M
+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 MAX3228/MAX3229 internal power supply consists
of a dual-mode regulated charge pump. For supply
voltages above +3.7V, the charge pump will generate
+5.5V at V+ and -5.5V at V-. The charge pumps oper-
ate 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 MAX3228/MAX3229 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.
PIN
MAX3228
MAX3229
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
D1
E1
C6
D1
E1
R_OUT
C1-
GND
Receiver Output(s)
Negative Terminal of Positive Regulated Charge-Pump Capacitor.
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
E5, E6
E3
E5
T_OUT
R_IN
RS-232 Transmitter Output(s)
RS-232 Receiver Input(s)
Pin Description (continued)
SYSTEM SUPPLY (V)
1 Li+ Cell
3 NiCad/NiMH Cells
Regulated Voltage Only
(V
CC
falling)
Regulated Voltage Only
(V
CC
falling)
V
CC
(V)
+2.4 to +4.2
+2.4 to +3.8
V
L
(V)
RS-232 MODE
Compliant/Compatible
Compliant/Compatible
Regulated System Voltage
Regulated System Voltage
+3.0 to +5.5
+3.0 to +5.5
Compliant
+2.5 to +3.0
+2.5 to +3.0
Compatible
Table 1. Operating Supply Options
Figure 1. V+ Switchover for Changing V
CC
0
20ms/div
6V
4V
V+
V
CC
0