Datasheet

MAX3250
±50V Isolated, 3.0V to 5.5V,
250kbps, 2 TX/2 RX, RS-232 Transceiver
8 _______________________________________________________________________________________
To achieve full isolation capability, C1, C2, and C9
should be rated for 100V or higher operation and be X7R
or X5R type or metalized film dielectric. Y5V and Z5U
dielectrics should be avoided as their voltage and tem-
perature coefficients make their power-transfer capabili-
ties insufficient.
Charge-Pump and Bypass Capacitors
Capacitors C3–C8 should be X7R or X5R type dielec-
tric. Their voltage rating needs to be 10V or higher.
Layout Information
Because the MAX3250 is intended for systems requir-
ing ±50V isolation, some consideration in component
placement is necessary.
A 20mil air gap should isolate the logic side and the
isolated RS-232 side, across the N.C. pins (pin num-
bers 6, 7, 22, and 23) of the MAX3250. The only com-
ponents that cross this air gap should be C1, C2, and
C9, which should all have a minimum 100V rating.
All capacitors should be located as close to the
MAX3250 as possible.
Maximum Voltage Between
ISOCOM and Logic GND
High values of applied isolation voltage and frequency
can cause ripple on ISOVCC, V+, V-, and in extreme
cases on V
CC
. Therefore, it is recommended that the
isolation-voltage and frequency be limited to the values
shown in the Typical Operating Characteristics .
Insert a 1kΩ 1/4W resistor in series with any isolation
test voltage when testing for maximum values of
applied isolation voltage. Exceeding the maximum lim-
its of voltage and frequency (see the Typical Operating
Characteristics ) could trigger a holding current in the
internal ESD-protection device if the ±80V isolation limit
is exceeded. This resistor should not be used in normal
application.
Transmitter Outputs when
Exiting Shutdown
Figure 3 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 2500pF.
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.
V
CC
(V) C1, C2 (µF) C3 (µF) C4, C7, C8 (µF)
C5 (µF)
C6 (µF) C9 (nF)
3.0 to 3.6 0.47 0.1 0.47
1
2.2 10
4.5 to 5.5 0.047 0.1 0.47
1
2.2 10
Table 1. Required Capacitor Values