1 µA S u p p ly Cu rre n t , 1 Mb p s , 3 .0 V t o 5 .5 V,
RS -2 3 2 Tra n s c e ive rs w it h Au t o S h u t d o w n P lu s
Hig h Da t a Ra t e s
__________Ap p lic a t io n s In fo rm a t io n
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
c irc uit. Fig ure 8 s hows a loop b a c k te s t re s ult a t
120kb p s , a nd Fig ure 9 s hows the s a me te s t a t
250kbps. For Figure 8, all transmitters were driven
Ca p a c it o r S e le c t io n
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,
FORCEON =
FORCEOFF
5V/div
0
T1OUT
and C
the other capacitors).
, to maintain the proper ratios (C1 to
BYPASS
2V/div
0
When using the minimum required capacitor values,
ma ke s ure the c a p a c itor va lue d oe s not d e g ra d e
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-.
T2OUT
V
= 3.3V
CC
5V/div
0
C1–C4 = 0.1µF
READY
5µs/div
P o w e r-S u p p ly De c o u p lin g
Figure 6. Transmitter Outputs when Exiting Shutdown or
Powering Up
In most circumstances, a 0.1µF V
bypass capacitor
CC
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.
V
CC
C
BYPASS
Tra n s m it t e r Ou t p u t s
w h e n Ex it in g S h u t d o w n
V
CC
C1+
V+
V-
C3*
C4
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.
C1
C1-
MAX3224
MAX3225
MAX3226
MAX3227
MAX3244
MAX3245
C2+
C2
C2-
–7/MAX3245
T_ OUT
T_ IN
R_ IN
5k
R_ OUT
Table 3. Required Minimum Capacitance
Values
1000pF
FORCEON
FORCEOFF
V
(V)
C1, C
C2, C3, C4
(µF)
BYPASS
CC
(µF)
V
CC
GND
3.0 to 3.6
4.5 to 5.5
3.0 to 5.5
0.1
0.1
0.33
1
0.047
0.22
*C3 CAN BE RETURNED TO V OR GND.
CC
Figure 7. Loopback Test Circuit
12 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
