ST AN1783 Application note
AN1783
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®
APPLICATION NOTE
HOW TO MAKE FIRE-WIRE COMMUNICATION PORT SAFE?
P. MERCERON
Video-mediaequipmentgrowthhasleadmanufacturers to use fasttransmission buses in orderto share information quickly and easily.
In many cases, video equipment use Fire-Wire link (IEEE1394) which brings the following features:
Up to 63 devices can be plugged on the bus
n
400Mb/s data transmission rate
n
Hot plugging
n
Power through the cable
n
Plug-and-play
n
Low cost cabling
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Low implementation cost
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As a matter of fact, this race for fast transmission and small compact video machines (cameras, mobiles,
videoequipment) has broughtmore and more integratedand sensitive electronicdevices at thesametime.
Asa result, on onehand, protection devices havebeen required tomakethe system morerobustregarding
ESD while, on the other hand, line adaptation have been implemented to prevent fastsignal distortion due
to high data rate transmission across non ideal elements.
ST1394-01SC6 is one of those devices that com-
Fig. 1: ST1394-01SC6 internal diagram.
bine both ESD protection and line termination at a
time in one chip especially designed for Fire-Wire
transmission.
Its ± 4kV ESD IEC61000-4-2 compliance and
small package (1.5x2.8mm) makes it very convenient for IEEE1394 link.
The following internal diagram gives an idea of its
characteristics.
We can notice on
n
ESD protection is performed with clamping
figure 1
the following points:
diodes located at each line side.
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RC elements realize line termination at each
data lines
n
Moreover, intrinsic clamping diode capacitors
offers RF rejection for GSM bandwidth.
October 2003 - Ed: 1
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AN1783 - APPLICATION NOTE
ST1394-01SC6 BOARD PLACEMENT RECOMMANDATIONS
Whatever the protection device at Fire Wire connector, it is known that upon ESD strike occurrence, a remaining over voltage is applied at sensitive IC side. In main cases a small part of this over voltage is coming from the clamping of the protection diode and a larger part from the copper track that connects
component altogether. For this reason it is important also to focus on the way component are routed.
Theway the ST1394-01SC6ismounted on theFire-Wire communication printedboardis also amajor concern. This will affect the ESD and line termination efficiency. Keep in mind when facing +4kV ESD surge
(IEC61000-4-2, level 2), a 10mm long; 0,5mn wide; 35µm thick copper track will induce about 70V!!
over-voltage due to track parasitic inductance.
Indeed, fast di/dt induced by the ESD IEC61000-4-2 level 2 brings the following calculation:
The ESD surge has a step rise time of 0.7ns minimum.
n
The level 2 of the ESD test table stands for 4kV air discharge and the internal series resistance of the
n
equivalent ESD generator is 330 ohms, thus the peak current induced by the 4kV surge is roughly 12A
A 1mm, 35µm thick, 0.5mm wide is 0.4nH
n
Then considering a 10mm; 35µm; 0.5mm copper track, the voltage drop along this track comes up with:
dI
=
; then it is
VV
=××
10 04 10
VL
dt
So shorter the tracks, better the ESD suppression.
.
−
9
07 10
.
121
.
×
−
9
=
70
!!
Next figures 2 and 3 give an idea of component placement on a printed circuit board:
Fig. 2: Bad layout. Fig. 3: Good layout.
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AN1783 - APPLICATION NOTE
ESD PROTECTION
The ST1394-01SC6 complies with IEC61000-4-2 standard, level 2, air discharge surge test (± 4kV).
The next figure shows the way the ST1394-01SC6 is connected to Fire Wire communication port:
Fig. 4: ST1394-01SC6 connection diagram.
Fire Wire
communication
bus
{
When the surge is applied at the Fire-Wire communication port, the ST1394-01SC6 eliminates the
over-voltage like shown further:
The way it works!
UponESD occurrence (
Considering
V
= Z1; ST1394-01SC6 internal diode breakdown voltage
BR
R
= dynamic resistance =1Ω for ST1394-01SC6
d
That means the remaining over-voltage at the ST1394-01SC6 input line is 12V above Z1 diode breaking
voltage.
Fig. 5: ESD occurence.
IA
112≈
cf.fig. 5
(≈4000V/330Ω per IEC61000-4-2 level 2)
),Z1 diode clampsthe surge andtheremaining voltage atits terminations is:
VV RIV V
1112=+×≡+
BR d BR
Vcc
I1
V1
Z1
IC to
protect
ESD surge
I1
V1
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