ISO15 datasheet(15/20 Pages) RENESAS

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APPLICATION INFORMATION

Transient Voltages

ISO

GND2

ISO

= V

(1)

GND2

ISO

6 10

(2)

ISO

GND2

= 0.94

(3)

CIN

RISO

RIN

A,B, Y,orZ

SystemGround (GND1)

BusReturn (GND2)

16V

CISO

ISO15

ISO35

www.ti.com................................................................................................................................................................ SLOS580B – MAY 2008 – REVISED JULY 2008

Isolation of a circuit insulates it from other circuits and earth so that noise develops across the insulation rather

than circuit components. The most common noise threat to data-line circuits is voltage surges or electrical fast

transients that occur after installation. The transient ratings of the ISO15 and ISO35 are sufficient for all but the

most severe installations. However, some equipment manufacturers use their ESD generators to test transient

susceptibility of their equipment, and can exceed insulation ratings. ESD generators simulate static discharges

that may occur during device or equipment handling with low-energy but high voltage transients.

Figure 21 models the ISO15 and ISO35 bus IO connected to a noise generator. CIN and RIN is capacitance or

resistance across the device and any other stray or added capacitance or resistance across the A or B pin to

GND2. CISO and RISO is the capacitance and resistance between GND1 and GND2 of the ISO15 and ISO35 plus

those of any other insulation (transformer, etc.). The stray inductance is assumed to be negligible. From this

model, the voltage at the isolated bus return is,

and will always be less than 16 V from VN. If the ISO15 and ISO35 are tested as a stand-alone device, RIN = 6

104

Ω, CIN = 16

10–12 F, RISO = 10

9Ω and C

ISO = 10

–12 F.

Note from Figure 21 that the resistor ratio determines the voltage ratio at low frequency and it is the inverse

capacitance ratio at high frequency. In the stand-alone case and for low frequency,

or essentially all of noise appears across the barrier. At high frequency,

and 94% of VN appears across the barrier. As long as RISO is greater than RIN and CISO is less than CIN, most of

transient noise appears across the isolation barrier.

It is not recommend for the user to test equipment transient susceptibility with ESD generators, or consider

product claims of ESD ratings above the barrier transient ratings of an isolated interface. ESD is best managed

through recessing or covering connector pins in a conductive connector shell and installer training.

Figure 21. Noise Model

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