LTC2873H Просмотр технического описания (PDF) - Analog Devices
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LTC2873H Datasheet PDF : 32 Pages
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LTC2873
APPLICATIONS INFORMATION
5V
0V ≤10µs
R1
1Ω
1/8W
C4
2.2µF
L1
10µH
C1
220nF
INRUSH
CURRENT
SW
12
VCC
14
CAP
9
13
VDD
C2
1µF
11
2873 F17
GND
Figure 17. Supply Current Overshoot Protection for
Input Supplies of 4.5V or Higher and Rise Times Faster
Than 10μs
VL Logic Supply and Logic Pins
A separate logic supply pin VL allows the LTC2873 to
interface with any logic signal from 1.7V to 5.5V. All logic
I/Os use VL as their high supply. For proper operation, VL
should not be greater than VCC. During power-up, if VL
is higher than VCC, the device will not be damaged, but
behavior of the device is not guaranteed. In particular,
supply currents can be somewhat higher than specified.
If VL is not connected to VCC, bypass VL with a 0.1µF
capacitor to GND.
RS232 and RS485 driver outputs are undriven and the
RS485 termination resistors are disabled when VL or VCC
is grounded or VCC is disconnected.
Although all logic input pins reference VL as their high
supply, they can be driven up to 7V, independent of VL
and VCC, with the exception of SHDN, which must not
exceed VL by more than 0.3V. Logic input pins do not
have internal biasing devices to pull them up or down.
They must be driven high or low to establish valid logic
levels; do not float.
RS485 Driver
The RS485 driver provides full RS485/RS422 compatibility.
When enabled, if DI is high, (A/DO to B/RI) is positive.
With the driver disabled, the A/DO and B/RI resistance is
greater than 96kΩ (typically 125kΩ) to ground over the
entire common mode range of –7V to +12V. This resistance
is actually the RS485 receiver input resistance, which is
connected to the same pins.
RS232 Driver with Speed Selection
The RS232 driver provides full compatibility with the TIA/
EIA-232-F (RS232) specification. When in RS232 mode,
the driver is automatically enabled. Like all RS232 driv-
ers, it is inverting, so that when the input, DI, is low, the
output, A/DO, is high, and vice-versa.
The RS232 driver slew rate can be selected to support
data rates of up to 250kbps or 1Mbps with the DE485/
F232 pin. Since RS232 signals are single ended and large
amplitude, compared with RS485, radiated emissions may
be a concern. To minimize emissions, the speed selection
should be set to Slow mode by setting DE485/F232 low
for data rates of 250kbps or less. For higher data rates, up
to 1Mbps, Fast mode must be engaged by setting DE485/
F232 high. Even in Fast mode the driver transitions are slew
controlled to minimize emissions. See "Typical Performance
Characteristics" section for examples of the waveforms.
Driver Overvoltage and Overcurrent Protection
The RS232 and RS485 driver outputs are protected from
short circuits to any voltage within the absolute maximum
range of ±15V. The maximum current in this condition
is 90mA for the RS232 driver and 250mA for the RS485
driver. If the RS485 driver output is shorted to a voltage
greater than VCC, when it is active, positive current of about
100mA can flow from the driver output back to VCC. If the
system power supply or loading cannot sink this excess
current, clamp VCC to GND with a Zener diode (e.g., 5.6V,
1W, 1N4734) to prevent an overvoltage condition on VCC.
All devices also feature thermal shutdown protection that
disables the drivers, receivers, and RS485 terminators in
case of excessive power dissipation during momentary
overload conditions. Overtemperature protection activates
at a junction temperature exceeding about 165°C (not
tested in production). NOTE: Continuous operation above
the specified maximum operating junction temperature
may result in device degradation or failure.
For more information www.analog.com
REV B
19
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