the logic device’s low VOH might not exceed the V<" />
參數(shù)資料
型號: ISL81334IAZ
廠商: Intersil
文件頁數(shù): 10/28頁
文件大?。?/td> 0K
描述: IC TXRX DUAL 2PORT ESD 28-SSOP
產(chǎn)品培訓模塊: Solutions for Industrial Control Applications
標準包裝: 47
類型: 收發(fā)器
驅(qū)動器/接收器數(shù): 2/2
規(guī)程: RS232,RS422,RS485
電源電壓: 4.5 V ~ 5.5 V
安裝類型: 表面貼裝
封裝/外殼: 28-SSOP(0.209",5.30mm 寬)
供應商設(shè)備封裝: 28-SSOP
包裝: 管件
18
FN6202.3
November 21, 2007
the logic device’s low VOH might not exceed the VIH of a 5V
powered dual protocol input. Connecting the VL pin to the
power supply of the logic device (as shown in Figure 12)
limits the ISL41334’s Rx output VOH to VL (see Figure 15),
and reduces the Tx and control input switching points to
values compatible with the logic device output levels.
Tailoring the logic pin input switching points and output levels
to the supply voltage of the UART, ASIC, or controller
eliminates the need for a level shifter/translator between the
two ICs.
VL can be anywhere from VCC down to 1.65V, but the input
switching points may not provide enough noise margin when
VL < 1.8V. Table 5 indicates typical VIH and VIL values for
various VL values so the user can ascertain whether or not a
particular VL voltage meets his needs.
The VL supply current (IL) is typically less than 100A, as
shown in Figures 20 and 21. All of the DC VL current is due
to inputs with internal pull-up resistors (DE, SP, LB, ON/OFF)
being driven to the low input state. The worst case IL current
occurs during SHDN (see Figure 20), due to the IL through
the ON/OFF pin pull-up resistor when that pin is driven low.
IIL through an input pull-up resistor is ~20A, so the IL in
Figure 20 drops by about 40A (at VL = 5V) when the two
SP inputs are high (middle vs top curve). IL is lowest in the
RS-232 mode, because only the ON/OFF pin should be
driven low. When all these inputs are driven high, IL drops to
<1A, so to minimize power dissipation drive these inputs
high when unneeded (e.g., SP inputs aren’t used in RS-232
mode, so drive them high).
Active Low Rx Enable (RXEN)
In many RS-485 applications, especially half duplex
configurations, users like to accomplish “echo cancellation”
by disabling the corresponding receiver while its driver is
transmitting data. This function is available on the QFN
package via an active low RXEN pin for each port. The
active low function also simplifies direction control, by
allowing a single Tx/Rx direction control line. If an active high
RXEN were used, either two valuable I/O pins would be
used for direction control, or an external inverter is required
between DE and RXEN. Figure 13 details the advantage of
using the RXEN pin.
TABLE 5. VIH AND VIL vs VL FOR VCC = 5V
VL (V)
VIH (V)
VIL (V)
1.65V
0.79
0.50
1.8V
0.82
0.60
2.0V
0.87
0.69
2.5V
0.99
0.86
3.3V
1.19
1.05
FIGURE 12. USING VL PIN TO ADJUST LOGIC LEVELS
GND
RXD
TXD
VCC = +2V
UART/PROCESSOR
GND
RA
DY
VCC = +5V
ISL81334
VOH ≤ 2V
VOH = 5V
VIH ≥ 2V
ESD
DIODE
GND
RXD
TXD
VCC = +2V
UART/PROCESSOR
GND
RA
DY
VCC = +5V
ISL41334
VOH ≤ 2V
VOH = 2V
VIH = 0.9V
ESD
DIODE
VL
FIGURE 13. USING ACTIVE LOW vs ACTIVE HIGH RX
ENABLE
0.1F
+
D
R
VCC
GND
RA
RXEN
DEN
DY
+5V
ISL81387
Tx/Rx
ACTIVE HIGH RX ENABLE
0.1F
+
D
R
VCC
GND
RA
RXEN *
DE
DY
+5V
ISL41334
Tx/Rx
* QFN ONLY
ACTIVE LOW RX ENABLE
Y
Z
A
B
Y
Z
A
B
ISL81334, ISL41334
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