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LTC2873

REV B

For more information www.analog.com

APPLICATIONS INFORMATION

Powering Multiple Devices

Multiple LTC2873 devices can be powered using the boost

regulator from only one of the devices, requiring only

one inductor (L1) and charge pump cap (C1). Since the

RS232 drivers provide the primary load to the circuit, the

following guidelines apply:

1. No more than four RS232 drivers can be supplied from

a single device.

2. If more than two RS232 drivers are being supplied

from a single device, then the inductor, L1, must be

increased to 22µH and the charge pump cap, C1, must

be increased to 470nH, and VDD and VEE bypass caps

must be increased to 2.2µF.

3. Ground the SW pin on devices with inactive boost

converters.

4. Connect CAP pins together for all devices.

5. Connect VEE pins together for all devices.

6. Connect VDD pins together for all devices.

Figure32showsanexampleofhowtoconnectfourdevices.

Inductor Selection

A 10µH or 22μH (±20%) inductor with a saturation current

(ISAT) rating of at least 220mA and a DCR (copper wire

resistance) of less than 1.3Ω is required. Some very small

inductors meeting these requirements are listed in Table 1.

Capacitor Selection

The small size of ceramic capacitors makes them ideal

for the LTC2873. Use X5R or X7R dielectric types; their

ESR is low and they retain their capacitance over relatively

wide voltage and temperature ranges. Use a voltage rating

of at least 10V.

Running with External VDD and VEE Supplies

The inductor and charge pump cap, C1, can be omitted

only if VDDandVEEareexternallysupplied.Bypasscapson

VDD and VEE must remain in place. In this circumstance,

ground the SW pin and float the CAP pin. External supplies

must not exceed the absolute maximum levels of ±7.5V.

Ideal supply levels are 7.2V and –6.5V as these are each

just wider than the regulation points of 7.0V and –6.3V so

the internal feedback is satisfied and the switching stops.

Lower voltages can be used even at –6V and +6V but the

internal boost regulator will be switching. This may cause

some switching noise but will not harm the part. VDD

and VEE supplies must be present for proper operation in

RS232 mode and in RS485 mode when the termination

is enabled. It is okay to run the LTC2873 in RS485 mode

with internal termination disabled (TE485 high), when VDD

and VEE are not present or fully settled.

Inrush Current and Supply Overshoot Precaution

Incertainapplications,fastsupplyslewratesaregenerated

when power is connected. If the VCC voltage is greater

than 4.5V and its rise time is faster than 10µs, the pins

VDD and SW can exceed their absolute maximum values

during start-up. When supply voltage is applied to VCC,the

voltage difference between VCC and VDD generates inrush

currentflowingthroughinductorL1andcapacitorsC1and

C2. The peak inrush current must not exceed 2A. To avoid

this condition, add a 1Ω resistor as shown in Figure 17.

This precaution is not relevant for supply voltages below

4.5V or rise times longer than 10µs.

Table 1. Recommended Inductors

PART NUMBER

L (μH)

ISAT (mA)

MAX DCR (Ω)

SIZE (mm)

MANUFACTURER

74479888310

250

0.5

2.5 × 2 × 1

Wurth Elektronik

CBC2016T100K (or M)

380

1.07

2 × 1.6 × 1.6

Taiyo Yuden

www.t-yuden.com

CBC2518T220K (or M)

320

1.0

2.5 × 1.8 × 1.8

BRC2016T220K (or M)

310

1.3

2 × 1.6 × 1.6

LQH32CN220K53

250

0.92

3.2 × 2.5 × 1.6

Murata

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