LTC2873H Ver la hoja de datos (PDF) - Analog Devices
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LTC2873H Datasheet PDF : 32 Pages
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LTC2873
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.
Figure 32 shows an example of how to connect four devices.
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 VDD and VEE are externally supplied. Bypass caps on
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
In certain applications, fast supply slew rates are generated
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
current flowing through inductor L1 and capacitors C1 and
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)
74479888310
10
CBC2016T100K (or M)
10
CBC2518T220K (or M)
22
BRC2016T220K (or M)
22
LQH32CN220K53
22
18
ISAT (mA)
250
380
320
310
250
MAX DCR (Ω)
0.5
1.07
1.0
1.3
0.92
SIZE (mm)
2.5 × 2 × 1
2 × 1.6 × 1.6
2.5 × 1.8 × 1.8
2 × 1.6 × 1.6
3.2 × 2.5 × 1.6
For more information www.analog.com
MANUFACTURER
Wurth Elektronik
Taiyo Yuden
www.t-yuden.com
Murata
www.murata.com
REV B
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