6
LT1342
PI FU CTIO S
U
UU
V
–
: Negative Supply Output (RS232 Drivers). V
–
≈
–(2V
CC
– 2.5V). This pin requires an external charge
storage capacitor C ≥ 0.1µF. To reduce supply ripple,
increase the size of the storage capacitor.
C1
+
, C1
–
, C2
+
, C2
–
: Commutating Capacitor Inputs. These
pins require two external capacitors C ≥ 0.2µF: one from
C1
+
to C1
–
, and another from C2
+
to C2
–
. The capacitor’s
effective series resistance should be less than 2Ω. For
C ≥ 1µF, low ESR tantalum capacitors work well in this
application, although small value ceramic capacitors may
be used with a minimal reduction in charge pump compli-
ance. For operation with an external 12V supply, omit C1
and connect the 12V supply to pin C1
+
. Pin V
+
may also be
shorted to C1
+
when a separate supply is used. The 12V
supply must be bypassed with a 0.1F capacitor.
DRIVER IN: RS232 Driver Input Pins. These inputs are
compatible with TTL or CMOS logic. Tie unused inputs to
V
CC
or V
L
.
DRIVER OUT:
Driver Outputs at RS232 Voltage Levels.
Driver output swing meets RS232 levels for loads up to
3k. Slew rates are controlled for lightly loaded lines.
Output current capability is sufficient for load condi-
tions up to 2500pF. Outputs are in a high impedance
state when in shutdown mode, V
CC
= 0V, or when the
DRIVER DISABLE pin is active. Outputs are fully short-
circuit protected from V
–
+ 30V to V
+
– 30V. Applying
higher voltages will not damage the device if the over-
drive is moderately current limited. Short circuits on
one output can load the power supply generator and
may disrupt the signal levels of the other outputs. The
driver outputs are protected against ESD to ±10V for
human body model discharges.
RX IN: Receiver Inputs. These pins accept RS232 level
signals (±30V) into a protected 5k terminating resistor.
The receiver inputs are protected against ESD to ±10kV for
human body model discharges. Each receiver provides
0.4V of hysteresis for noise immunity. Open receiver
inputs assume a logic low state.
RX OUT: Receiver Outputs with 3.3V Logic Compatible
Voltage Levels. Outputs are in a high impedance state
when in
shutdown
mode to allow data line sharing.
Outputs are fully short-circuit protected to ground or V
CC
or V
L
with the power on, off, or in
shutdown
mode.
Receiver output level is determined by V
L
supply voltage.
Use V
L
= 3.3V for interfacing with 3.3V logic, V
L
= 5V for
interfacing with 5V logic.
ESD PROTECTIO
U
The RS232 line inputs of the LT1342 have on-chip protec-
tion from ESD transients up to ±10kV. The protection
structures act to divert the static discharge safely to
system ground. In order for the ESD protection to function
effectively, the power supply and ground pins of the
LT1342 must be connected to ground through low imped-
ances. The power supply decoupling capacitors and charge
pump storage capacitors provide this low impedance in
normal application of the circuit. The only constraint is that
low ESR capacitors must be used for bypassing and
charge storage. ESD testing must be done with pins V
CC
,
V
+
, V
–
and GND shorted to ground or connected with low
ESR capacitors.
ESD Test Circuit
LT1342 • ESD TC
5V V
CC
0.1µF
0.2µF
0.1µF
RS232
LINE PINS
PROTECTED
TO ±10kV
LT1342
1
2
3
4
5
6
7
8
9
10
11
12
13
14
DRIVER 1 OUT
RX1 IN
DRIVER 2 OUT
RX2 IN
RX3 IN
RX4 IN
DRIVER 3 OUT
RX5 IN
3V V
L
28
27
26
25
24
23
22
21
20
19
18
17
16
15
0.2µF
DRIVER 1 IN
RX1 OUT
DRIVER 2 IN
RX2 OUT
RX3 OUT
RX4 OUT
DRIVER 3 IN
RX5 OUT
V
–
V
+
0.1µF
GND
DRIVER DISABLE
0.1µF
NC
ON/OFF
