited), and the

MAX3293 up to 250kbps (slew-rate limited). These

devices are enabled using an active-high driver enable

(DE) input. When disabled, outputs enter a high-imped-

ance state, and the supply current reduces to 1µA.

The MAX3293/MAX3294/MAX3295 have a hot-swap

input structure that prevents disturbance on the differ-

ential signal lines when a circuit board is plugged into

a “hot” backplane (see the Hot-Swap Capability sec-

tion). Drivers are also short-circuit current limited and

are protected against excessive power dissipation by

thermal-shutdown circuitry.

Driver

The driver accepts a single-ended, logic-level input

(DI) and translates it to a differential RS-485/RS-422

level output (Y and Z). Driving DE high enables the dri-

ver, while pulling DE low places the driver outputs

(Y and Z) into a high-impedance state (see Table 1).

Low-Power Shutdown

Force DE low to disable the MAX3293/MAX3294/

MAX3295. In shutdown mode, the device consumes a

maximum of 10µA of supply current.

Hot-Swap Capability

Hot-Swap Input

When circuit boards are inserted into a “hot” or pow-

ered backplane, disturbances to the enable can lead to

data errors. Upon initial circuit board insertion, the

processor undergoes its power-up sequence. During

this period, the output drivers are high impedance and

are unable to drive the DE input of the MAX3293/

MAX3294/MAX3295 to a defined logic level. Leakage

currents up to 10µA from the high-impedance output

could cause DE to drift to an incorrect logic state.

Additionally, parasitic circuit board capacitance could

cause coupling of V

or GND to DE. These factors

could improperly enable the driver.

The MAX3293/MAX3294/MAX3295 eliminate all above

issues with hot-swap circuitry. When V

rises, an

internal pulldown circuit holds DE low for approximately

10µs. After the initial power-up sequence, the pulldown

circuit becomes transparent, resetting the hot-swap tol-

erable input.

Table 1. MAX3293/MAX3294/

MAX3295 (RS-485/RS-422) Transmitting

Function Table

X = Don’t care.

INPUTS OUTPUTS

DE DI Y Z

0 X Shutdown Shutdown

1001

1110

TIMER

(HOT SWAP)

10µs

100µA

M1 M2

5.6kΩ

2mA

Figure 6. Simplified Structure of the Driver Enable Input (DE)

Figure 7. Differential Power-Up Glitch (0.1V/µs)

DIFFERENTIAL POWER-UP GLITCH

(0.1V/µs)

4µs/div

2V/div

Y-Z

10mV/div

AC-COUPLED

10mV/div

AC-COUPLED

20mV/div

1 2 3 4 5 6 7 8 9 10 11

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