RC

RD

CS

High-Voltage

Pulse

Generator

Device

Under

Test

40

35

30

25

20

15

10

5

0

Time - ns

0

50

100

150

200

250

300

10kV IEC

10kV HBM

330

(1.5k)

150pF

(100pF)

50M

(1M)

RxD

TxD

DRV

MCU

R

RE

DE

D

A

B

Vcc

GND

1

2

3

4

7

6

5

3.3V

10k

XCVR

8

5V

0.1µF

BAS70

RCV

SN888C

www.ti.com

SLLSEI4 – SEPTEMBER 2013

3-V to 5-V Interface

Interfacing the SN888C device to a 3-V controller is easy. Because the 5-V logic inputs of the transceiver accept

3-V input signals, they can be directly connected to the controller I/O. The 5-V receiver output, R, however, must

be level-shifted by a Schottky diode and a 10-k resistor to connect to the controller input (see Figure 15). When

R is high, the diode is reverse biased and the controller supply potential lies at the controller RxD input. When R

is low, the diode is forward biased and conducts. Only in this case, the diode forward voltage of 0.2 V lies at the

controller RxD input.

Figure 15. 3-V to 5-V Interface

Noise Immunity

The input sensitivity of a standard RS-485 transceiver is ±200 mV. When the differential input voltage, VID, is

greater than +200 mV, the receiver output turns high, for VID < –200 mV the receiver outputs low.

The SN888C transceiver implements high receiver noise-immunity by providing a typical positive-going input

threshold of 35 mV and a minimum hysteresis of 40 mV. In the case of a noisy input condition, a differential

noise voltage of up to 40 mVPP can be present without causing the receiver output to change states from high to

low.

Transient Protection

The bus terminals of the SN888C transceiver family possess on-chip ESD protection against ±16 kV HBM and

±12 kV IEC61000-4-2 contact discharge. The International Electrotechnical Commision (IEC) ESD test is far

more severe than the HBM ESD test. The 50% higher charge capacitance, CS, and 78% lower discharge

resistance, RD of the IEC model produce significantly higher discharge currents than the HBM model.

As stated in the IEC 61000-4-2 standard, contact discharge is the preferred transient protection test method.

Although IEC air-gap testing is less repeatable than contact testing, air discharge protection levels are inferred

from the contact discharge test results.

Figure 16. HBM and IEC-ESD Models and Currents in Comparison (HBM Values in Parenthesis)

Copyright © 2013, Texas Instruments Incorporated

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