LT1332CS datasheet(7/8 Pages) LINER

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LT1332

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tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

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 Mode

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 0.1

µFceramicdecouplingcapacitorsareneeded

on the positive supply inputs, low ESR tantalum capaci-

Figure 1. Multiple LT1332s Powered from a Single LT1172 DC/DC Converter

tors should be used in the charge pump to reduce voltage

losses. The C – capacitor should be at least 1

µF 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-

ON/OFF

GND

V–

LT1332

C–

V+

3V VCC GND

0.1

µF10µF

ON/OFF

GND

V–

LT1332

C–

V+

3V VCC GND

0.1

µF10µF

0.1

µF

3V VCC

INPUT

ON/OFF

µF

1N5817

L1*

100

µH

135k**

VSW

LT1172

VN2222LL

VIN

GND

25k**

µF

VIN

0.1

µF

OPTIONAL

SHUTDOWN

2.7V < VIN < 5.5V

I(VIN) < 14mA (UNLOADED)

L1 = SUMIDA CD105-101K OR COILCRAFT DO3316-104

1% METAL FILM

LT1332 • F01

0.1

µF10µF

ON/OFF

GND

V–

LT1332

C–

V+

3V VCC GND

0.1

µF10µF

ON/OFF

GND

V–

LT1332

C–

V+

3V VCC GND

0.1

µF

0.1

µF

0.1

µF

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