8
FN6001.4
January 16, 2005
15kV ESD Protection
All pins on the 3V interface devices include ESD protection
structures, but the ISL83385E incorporates advanced
structures which allow the RS-232 pins (transmitter outputs
and receiver inputs) to survive ESD events up to 15kV. The
RS-232 pins are particularly vulnerable to ESD damage
because they typically connect to an exposed port on the
exterior of the finished product. Simply touching the port
pins, or connecting a cable, can cause an ESD event that
might destroy unprotected ICs. These new ESD structures
protect the device whether or not it is powered up, protect
without allowing any latchup mechanism to activate, and
don’t interfere with RS-232 signals as large as 25V.
Human Body Model (HBM) Testing
As the name implies, this test method emulates the ESD
event delivered to an IC during human handling. The tester
delivers the charge through a 1.5k current limiting resistor,
making the test less severe than the IEC61000 test which
utilizes a 330 limiting resistor. The HBM method
determines an ICs ability to withstand the ESD transients
typically present during handling and manufacturing. Due to
the random nature of these events, each pin is tested with
respect to all other pins. The RS-232 pins on “E” family
devices can withstand HBM ESD events to 15kV.
IEC61000-4-2 Testing
The IEC61000 test method applies to finished equipment,
rather than to an individual IC. Therefore, the pins most likely
to suffer an ESD event are those that are exposed to the
outside world (the RS-232 pins in this case), and the IC is
tested in its typical application configuration (power applied)
rather than testing each pin-to-pin combination. The lower
current limiting resistor coupled with the larger charge
storage capacitor yields a test that is much more severe than
the HBM test. The extra ESD protection built into this
device’s RS-232 pins allows the design of equipment
meeting level 4 criteria without the need for additional board
level protection on the RS-232 port.
AIR-GAP DISCHARGE TEST METHOD
For this test method, a charged probe tip moves toward the
IC pin until the voltage arcs to it. The current waveform
delivered to the IC pin depends on approach speed,
humidity, temperature, etc., so it is difficult to obtain
repeatable results. The “E” device RS-232 pins withstand
15kV air-gap discharges.
CONTACT DISCHARGE TEST METHOD
During the contact discharge test, the probe contacts the
tested pin before the probe tip is energized, thereby
eliminating the variables associated with the air-gap
discharge. The result is a more repeatable and predictable
test, but equipment limits prevent testing devices at voltages
higher than 8kV. All “E” family devices survive 8kV contact
discharges on the RS-232 pins.
TABLE 4. LOGIC FAMILY COMPATIBILITY WITH VARIOUS
SUPPLY VOLTAGES
SYSTEM
POWER-SUPPLY
VOLTAGE
(V)
V
CC
SUPPLY
VOLTAGE
(V) COMPATIBILITY
3.3 3.3 Compatible with all CMOS families.
5 5 Compatible with all TTL and CMOS
logic families.
5 3.3 Compatible with ACT and HCT
CMOS, and with TTL.
ISL83385E
outputs are incompatible with AC,
HC, and CD4000 CMOS inputs.
Typical Performance Curves V
CC
= 3.3V, T
A
= 25°C
FIGURE 7. TRANSMITTER OUTPUT VOLTAGE vs LOAD
CAPACITANCE
FIGURE 8. SLEW RATE vs LOAD CAPACITANCE
-6.0
-4.0
-2.0
0
2.0
4.0
6.0
1000 2000 3000 4000 50000
LOAD CAPACITANCE (pF)
TRANSMITTER OUTPUT VOLTAGE (V)
1 TRANSMITTER AT 250kbps
V
OUT
+
V
OUT
-
1 TRANSMITTER AT 30kbps
LOAD CAPACITANCE (pF)
SLEW RATE (V/s)
0 1000 2000 3000 4000 5000
5
10
15
20
25
+SLEW
-SLEW
ISL83385E
