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参数资料
| 型号： | DC1786A |
| 厂商： | Linear Technology |
| 文件页数： | 13/36页 |
| 文件大小： | 0K |
| 描述： | BOARD DEMO LTC2871 |
| 设计资源： | DC1786A Design Files DC1786A Schematic |
| 标准包装： | 1 |
| 主要目的： | 接口，收发器，RS-232/RS-485 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | LTC2871 |
| 主要属性： | 3 个驱动器，3 个接收器 |
| 次要属性： | 3 V ~ 5.5 V 电源 |
| 已供物品： | 板 |
| 相关产品： | LTC2871CUHF#PBF-ND - IC TXRX RS485/RS422 38-QFN LTC2871IUHF#PBF-ND - IC TXRX RS485/RS422 38-QFN LTC2871CFE#PBF-ND - IC TXRX RS485/RS422 38-TSSOP LTC2871IFE#PBF-ND - IC TXRX RS485/RS422 38-TSSOP LTC2871CUHF#TRPBF-ND - IC TXRX RS485/RS422 38-QFN LTC2871IFE#TRPBF-ND - IC TXRX RS485/RS422 38-TSSOP LTC2871IUHF#TRPBF-ND - IC TXRX RS485/RS422 38-QFN LTC2871CFE#TRPBF-ND - IC TXRX RS485/RS422 38-TSSOP |
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LTC2870/LTC2871
20
28701f
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 14.
This precaution is not relevant for supply voltages below
4.5V or rise times longer than 10μs.
VL Logic Supply and Logic Pins
A separate logic supply pin VL allows the LTC2870 and
LTC2871 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.
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 FEN, which must not exceed VL
by more than 1V for proper operation. 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 compati-
bility. When enabled, if DI is high, Y – Z is positive. With
the driver disabled the Y and Z output resistance is greater
than 96kΩ (typically 125kΩ) to ground over the entire
common mode range of –7V to +12V. This resistance is
equivalent to the input resistance on these lines when the
driver is configured in half-duplex mode and Y and Z act
as the RS485 receiver inputs.
Driver Overvoltage and Overcurrent Protection
The RS232 and RS485 driver outputs are protected from
short circuits to any voltage within the absolute maximum
range ±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 up to 100mA
may 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.
APPLICATIONS INFORMATION
Figure 14. 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
VCC
VDD
GND
SW
CAP
C2
1μF
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