±15kV ESD-Protected +2.5V to +5.5V

RS-232 Transceivers in UCSP

6

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Detailed Description

Dual-Mode Regulated Charge-Pump

Voltage Converter

The MAX3228E/MAX3229E internal power supply con-

sists of a dual-mode regulated charge pump. For sup-

ply voltages above +3.7V, the charge pump will

generate +5.5V at V+ and -5.5V at V-. The charge

pumps operate in a discontinuous mode. If the output

voltages are less than ±5.5V, the charge pumps are

enabled, if the output voltages exceed ±5.5V, the

charge pumps are disabled.

For supply voltages below +2.85V, the charge pump

will generate +4.0V at V+ and -4.0V at V-. The charge

pumps operate in a discontinuous mode. If the output

voltages are less than ±4.0V, the charge pumps are

enabled, if the output voltages exceed ±4.0V, the

charge pumps are disabled.

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

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

and V- supply voltages.

Voltage Generation in the

Switchover Region

The MAX3228E/MAX3229E include a switchover circuit

between these two modes that have approximately

400mV of hysteresis around the switchover point. The

hysteresis is shown in Figure 1. This large hysteresis

eliminates mode changes due to power-supply

bounce.

For example, a three-cell NiMh battery system starts at

output voltage of ±5.5V. As the battery discharges, the

PIN

MAX3228E

MAX3229E

NAME

FUNCTION

C5

C5

FORCEOFF

FORCEOFF Input, Active-Low. Drive FORCEOFF low to shut down transmitters,

receivers, and on-board charge pump. This overrides all automatic circuitry and

FORCEON. Pulls itself high internally if not connected.

C6, D6

C6

R_OUT

Receiver Output(s)

D1

D1

C1-

Negative Terminal of Positive Regulated Charge-Pump Capacitor

E1

E1

GND

Ground

E2

E2

INVALID

Output of Valid Signal Detector.

INVALID is enabled low if no valid RS-232 level is

present on any receiver input.

E3, E4

E3

T_OUT

RS-232 Transmitter Output(s)

E5, E6

E5

R_IN

RS-232 Receiver Input(s)

Pin Description (continued)

SYSTEM SUPPLY (V)

RS-232 MODE

1 Li+ Cell

+2.4 to +4.2

Regulated System Voltage

Compliant/Compatible

3 NiCad/NiMh Cells

+2.4 to +3.8

Regulated System Voltage

Compliant/Compatible

Regulated Voltage Only

+3.0 to +5.5

+3.0 to +5.5

Compliant

Regulated Voltage Only

+2.5 to +3.0

+2.5 to +3.0

Compatible

Table 1. Operating Supply Options

0

6V

4V

0

20ms/div

V+