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

up to 250kbps, True RS-232 Transceiver

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5

_______________Detailed Description

Dual Charge-Pump Voltage Converter

The MAX3385E’s internal power supply consists of a

regulated dual charge pump that provides output volt-

ages of +5.5V (doubling charge pump) and -5.5V

range. The charge pump operates in discontinuous

mode; if the output voltages are less than 5.5V, the

charge pump is enabled, and if the output voltages

exceed 5.5V, the charge pump is disabled. Each

charge pump requires a flying capacitor (C1, C2) and a

reservoir capacitor (C3, C4) to generate the V+ and V-

supplies (Figure 1).

RS-232 Transmitters

The transmitters are inverting level translators that con-

vert CMOS-logic levels to ±5.0V EIA/TIA-232 levels.

The MAX3385E transmitters guarantee a 250kbps data

rate with worst-case loads of 3k

Ω in parallel with 1000pF,

providing compatibility with PC-to-PC communication

software (such as LapLink™). Transmitters can be paral-

leled to drive multiple receivers or mice.

The MAX3385E’s transmitters are disabled and the out-

puts are forced into a high-impedance state when the

device is in shutdown (SHDN = GND). The MAX3385E

permits the outputs to be driven up to ±12V in shut-

down.

The transmitter inputs do not have pull-up resistors.

RS-232 Receivers

The receivers convert RS-232 signals to CMOS-logic

output levels (Table 1).

Shutdown Mode

Supply current falls to less than 1µA in shutdown mode

(SHDN = low). When shut down, the device’s charge

pulled to ground, and the transmitter outputs are dis-

abled (high impedance). The time required to exit shut-

0

SHDN

1

High-Z

T_OUT

Active

Active

R_OUT

Active

MAX3385E

5k

R_ IN

R_ OUT

C2-

C2+

C1-

C1+

V-

V+

C4

C1

C2

0.1

µF

SHDN

T_ OUT

T_ IN

GND

7k

150pF

MAX3385E

5k

R_ IN

R_ OUT

C2-

C2+

C1-

C1+

V-

V+

C4

C3

3k

C3

3k

C1

C2

0.1

µF

SHDN

T_ OUT

T_ IN

GND

3k

2500pF

MINIMUM SLEW-RATE TEST CIRCUIT

MAXIMUM SLEW-RATE TEST CIRCUIT

Figure 1. Slew-Rate Test Circuits

Table 1. Shutdown Truth Table

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