ISL4270E Datasheet, PDF (8/13 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +3V to +5.5V, 300nA, 250kbps, RS-232 Transceivers with Enhanced Automatic Powerdown and a Separate Logic Supply

ISL4270E
INVALID Output
The INVALID output always indicates (see Table 2) whether
or not 30μs have elapsed with invalid RS-232 signals (see
Figures 4 and 7) persisting on all of the receiver inputs,
giving the user an easy way to determine when the interface
block should power down. Invalid receiver levels occur
whenever the driving peripheral’s outputs are shut off
(powered down) or when the RS-232 interface cable is
disconnected. In the case of a disconnected interface cable
where all the receiver inputs are floating (but pulled to GND
by the internal receiver pull down resistors), the INVALID
logic detects the invalid levels and drives the output low. The
power management logic then uses this indicator to power
down the interface block. Reconnecting the cable restores
valid levels at the receiver inputs, INVALID switches high,
and the power management logic wakes up the interface
block. INVALID can also be used to indicate the DTR or
RING INDICATOR signal, as long as the other receiver
inputs are floating, or driven to GND (as in the case of a
powered down driver).
INVALID switches high 1μs after detecting a valid RS-232
level on a receiver input. INVALID operates in all modes
(forced or automatic powerdown, or forced on), so it is also
useful for systems employing manual powerdown circuitry.
2.7V
VALID RS-232 LEVEL - INVALID = 1
INDETERMINATE
0.3V
-0.3V
INVALID LEVEL - INVALID = 0
INDETERMINATE
Figure 5 illustrates the enhanced powerdown control logic.
Note that once the ISL4270E enters powerdown (manually
or automatically), the 30 second timer remains timed out
(set), keeping the ISL4270E powered down until FORCEON
transitions high, or until a transition occurs on a receiver or
transmitter input.
As stated previously, the INVALID output switches low
whenever invalid levels have persisted on all of the receiver
inputs for more than 30μs (see Figure 7), but this has no
direct effect on the state of the ISL4270E (see the next
sections for methods of utilizing INVALID to power down the
device).
The time to recover from automatic powerdown mode is
typically 100μs.
FORCEOFF
T_IN
EDGE
DETECT
S
30s
TIMER
AUTOPWDN
R_IN
EDGE
DETECT
R
FORCEON
FIGURE 5. ENHANCED AUTOMATIC POWERDOWN LOGIC
Emulating Standard Automatic Powerdown
ISL4270E
-2.7V
VALID RS-232 LEVEL - INVALID = 1
FIGURE 4. DEFINITION OF VALID RS-232 RECEIVER LEVELS
Enhanced Automatic Powerdown
Even greater power savings is available by using the
enhanced automatic powerdown function. When the
enhanced powerdown logic determines that no transitions
have occurred on any of the transmitter nor receiver inputs
for 30 seconds, the charge pump and transmitters
powerdown, thereby reducing supply current to 1μA. The
ISL4270E automatically powers back up whenever it detects
a transition on one of these inputs. This automatic
powerdown feature provides additional system power
savings without changes to the existing operating system.
Enhanced automatic powerdown operates when the
FORCEON input is low, and the FORCEOFF input is high.
Tying FORCEON high disables automatic powerdown, but
manual powerdown is always available via the overriding
FORCEOFF input. Table 2 summarizes the enhanced
automatic powerdown functionality.
CPU
I/O
UART
FIGURE 6. CONNECTIONS FOR AUTOMATIC POWERDOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
If enhanced automatic powerdown isn’t desired, the user can
implement the standard automatic powerdown feature
(mimics the function on the ICL3221E/23E/43E) by
connecting the INVALID output to the FORCEON and
FORCEOFF inputs, as shown in Figure 6. After 30μs of
invalid receiver levels, INVALID switches low and drives the
ISL4270E into a forced powerdown condition. INVALID
switches high as soon as a receiver input senses a valid
RS-232 level, forcing the ISL4270E to power on. See the
8
FN6041.2
June 16, 2010