LT1332 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT1332
Linear Technology
'LT1332' PDF : 8 Pages View PDF
UU W U
APPLICATIO S I FOR ATIO
Operation with a Switching Regulator
The LT1332 is designed to be powered from an external
switching regulator which may be used elsewhere for
power conditioning. In a typical application, the LT1332
shares the regulator’s positive output, while charge is
capacitively pumped from the regulator’s switch pin to the
negative supply. Schottky rectifiers built into the LT1332
simplify the charge pump design. When used with a
micropower switcher like the LT1109A, the Burst ModeTM
operation of the charge pump resembles the switching
characteristics of the LT1237 and similar devices.
The V – supply is not directly regulated. The circuit relies
on cross regulation and the regulator’s minimum duty
cycle to control V –. Select the C – and V – storage capaci-
tors so that when the regulator operates at minimum duty
cycle, sufficient charge will transfer to the V – storage cap
to maintain a voltage of at least – 6.5V.
While only 0.1µF ceramic decoupling capacitors are needed
on the positive supply inputs, low ESR tantalum capaci-
LT1332
tors should be used in the charge pump to reduce voltage
losses. The C – capacitor should be at least 1µF and the V –
capacitor should be 5 to 10 times bigger. As a rule of
thumb, make the V – capacitor at least 1/DCMIN times
bigger than the C – capacitor where DCMIN is the regulator’s
minimum duty cycle. Using large values for the V – capaci-
tor reduces ripple on the V – supply.
Multiple Transceivers
The circuit in Figure 1 demonstrates how the LT1332 may
be used with different types of switching regulators. Four
LT1332s are powered from a single PWM DC/DC con-
verter using an LT1172. Even with all twelve drivers
heavily loaded (RL = 3k, CL = 2500pF), the circuit generates
fully compliant RS232 signals at 120k baud.
Operations with External Supplies
When external RS232 supplies are available (6.5V ≤ V +
≤ 13.2V, – 13.2V ≤ V – ≤ – 6.2V) the LT1332 can be used
as a stand-alone unit. Capacitor selection is consider-
1µF
0.1µF
V+
V–
LT1332
C–
GND
1µF
10µF 0.1µF
V+
V–
LT1332
C–
GND
1µF
10µF 0.1µF
V+
V–
LT1332
C–
GND
1µF
10µF 0.1µF
3V VCC
INPUT
ON/OFF
0.1µF
ON/OFF
3V VCC GND
2.7V < VIN < 5.5V
I(VIN) < 14mA (UNLOADED)
VIN
68µF
VIN
L1*
100µH
VSW
LT1172
FB
GND
VC
0.1µF
ON/OFF
3V VCC GND
0.1µF
ON/OFF
3V VCC GND
0.1µF
1N5817 8V
+
22µF
135k**
25k**
*L1 = SUMIDA CD105-101K OR COILCRAFT DO3316-104
**1% METAL FILM
V+
V–
LT1332
C–
GND
ON/OFF
3V VCC GND
10µF
OPTIONAL
SHUTDOWN
VN2222LL
1k
0.1µF
LT1332 • F01
Figure 1. Multiple LT1332s Powered from a Single LT1172 DC/DC Converter
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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