±15kV ESD-Protected, +5V, 1µA, Single RS-232

Transceiver with AutoShutdown

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5

_______________Detailed Description

Dual Charge-Pump Voltage Converter

The MAX221E’s internal power supply consists of a

dual charge pump that provides a positive output volt-

age (doubling charge pump) and negative output volt-

age (inverting charge pump) from a single +5V supply.

The charge pumps operate in continuous mode. Each

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

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

supplies.

RS-232 Transmitter

The transmitter is an inverting level translator that con-

verts CMOS-logic levels to 5.0V EIA/TIA-232 levels. It

guarantees a 250kbps data rate with worst-case loads

of 3k

Ω in parallel with 1000pF.

When FORCEOFF is driven to ground, or when the

AutoShutdown circuitry senses invalid voltage levels on

the receiver input, the transmitter is disabled and the

output is forced into a high-impedance state. The trans-

mitter input does not have a pull-up resistor.

RS-232 Receiver

The MAX221E’s receiver converts RS-232 signals to

CMOS-logic output levels. The receiver has an inverting

three-state output and can be active or inactive. In

shutdown (FORCEOFF = low) or in AutoShutdown, the

receiver is active (Table 1). Drive EN high to place the

receiver in a high-impedance state. The receiver is

high-impedance when the MAX221E is in shutdown

(FORCEOFF = low).

The MAX221E’s INVALID output is pulled low when

there is no valid RS-232 signal level detected on the

receiver input. INVALID is functional in any mode

(Figures 2 and 3).

AutoShutdown

The MAX221E achieves 1µA supply current with

Maxim’s AutoShutdown feature, which operates when

FORCEON is low and FORCEOFF is high. When the

device senses no valid signal levels on the receiver

input for 30µs, the on-board charge pump and driver

are shut off, reducing supply current to 1µA. This

occurs if the RS-232 cable is disconnected or the con-

nected peripheral transmitter is turned off. The

MAX221E turns on again when a valid level is applied

to the RS-232 receiver input. As a result, the system

saves power without changes to the existing BIOS or

operating system.

Table 1 and Figure 2c summarize the MAX221E operat-

ing modes. FORCEON and FORCEOFF override

AutoShutdown. When neither control is asserted, the

device selects between these states automatically,

based on the receiver input level. Figures 2a, 2b, and

3a depict valid and invalid RS-232 receiver levels.

Figure 3 shows the input levels and timing diagram for

AutoShutdown operation.

A device or another system with AutoShutdown con-

nected to the MAX221E may need time to wake up.

Figure 4 shows a circuit that forces the transmitter on

for 100ms, allowing enough time for the other system to

realize that the MAX221E is awake. If the other system

transmits valid RS-232 signals within that time, the

RS-232 ports on both systems remain enabled.

When shut down, the device’s charge pumps turn off,

transmitter output is high impedance. The time required

to exit shutdown is typically 100µs (Figure 3b).

MAX221E

FORCEON

EN

12

1

ROUT

9

FORCEOFF

16

INVALID

10

GND

14

TIN

11

C2-

6

C2+

5

C1-

4

C1+

2

RIN 8

TOUT 13

V-

7

V+

3

C4

0.1

µF

15

C1

0.1

µF

C2

0.1

µF

0.1

µF

+5V

TO POWER-

MANAGEMENT

UNIT

5k

3k

C3

0.1

µF

Figure 1. Shutdown Current Test Circuit