The major difference between tests done using the

Human Body Model and IEC1000-4-2 is higher peak

current in IEC1000-4-2, because series resistance is

lower in the IEC1000-4-2 model. Hence, the ESD with-

stand voltage measured to IEC1000-4-2 is generally

lower than that measured using the Human Body

Model. Figure 7a shows the IEEE1000-4-2 model and

Figure 7b shows the current waveform for the ±8kV

IEC1000-4-2 Level 4 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 resistance. Its

objective is to emulate the stress caused by contact that

occurs with handling and assembly during manufactur-

ing. Of course, all pins require this protection during

manufacturing, not just RS-232 inputs and outputs.

Therefore, after PC board assembly, the Machine Model

is less relevant to I/O ports.

___________Applications Information

Capacitor Selection

The capacitor type used for C1–C4 is not critical for

proper operation; either polarized or nonpolarized

capacitors may be used. The charge pump requires

0.1µF capacitors for 3.3V operation. For other supply

voltages, refer to Table 3 for required capacitor values.

Do not use values smaller than those listed in Table 3.

Increasing the capacitor values (e.g., by a factor of 2)

reduces ripple on the transmitter outputs and slightly

reduces power consumption. C2, C3, and C4 can be

increased without changing C1’s value. However, do

not increase C1 without also increasing the values of

C2, C3, and C4 to maintain the proper ratios (C1 to

the other capacitors).

When using the minimum required capacitor values,

make sure the capacitor value does not degrade exces-

sively with temperature. If in doubt, use capacitors with a

larger nominal value. The capacitor’s equivalent series

resistance (ESR) usually rises at low temperatures and

influences the amount of ripple on V+ and V-.

Power-Supply Decoupling

adequate. In applications that are sensitive to power-

supply noise, use a capacitor of the same value as the

charge-pump capacitor C1. Connect bypass capacitors

as close to the IC as possible.

Transmitter Outputs

when Exiting Shutdown

Figure 8 shows two transmitter outputs when exiting

shut down mode. As they become active, the two trans-

mitter outputs are shown going to opposite RS-232 lev-

els (one transmitter output is high, the other is low).

Each transmitter is loaded with 3k

Ω in parallel with

1000pF. The transmitter outputs display no ringing or

undesirable transients as they come out of shutdown,

and are enabled only when the magnitude of V-

exceeds approximately -3V.

±15kV ESD-Protected, 1µA, 3.0V to 5.5V, 250kbps,

RS-232 Transceivers with AutoShutdown

______________________________________________________________________________________

11

0.33

0.047

4.5 to 5.5

0.1

0.1

3.15 to 3.6

0.22

0.22

3.0 to 3.6

C2, C3, C4

(µF)

(µF)

(V)

Table 3. Required Capacitor Values

40

µs/div

T2OUT

FORCEON =

FORCEOFF

T1OUT

5V/div

2V/div

C1–C4 = 0.1

µF

Figure 8. Transmitter Outputs Exiting Shutdown or Powering Up

1

0.22

3.0 to 5.5

Table 4. Logic Family Compatibility with

Various Supply Voltages

Compatible with ACT and

HCT CMOS, and with TTL.

Incompatible with AC, HC,

or CD4000 CMOS.

3.3

5

SYSTEM

POWER-

SUPPLY

VOLTAGE

(V)

Compatible with all TTL and

CMOS-logic families.

5

5

Compatible with all CMOS

families.

3.3

3.3

COMPATIBILITY

VOLTAGE

(V)