Datasheet

LTC2870/LTC2871
21
28701fb
For more information www.linear.com/LTC2870
Multiple LTC2870 or LTC2871 devices can be powered
using the boost regulator from only one of the devices,
requiring only one inductor and flying cap. 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 flying cap, C1, must be
increased to 470nH, and V
DD
and V
EE
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 V
EE
pins together for all devices.
6. Connect V
DD
pins together for all devices.
Figures 51 and 52 show examples of connecting two
devices and four devices.
Inductor Selection
A 10μH inductor with a saturation current (I
SAT
) 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 12.
Table 12. Recommended Inductors
PART NUMBER
I
SAT
(mA)
MAX
DCR (Ω) SIZE(mm) MANUFACTURER
LBC2016T100K
CBC2016T100M
245
380
1.07
1.07
2 ×
1.6 × 1.6
2 × 1.6 × 1.6
Taiyo Yuden
www.t-yuden.com
FSLB2520-100K 220 1.1 2.5 × 2 × 1.6 Toko
www.tokoam.com
Capacitor Selection
The small size of ceramic capacitors makes them ideal
for the LTC2870 and LTC2871. 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 V
DD
and V
EE
Supplies
The inductor and flying cap, C1, can be omitted only if
V
DD
and V
EE
are externally supplied. Bypass caps on
V
DD
and V
EE
must remain in place. In this circumstance,
ground the SW pin and float the CAP pin. External sup-
plies must not exceed the ABSMAX levels of ±7.5V. Ideal
supply levels are
7.3V
and –6.5V as these are each just
wider than the regulation points of 7.2V 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. V
DD
and
V
EE
supplies must be present for proper operation in all
modes except shutdown, including RS485-only mode.
Inrush Current and Supply Overshoot Precaution
In certain applications fast supply slew rates are gener
-
ated when power is connected. If the V
CC
voltage is greater
than 4.5V and its rise time is faster than 10μs, the pins
V
DD
and SW can exceed their absolute maximum values
during start-up. When supply voltage is applied to V
CC
,
the voltage difference between V
CC
and V
DD
generates
inrush current flowing through inductor L1 and capaci-
tors C1 and
C2. The peak inrush current must not exceed
2A. To avoid this condition, add a 1Ω resistor as shown
in Figure15. This precaution is not relevant for supply
voltages below 4.5V or rise times longer than 10μs.
applicaTions inForMaTion
Figure15. Supply Current Overshoot Protection
for Input Supplies of 4.5V of Higher
28701 F14
0V
5V
≤10µs
C1
220nF
L1
10µH
INRUSH
CURRENT
C4
2.2µF
R1
1Ω
1/8W
V
CC
V
DD
GND
SW CAP
C2
1µF
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