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

RS-422 Transmitters

_______________________________________________________________________________________

9

Hot-Swap Input Circuitry

The MAX3293/MAX3294/MAX3295 enable input fea-

tures hot-swap capability. At the input, there are two

ramps from zero, an internal 10µs timer turns on M2

and sets the SR latch, which also turns on M1.

Transistors M2, a 2mA current sink, and M1, a 100µA

current sink, pull DE to GND through a 5.6k

Ω resistor.

M2 is designed to pull DE to the disabled state against

an external parasitic capacitance up to 100pF that may

drive DE high. After 10µs, the timer deactivates M2

while M1 remains on, holding DE low against three-

state leakages that can drive DE high. M1 remains on

until an external source overcomes the required input

current. At this time, the SR latch resets and M1 turns

off. When M1 turns off, DE reverts to a standard, high-

1V, the hot-swap input is reset.

Hot-Swap Line Transient

During a hot-swap event when the driver is connected to

the line and is powered up, the driver must not cause the

differential signal to drop below 200mV. Figures 7, 8, and

9 show the results of the MAX3295 during power-up for

ended signal on each side of the 100

Ω termination, as

well as the differential signal across the termination.

ESD Protection

Human Body Model

Figure 10 shows the Human Body Model, and Figure 11

shows the current waveform it generates when dis-

charged into low impedance. This model consists of a

100pF capacitor charged to the ESD voltage of interest,

which is then discharged into the device through a

1.5k

Ω resistor.

Figure 8. Differential Power-Up Glitch (1V/µs)

Figure 9. Differential Power-Up Glitch (10V/µs)

Figure 11. Current Waveform

DIFFERENTIAL POWER-UP GLITCH

(10V/

μs)

200ns/div

2V/div

Y

Z

Y-Z

0V

50mV/div

AC-COUPLED

50mV/div

AC-COUPLED

100mV/div

DIFFERENTIAL POWER-UP GLITCH

(1V/

μs)

1

μs/div

2V/div

Y

Z

Y-Z

0V

100mV/div

AC-COUPLED

100mV/div

AC-COUPLED

200mV/div

Figure 10. Human Body ESD Test

CHARGE-CURRENT-

LIMIT RESISTOR

DISCHARGE

RESISTANCE

STORAGE

CAPACITOR

Cs

100pF

1M

Ω

1.5k

Ω

HIGH-

VOLTAGE

DC

SOURCE

DEVICE

UNDER

TEST

90%

36.8%

TIME

CURRENT WAVEFORM

PEAK-TO-PEAK RINGING

(NOT DRAWN TO SCALE)

Ir

10%

0V

0V

AMPERES