LTC2803CDHC datasheet(10/18 Pages) LINER

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LTC2801/LTC2802/

LTC2803/LTC2804

2801234fe

BOOST

REGULATOR

VEE

VDD

1µF

CAP

220nF

10µH

VCC

1µF

220nF

*OMIT IF VL IS CONNECTED TO VCC

1.8V TO 5.5V

2801 F06

aPPlicationS inForMation

Overview

The LTC2801 family of RS-232 transceivers operates on

a VCC supply of 1.8V to 5.5V, utilizing a switching regula-

tor to generate the necessary higher voltage rails for the

drivers. The transceivers interface with logic operating

on any supply from 1.8V to 5.5V, independent of the VCC

voltage. Depending on the device, one or two control pins

are available to invoke Shutdown, Receiver Active and

Driver Disable features.

DC-DC Converter

The on-chip DC-DC converter operates from the VCC

input, generating a 7V VDD supply and a charge pumped

–6.3V VEE supply, as shown in Figure 6. VDD and VEE

power the output stage of the drivers and are regulated

to levels that guarantee greater than

±5V output swing.

The DC-DC converter requires a 10µH inductor (L1) and

a bypass capacitor (C4) of at least 1µF. The recommended

size for the charge pump capacitor (C1) is 220nF and for

the storage capacitors (C2 and C3) is 1µF. Larger storage

capacitors up to 4.7µF may be used if C1 is kept at 20% to

50% their size and C4 is also scaled. Locate C1-C4 close

to their associated pins.

Power-Saving Modes

When the DC-DC converter and drivers are turned off (PS

and MODE or ON/OFF = logic low), VCC supply current is

reduced to 1µA. Tables 1 and 2 summarize the modes for

each device.

In Shutdown mode, VL supply current is reduced to 1µA,

and both receiver and driver outputs assume a high im-

pedance state.

In Receivers Active mode, the quiescent VL supply current

is reduced to 15µA and the driver outputs assume a high

impedance state. The receivers operate at a reduced rate

(typically 100 kbps) with hysteresis turned off.

Half-Duplex Operation

When the DC-DC converter is kept on (PS = logic high),

MODEservesasalow-latencydriverenableforhalf-duplex

operation. Each driver is enabled and disabled in less than

2µs, while each receiver remains continuously active. This

mode of operation is illustrated in Figures 15-17.

Battery Operation

To maximize battery life, connect VCC (and L1) directly to

the unregulated battery voltage and VL to the regulated

supply, as shown in Figure 22. This configuration typically

minimizes conversion loss while providing compatibility

with system logic levels.

Inductor Selection

A 10µH inductor with a saturation current (ISAT) rating

of at least 200mA and low DCR (copper wire resistance)

is recommended. Some small inductors meeting these

requirements are listed in Table 3.

Figure 6. DC/DC Converter and Recommended Bypassing

VL Logic Supply

AseparatelogicsupplypinVLallowstheLTC2801familyto

interface with any logic signal from 1.8V to 5.5V, as shown

in Figure 7. Simply connect the desired logic supply to

VL. There is no interdependency between VCC and VL; they

may simultaneously operate at any voltage from 1.8V to

5.5Vandsequenceinanyorder.IfVLispoweredseparately

from VCC, bypass VL with a 220nF capacitor (C5).

Figure 7. VCC and VL Are Independent

ANY VOLTAGE FROM

1.8V TO 5.5V

ANY VOLTAGE FROM

1.8V TO 5.5V

VCC

EXTERNAL

DEVICE

RS-232

2801 F07

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