MAX3224 Maxim, MAX3224 Datasheet - Page 12

• Manufacturer Part Number: MAX3224
• Description: 1A Supply Current / 1Mbps / 3.0V to 5.5V / RS-232 Transceivers with AutoShutdown Plus
• Manufacturer: Maxim

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, see
Table 3 for required capacitor values. Do not use val-
ues smaller than those listed in Table 3. Increasing the
capacitor values (e.g., by a factor of 2) reduces ripple
on the transmitter outputs and slightly reduces power
consumption. C2, C3, and C4 can be increased without
changing C1’s value. However, do not increase C1
without also increasing the values of C2, C3, C4,
and C
the other capacitors).
When using the minimum required capacitor values,
make sure the capacitor value does not degrade
excessively with temperature. If in doubt, use capaci-
tors with a larger nominal value. The capacitor’s equiv-
alent series resistance (ESR), which usually rises at low
temperatures, influences the amount of ripple on V+
and V-.
In most circumstances, a 0.1µF V
is adequate. In applications that are sensitive to power-
supply noise, use a capacitor of the same value as
charge-pump capacitor C1. Connect bypass capaci-
tors as close to the IC as possible.
Figure 6 shows two transmitter outputs when exiting
shutdown mode. As they become active, the two trans-
mitter outputs are shown going to opposite RS-232 lev-
els (one transmitter input is high; the other is low). Each
transmitter is loaded with 3kΩ in parallel with 1000pF.
The transmitter outputs display no ringing or undesir-
able transients as they come out of shutdown. Note that
the transmitters are enabled only when the magnitude
of V- exceeds approximately -3V.
1µA Supply Current, 1Mbps, 3.0V to 5.5V,
RS-232 Transceivers with AutoShutdown Plus
Table 3. Required Minimum Capacitance
Values
12
__________Applications Information
3.0 to 3.6
4.5 to 5.5
3.0 to 5.5
______________________________________________________________________________________
BYPASS
V
(V)
CC
, to maintain the proper ratios (C1 to
C1, C
Power-Supply Decoupling
0.047
(µF)
0.22
when Exiting Shutdown
0.1
BYPASS
Transmitter Outputs
Capacitor Selection
CC
bypass capacitor
C2, C3, C4
(µF)
0.33
0.1
1
The MAX3224/MAX3226/MAX3244 maintain the RS-232
±5.0V minimum transmitter output voltage even at high
data rates. Figure 7 shows a transmitter loopback test
circuit. Figure 8 shows a loopback test result at
120kbps, and Figure 9 shows the same test at
250kbps. For Figure 8, all transmitters were driven
Figure 6. Transmitter Outputs when Exiting Shutdown or
Powering Up
Figure 7. Loopback Test Circuit
*C3 CAN BE RETURNED TO V
5V/div
2V/div
5V/div
C1
C2
0
0
0
V
V
CC
CC
V
C1–C4 = 0.1 F
CC
C
BYPASS
= 3.3V
FORCEOFF
T_ IN
R_ OUT
FORCEON
C1+
C1-
C2+
C2-
CC
OR GND.
MAX3224
MAX3225
MAX3226
MAX3227
MAX3244
MAX3245
5 s/div
GND
V
CC
T_ OUT
5k
High Data Rates
R_ IN
V+
V-
C3*
C4
FORCEON =
FORCEOFF
T1OUT
T2OUT
READY
1000pF