Rainbow Electronics MAX3079E - Manual (Page 17)

current in IEC 1000-4-2, because series resistance is
lower in the IEC 1000-4-2 model. Hence, the ESD with-
stand voltage measured to IEC 1000-4-2 is generally
lower than that measured using the Human Body Model.
Figure 10c shows the IEC 1000-4-2 model, and Figure
10d shows the current waveform for IEC 1000-4-2 ESD
Contact Discharge test.
The air-gap test involves approaching the device with a
charged probe. The contact-discharge method connects
the probe to the device before the probe is energized.
Machine Model
The machine model for ESD tests all pins using a
200pF storage capacitor and zero discharge resis-
tance. The objective is to emulate the stress caused
when I/O pins are contacted by handling equipment
during test and assembly. Of course, all pins require
this protection, not just RS-485 inputs and outputs.
Applications Information
256 Transceivers on the Bus
The standard RS-485 receiver input impedance is 12k Ω
(1-unit load), and the standard driver can drive up to 32-
unit loads. The MAX3070E family of transceivers has a
1/8-unit load receiver input impedance (96k
Ω), allowing
up to 256 transceivers to be connected in parallel on one
communication line. Any combination of these devices
as well as other RS-485 transceivers with a total of 32-
unit loads or fewer can be connected to the line.
Reduced EMI and Reflections
The MAX3070E/MAX3071E/MAX3072E feature reduced
slew-rate drivers that minimize EMI and reduce reflec-
tions caused by improperly terminated cables, allowing
error-free data transmission up to 250kbps. The
MAX3073E/MAX3074E/MAX3075E offer higher driver
output slew-rate limits, allowing transmit speeds up to
500kbps. The MAX3079E with SRL = V
CC or uncon-
nected, are slew-rate limited. With SRL unconnected,
the MAX3079E error-free data transmission is up to
250kbps; with SRL connected to V
CC the data transmit
speeds up to 500kbps.
MAX3070E–MAX3079E
+3.3V, ±15kV ESD-Protected, Fail-Safe,
Hot-Swap, RS-485/RS-422 Transceivers
______________________________________________________________________________________ 17
CHARGE-CURRENT-
LIMIT RESISTOR
DISCHARGE
RESISTANCE
STORAGE
CAPACITOR Cs
100pF
R C
1M Ω
R D
1500 Ω
HIGH-
VOLTAGE
DC
SOURCE
DEVICE
UNDER
TEST
Figure 10a. Human Body ESD Test Model
I
P
100%
90%
36.8%
t
RL
TIME
t DL
CURRENT WAVEFORM
PEAK-TO-PEAK RINGING
(NOT DRAWN TO SCALE)
Ir
10%
0
0
AMPS
Figure 10b. Human Body Current Waveform
CHARGE-CURRENT-
LIMIT RESISTOR
DISCHARGE
RESISTANCE
STORAGE
CAPACITOR Cs
150pF
R C
50M Ω TO 100MΩ
R D
330 Ω
HIGH-
VOLTAGE
DC
SOURCE
DEVICE
UNDER
TEST
Figure 10c. IEC 1000-4-2 ESD Test Model
t r = 0.7ns TO 1ns
30ns
60ns
t
100%
90%
10%
I PEAK
I
Figure 10d. IEC 1000-4-2 ESD Generator Current Waveform