LTC1323
APPLICATIO S I FOR ATIO
Driving Differential AppleTalk or Single-Ended Loads
The differential driver is able to drive either an AppleTalk
load or a single-ended load such as a printer or modem.
With a differential AppleTalk load, TXD
+
and TXD
–
will
typically swing between 1.2V and 3.5V (Figure 14a). With
a single-ended 3k load such as a printer, either TXD
+
or
TXD
–
will meet the single-ended voltage swing require-
ment of
±3.7V
(Figure 14b). An automatic switching circuit
prevents the differential driver from overloading the charge
pump if the outputs are shorted to ground while driving
single-ended signals. This allows the second single-ended
driver to continue to operate normally when the first is
shorted, and allows external circuitry attached to the charge
pump output to continue to operate even if there are faults
at the driver outputs.
V
CC
= 5V
+
1µF
24
12
LTC1323
13
V
CC
EXTERNAL
CHIP
GND
V
EE
I
VEE
4.7µF
–5.5V
≤
V
EE
≤
–4.5V
I
VEE
≤
10mA
21
LTC1323 • F15
Figure 14
Thermal Shutdown Protection
The LTC1323 includes a thermal shutdown circuit which
protects against prolonged shorts at the driver outputs. If
a driver output is shorted to another output or to the power
supply, the current will be initially limited to a maximum of
500mA. When the die temperature rises above 150°C, the
thermal shutdown circuit disables the driver outputs.
When the die cools to about 130°C, the outputs are re-
enabled. If the short still exists, the part will heat again and
the cycle will repeat. This oscillation occurs at about 10Hz
and prevents the part from being damaged by excessive
power dissipation. When the short is removed, the part will
return to normal operation.
10
U
Power Shutdown
The power shutdown feature of the LTC1323 is designed
for battery-powered systems. When SHDN is forced
high the part enters shutdown mode. In shutdown the
supply current typically drops from 2.4mA to 0.5µA , the
charge pump turns off, and the driver and receiver
outputs are three-stated.
Receiver Keep-Alive Mode (24-Pin SO Wide Only)
The 24-pin SO Wide version of the LTC1323 also features
a power saving receiver keep-alive mode. When CPEN is
pulled high the charge pump is turned off and the outputs
of both drivers, the noninverting single-ended receiver and
the differential receiver are forced into three-state. The
inverting single-ended receiver (RXI) is kept alive with I
CC
dropping to 65µA and the receiver delay time increasing to
a maximum of 400ns. The receiver can then be used to
monitor a wake-up control signal.
Charge Pump Capacitors and Supply Bypassing
The LTC1323 requires two external 0.33µF capacitors for
the charge pump to operate: one from C1
+
to C1
–
and one
from C2
+
to C2
–
. These capacitors should be low ESR
types and should be mounted as close as possible to the
LTC1323. Monolithic ceramic capacitors work well in this
application. Do not use capacitors greater than 2µF at the
charge pump pins or internal peak currents can rise to
destructive levels. The LTC1323 also requires that both V
CC
and V
EE
be well bypassed to ensure proper charge pump
operation and prevent data errors. A 1µF capacitor from
V
CC
to ground is adequate. A 1µF capacitor is required from
V
EE
to ground and should be increased to 4.7µF if an
external load is connected to the V
EE
pin. Ceramic or
tantalum capacitors are adequate for power supply by-
passing; aluminum electrolytic capacitors should only be
used if their ESR is low enough for proper charge pump
operation. Inadequate bypass or charge pump capacitors
will cause the charge pump output to go out of regulation
prematurely, degrading the output swing at the SINGLE-
ENDED driver outputs.
C1
W
U U
+
LINER [ LINEAR TECHNOLOGY ]
