down is typically 100µs, as shown in Figure 2. Connect

±15kV ESD Protection

As with all Maxim devices, ESD-protection structures

are incorporated on all pins to protect against electro-

static discharges encountered during handling and

assembly. The driver outputs and receiver inputs of the

MAX3385E have extra protection against static electric-

ity. Maxim’s engineers have developed state-of-the-art

structures to protect these pins against ESD of ±15kV

without damage. The ESD structures withstand high

ESD in all states: normal operation, shutdown, and

powered down. After an ESD event, Maxim’s “E” ver-

sions keep working without latchup, whereas compet-

ing RS-232 products can latch and must be powered

down to remove latchup.

ESD protection can be tested in various ways; the

transmitter outputs and receiver inputs of this product

family are characterized for protection to the following

limits:

1) ±15kV using the Human Body Model

2) ±8kV using the contact-discharge method specified

in IEC 1000-4-2

3) ±15kV using IEC 1000-4-2’s air-gap method.

ESD Test Conditions

ESD performance depends on a variety of conditions.

Contact Maxim for a reliability report that documents

test setup, test methodology, and test results.

Human Body Model

Figure 3a shows the Human Body Model, and Figure

3b shows the current waveform it generates when dis-

charged into a low impedance. This model consists of a

100pF capacitor charged to the ESD voltage of interest,

which is then discharged into the test device through a

1.5k

Ω resistor.

IEC 1000-4-2

The IEC 1000-4-2 standard covers ESD testing and per-

formance of finished equipment; it does not specifically

refer to integrated circuits. The MAX3385E helps you

design equipment that meets Level 4 (the highest level) of

IEC 1000-4-2, without the need for additional ESD-pro-

tection components.

The major difference between tests done using the

Human Body Model and IEC 1000-4-2 is higher peak

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 4a shows the IEC 1000-4-2 model, and

Figure 4b shows the current waveform for the 8kV IEC

1000-4-2 Level 4 ESD contact-discharge test.

The air-gap test involves approaching the device with a

charged probe. The contact-discharge method con-

nects the probe to the device before the probe is ener-

gized.

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. Of course, all pins require this protec-

tion 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 capaci-

tors for 3.3V operation. For other supply voltages, refer

to Table 2 for required capacitor values. Do not use val-

±15kV ESD-Protected, 3.0V to 5.5V, Low-Power,

up to 250kbps, True RS-232 Transceiver

40

µs/div

SHDN

T2OUT

T1OUT

5V/div

0

2V/div

0

C1–C4 = 0.1

µF

Figure 2. Transmitter Outputs Exiting Shutdown or

Powering Up

0.1

0.047

(µF)

0.1

0.1

0.33

C2, C3, C4

(µF)

0.47

3.0 to 3.6

4.5 to 5.5

(V)

3.0 to 5.5

Table 2. Required Minimum Capacitance

Values

6

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