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. Its objective is to emulate the stress caused by

contact that occurs with handling and assembly during

manufacturing. 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; polarized or nonpolarized capacitors

can be used. The charge pump requires 0.1µF capacitors

for 2.5V operation (Table 2). Do not use values smaller

than those listed in Table 2. Increasing the capacitor

values (e.g., by a factor of 2) reduces ripple on the

transmitter outputs and slightly reduces power con-

sumption. 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

excessively with temperature. If in doubt, use capacitors

with a larger nominal value. The capacitor’s equivalent

series resistance (ESR), which usually rises at low tem-

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

Power-Supply Decoupling

In most circumstances, a 0.1µF bypass capacitor is

adequate. In applications that are sensitive to power-

the same value as charge-pump capacitor C1. Connect

bypass capacitors as close to the IC as possible.

Transmitter Outputs when

Exiting Shutdown

Figure 2 shows two transmitter outputs when exiting

shutdown mode. As they become active, the two

transmitter outputs are shown going to opposite RS-232

levels (one transmitter input 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.

Note that the transmitters are enabled only when the

magnitude of V- exceeds approximately 2.5V.

2.5V, ±15kV ESD-Protected RS-232

Transceivers for PDAs and Cell Phones

8

_______________________________________________________________________________________

CHARGE-CURRENT

LIMIT RESISTOR

DISCHARGE

RESISTANCE

STORAGE

CAPACITOR

Cs

150pF

50M

Ω to 100MΩ

330

Ω

HIGH-

VOLTAGE

DC

SOURCE

DEVICE

UNDER

TEST

Figure 4a. IEC 1000-4-2 ESD Test Model

tR = 0.7ns to 1ns

30ns

60ns

t

100%

90%

10%

I

Figure 4b. IEC 1000-4-2 ESD Generator Current Waveform

Table 2. Minimum Required Capacitor

Values

(V)

C1–C4

(µF)

2.5 to 3.0

0.1