ISL4238E Datasheet, PDF (10/17 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps/1Mbps, RS-232 Transceivers with Enhanced Automatic Power-down

ISL4238E, ISL4244E, ISL4245E
V_CC
TRANSITION
DETECTOR
TO
WAKE-UP
LOGIC
V_CC
R_OUTB
R_X
POWERED
DOWN
UART
T_X
V_OUT = HI-Z
R_OUT
T1_IN
FORCEOFF = GND
ISL4238E
ISL4244E
ISL4245
R_IN
T1_OUT
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN
Power-down Functionality
The already low current requirement drops significantly
when the device enters power-down mode. In power-down,
supply current drops to 10nA, because the on-chip charge
pump turns off (V+ collapses to V_CC, V- collapses to GND),
and the transmitter outputs tristate. Inverting receiver
outputs disable only in manual power-down; refer to Table 2
for details. This micro-power mode makes these devices
ideal for battery powered and portable applications.
Software Controlled (Manual) Power-down
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 power-down 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
power-down is desired, the user must strap FORCEON high
to disable the enhanced automatic power-down circuitry.
ISL4238E, ISL4244E, ISL4245 inverting (standard) receiver
outputs also disable when the device is in manual power-
down, 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 power-down feature, enabling them
to function as a manual SHUTDOWN input (see Figure 4).
With any of the previous mentioned control schemes, the
time required to exit power-down, and resume transmission
is only 100µs.
PWR
MGT
LOGIC
FORCEOFF
FORCEON
INVALID
ISL4238E
ISL4244E
ISL4245
CPU
I/O
UART
FIGURE 4. CONNECTIONS FOR MANUAL POWER-DOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
When using both manual and enhanced automatic power-
down (FORCEON = 0), the ISL4238E, ISL4244E, ISL4245
won’t power up from manual power-down 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,
ISL4244E, ISL4245 powers up as soon as FORCEOFF
switches high. The rising edge of the Master Power-down
signal forces the device to power up, and the ISL4238E,
ISL4244E, ISL4245 returns to enhanced automatic power-
down mode an RC time constant after this rising edge. The
time constant isn’t critical, because the ISL4238E,
ISL4244E, ISL4245 remains powered up for 30s 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 30s both systems remain enabled.
POWER
MANAGEMENT
UNIT
MASTER POWER-DOWN LINE
0.1µF
1MΩ
FORCEOFF FORCEON
ISL4238E, ISL4244E, ISL4245
FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP
WHEN EXITING FORCED POWER-DOWN
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
10
FN8038.5
May 13, 2010