ISL3241E, ISL3243E

10

FN6768.2

June 18, 2012

Software Controlled (Manual) Powerdown

The devices in this family provide pins that allow the user to

force the IC into the low power, standby state.

On the ISL3241E, the powerdown control is via a simple

shutdown (SHDN) pin. Driving this pin high enables normal

operation, while driving it low forces the IC into it’s powerdown

needed. Note that all the receiver outputs remain controlled by

EN during shutdown (see Table 2). For the lowest power

consumption during powerdown, the receivers should also be

disabled by driving the EN input high (see next section, and

Figures 5 and 6).

The ISL3243E utilizes 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 automatic powerdown circuitry.

ISL3243E 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 5 and 6).

Connecting FORCEOFF and FORCEON together disables the

automatic powerdown feature, enabling them to function as a

manual SHUTDOWN input (see Figure 7).

With any of the previous mentioned control schemes, the time

required to exit powerdown, and resume transmission is only

20µs. A mouse, or other application, may need more time to

wake up from shutdown. If automatic power-down is being

utilized, the RS-232 device will reenter power-down if valid

receiver levels aren’t reestablished within 20µs of the

ISL3243E powering up. Figure 8 illustrates a circuit that keeps

the ISL3243E from initiating automatic power-down for 100ms

after powering up. This gives the slow-to-wake peripheral

circuit time to reestablish valid RS-232 output levels.

output pins. These pins interface with “logic” devices such as

UARTs, ASICs, and µcontrollers, and today most of these

devices use power supplies significantly lower than 3.3V. Thus,

powered ISL324xE logic input, or a 3.3V receiver output high

level might overdrive and damage the input diodes on an input

of a 1.8V powered logic device, as shown in Figure 9.

(see Figure 9) reduces the ISL324xE’s logic input switching

points, and limits the receiver output high voltage, to values

compatible with the logic device’s I/O levels. Tailoring the

ISL324XE’s logic pin input switching points and output levels

FIGURE 6. DISABLED RECEIVERS PREVENT POWER DRAIN

ISL324XE

TRANSITION

FORCEOFF = GND

TO

POWERED

DETECTOR

DOWN

UART

WAKE-UP

LOGIC

OR

FIGURE 7. CONNECTIONS FOR MANUAL POWERDOWN WHEN NO

VALID RECEIVER SIGNALS ARE PRESENT

PWR

FORCEOFF

INVALID

CPU

I/O

FORCEON

ISL3243E

MGT

LOGIC

UART

FIGURE 8. CIRCUIT TO PREVENT AUTO POWERDOWN FOR

100ms AFTER FORCED POWERUP

ISL3243E

FORCEOFF

FORCEON

POWER

MASTER POWERDOWN LINE

1MΩ

0.1µF

MANAGEMENT

UNIT