ISL4238E Intersil Corporation, ISL4238E Datasheet - Page 10

• Manufacturer Part Number: ISL4238E
• Description: QFN Packaged, 15kV Esd Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps/ 1Mbps, RS-232 Transceivers With Enhanced Automatic Powerdown
• Manufacturer: Intersil Corporation

Powerdown Functionality
The already low current requirement drops significantly
when the device enters powerdown mode. In powerdown,
supply current drops to 10nA, because the on-chip charge
pump turns off (V+ collapses to V
and the transmitter outputs tristate. Inverting receiver
outputs disable only in manual powerdown; refer to Table 2
for details. This micro-power mode makes these devices
ideal for battery powered and portable applications.
Software Controlled (Manual) Powerdown
These devices allow the user to force the IC into the low
power, standby state, and utilize a two pin approach where
the FORCEON and FORCEOFF inputs determine the IC’s
mode. For always enabled operation, FORCEON and
FORCEOFF are both strapped high. To switch between
active and powerdown modes, under logic or software
control, only the FORCEOFF input need be driven. The
FORCEON state isn’t critical, as FORCEOFF dominates
over FORCEON. Nevertheless, if strictly manual control over
powerdown is desired, the user must strap FORCEON high
to disable the enhanced automatic powerdown circuitry.
ISL4238E/4XE inverting (standard) receiver outputs also
disable when the device is in manual powerdown, thereby
eliminating the possible current path through a shutdown
peripheral’s input protection diode (see Figures 2 and 3).
Connecting FORCEOFF and FORCEON together disables
the enhanced automatic powerdown feature, enabling them
to function as a manual SHUTDOWN input (see Figure 4).
With any of the above control schemes, the time required to
exit powerdown, and resume transmission is only 100µs.
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN
POWERED
DOWN
UART
V
WAKE-UP
CC
LOGIC
TO
R
T
X
X
FORCEOFF = GND
TRANSITION
DETECTOR
V
OUT
R
= HI-Z
V
R
OUTB
T1
CC
OUT
10
IN
CC
, V- collapses to GND),
ISL4238E/4XE
ISL4238E, ISL4244E, ISL4245E
R
T1
IN
OUT
When using both manual and enhanced automatic
powerdown (FORCEON = 0), the ISL4238E/4XE won’t
power up from manual powerdown until both FORCEOFF
and FORCEON are driven high, or until a transition occurs
on a receiver or transmitter input. Figure 5 illustrates a circuit
for ensuring that the ISL4238E/4XE powers up as soon as
FORCEOFF switches high. The rising edge of the Master
Powerdown signal forces the device to power up, and the
ISL4238E/4XE returns to enhanced automatic powerdown
mode an RC time constant after this rising edge. The time
constant isn’t critical, because the ISL4238E/4XE remains
powered up for 30 seconds after the FORCEON falling edge,
even if there are no signal transitions. This gives slow-to-
wake systems (e.g., a mouse) plenty of time to start
transmitting, and as long as it starts transmitting within 30
seconds both systems remain enabled.
INVALID Output
The INVALID output always indicates (see Table 2) whether
or not 30µs have elapsed with invalid RS-232 signals (see
Figures 6 and 9) 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
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN
FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP
MANAGEMENT
POWER
CPU
UNIT
LOGIC
PWR
MGT
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
WHEN EXITING FORCED POWERDOWN
UART
MASTER POWERDOWN LINE
I/O
FORCEOFF
FORCEON
INVALID
FORCEOFF
ISL4238E/4XE
0.1µF
ISL4238E/4XE
FORCEON
1MΩ