6
LT1237
PI FU CTIO S
U
UU
C1
+
, C1
–
, C2
+
, C2
–
: Commutating Capacitor Inputs, re-
quire two external capacitors C ≥ 0.2µF: one from C1
+
to
C1
–
, and another from C2
+
to C2
–
. The capacitor’s effec-
tive 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 compliance.
DRIVER IN: RS232 Driver Input Pins. These inputs are
TTL/CMOS compatible. Inputs should not be allowed to
float. Tie unused inputs to V
CC
.
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 conditions 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 overdrive 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 ±15kV for human body model discharges, ±8kV
for IEC 1000-4-2 contact mode discharges and ±15kV for
IEC 1000-4-2 air gap 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 ±15kV for
human body model discharges, ±8kV for IEC 1000-4-2
contact mode discharges and ±15kV for IEC 1000-4-2 air
gap discharges. Each receiver provides 0.4V of hysteresis
for noise immunity. Open receiver inputs assume a logic
low state.
RX OUT: Receiver Outputs with TTL/CMOS Voltage Lev-
els. Outputs RX1, RX2, RX3, and RX4 are in a high
impedance state when in shutdown mode to allow data
line sharing. Outputs, including LOW-Q RX OUT, are fully
short-circuit protected to ground or V
CC
with the power
on, off, or in shutdown mode.
LOW Q-CURRENT RX IN: Low Power Receiver Input. This
special receiver remains active when the part is in shut-
down mode, consuming typically 60µA. This receiver has
the same 5k input impedance and ±10kV ESD protection
characteristics as the other receivers.
LOW Q-CURRENT RX OUT: Low Power Receiver Output.
This pin produces the same TTL/CMOS output voltage
levels as receivers RX1, RX2, RX3, and RX4 with slightly
decreased speed and short-circuit current. Data rates to
120kbaud are supported by this receiver.
ESD PROTECTIO
U
The RS232 line inputs of the LT1237 have on-chip protec-
tion from ESD transients up to ±15kV for human body
discharges, ±8kV for IEC 1000-4-2 contact mode dis-
charges and ±15V for IEC 1000-4-2 air gap discharges.
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 LT1237 must be connected to ground through low
impedances. The power supply decoupling capacitors and
charge pump storage capacitors provide this low imped-
ance in normal application of the circuit. The only con-
straint is that low ESR capacitors must be used for
bypassing and charge storage. ESD testing must be done
with pins V
CC
, V
L
, V
+
, V
–
and GND shorted to ground or
connected with low ESR capacitors.
1237 TC01
5V V
CC
1µF
0.2µF
0.1µF
RS232
LINE PINS
PROTECTED
TO ±10kV
LT1237
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 (LOW-Q)
ON/OFF
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 (LOW-Q)
V
–
V
+
0.1µF
GND
DRIVER DISABLE
ESD Test Circuit
