LT1330
7
1330fb
ESD PROTECTION
The RS232 line inputs of the LT1330 have on-chip protection
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 LT1330 must
be connected to ground through low impedances. 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
L
, V
+
,
V
–
, and GND shorted to ground or connected with low
ESR capacitors.
1330 TC01
5V V
CC
1μF
0.2μF
0.1μF
RS232
LINE PINS
PROTECTED
TO ±10kV
LT1330
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
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 (LOW-Q)
V
–
V
+
0.1μF
GND
DRIVER DISABLE
0.1μF
ESD Test Circuit
APPLICATIONS INFORMATION
Storage Capacitor Selection
The V
+
and V
–
storage capacitors must be chosen care-
fully to insure low ripple and stable operation. The LT1330
charge pump operates in a power effi cient Burst Mode
®
operation. When storage capacitor voltage drops below
a preset threshold, the oscillator is gated on until V
+
and
V
–
are boosted up to levels exceeding a second threshold.
The oscillator then turns off, and current is supplied from
the V
+
and V
–
storage capacitors.
The V
–
potential is monitored to control charge pump
operation. It is therefore important to insure lower V
+
ripple
than V
–
ripple, or erratic operation of the charge pump will
result. Proper operation is insured in most applications
by choosing the V
+
fi lter capacitor to be at least 5 times
the V
–
fi lter capacitor value. If V
+
is more heavily loaded
than V
–
, a larger ratio may be needed.
The V
–
fi lter capacitor should be selected to obtain low
ripple when the drivers are loaded, forcing the charge
pump into continuous mode. A minimum value 0.1μF is
suggested.
Do not attempt to reduce V
–
ripple when the charge pump
is in discontinuous Burst Mode operation. The ripple in this
mode is determined by internal comparator thresholds.
Larger storage capacitor values increase the burst period,
and do not reduce ripple amplitude.
Power Saving Operational Modes
The LT1330 has both shutdown and driver disable operating
modes. These operating modes can optimize power
consumption based upon applications needs.
The On/Off shutdown control turns off all circuitry except
for Low-Q RX5. When RX5 detects a signal, this information
can be used to wake up the system for full operation.
If more than one line must be monitored, the driver
disable mode provides a power effi cient operating option.
The driver disable mode turns off the charge pump and
RS232 drivers, but keeps all fi ve receivers active. Power
consumption in driver disable mode is 3mA from V
CC
.
Burst Mode is a registered trademark of Linear Technology Corporation.
