LT1332
6
work well in this application. When V– is powered from
an external supply, the filter capacitor can be considerably
smaller (C ≥ 0.1F). Ceramic capacitors work well under
these conditions. V– should be greater than –13.2V and
less than –6.5V.
C–: Commutating Capacitor Input. When the LT1332 is
used with a switching regulator, a charge pump capacitor
should be connected from the regulator’s switch pin to the
C– pin. Make the external capacitor 1F or larger with low
effective series resistance to maintain good charge pump
efficiency. Low ESR tantalum capacitors (ESR < 2Ω) work
well in this application. The C– pin should be left open
when V– is powered from an external supply.
DRIVER IN: RS232 Driver Input Pins. Inputs are TTL/
CMOS compatible, with threshold set to 1.2V. Unused
inputs should not float; tie them to VCC.
DRIVER OUT: Driver Outputs at RS232 Voltage Levels.
Outputs are in a high impedance state when in SHUT-
DOWN mode, or VCC = 0V. Outputs are fully short-circuit
protected from V– + 30V to V+ – 30V with the power on,
off or SHUTDOWN. Typical breakdowns are ±45V.
Applying higher voltages will not damage the device if
the overdrive is moderately current limited. Although
the outputs are protected, 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 ±10kV for human body
model discharges.
RX IN: Receiver Inputs. These pins accept RS232 level
signals (±5V to ±30V) into a protected 5k terminating
resistor. The receiver inputs are protected against ESD
to ±10kV for human body model discharges. Each re-
ceiver provides 0.4V of hysteresis for noise immunity.
The receiver thresholds are specified at VCC = 3V. When
VCC varies from 2V to 6V, the lower threshold increases
about 3V. Regardless of these shifts, the device provides
accurate data from valid RS232 input signals. A graph
in the performance characteristics section shows typi-
cal changes in the thresholds. The active receiver (RXA,
Pin 10, S Package) remains functional in SHUTDOWN.
RX OUT: Receiver Outputs with TTL/CMOS Voltage Levels.
OutputsareinahighimpedancestagewheninSHUTDOWN
mode to allow data line sharing. Outputs are fully short-
circuit protected to ground or VCC with the power on, off
or in SHUTDOWN mode. The active receiver (RXA, Pin 15,
S Package) remains functional in SHUTDOWN.
PIN FUNCTIONS
ESD PROTECTION
ESD Test Circuit
TheRS232lineinputsoftheLT1332haveon-chipprotection
fromESDtransientsupto±10kV.Theprotectionstructures
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 LT1332 must
be connected to ground through low impedances. The
power supply decoupling capacitors and charge pump
storage capacitors provide this low impedance in normal
applications 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 VCC, V+,
V– and GND shorted to ground or connected with low
ESR capacitors.
LT1332 TC
LT1332
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
–12V
0.1
F
0.1
F
3V
RS232
LINE PINS
PROTECTED
TO ±10kV
C–
DRIVER OUT
RX IN
DRIVER OUT
RX IN
DRIVER OUT
RXA IN
NC
V–
GND
DRIVER IN
RX OUT
DRIVER IN
RX OUT
DRIVER IN
RXA OUT
RS232 ON/OFF
V+
3V VCC
0.1
F
12V