6

FN6045.2

April 13, 2007

voltage doubler and inverter functions. The charge pumps

operate discontinuously (i.e., they turn off as soon as the V+

and V- supplies are pumped up to the nominal values),

resulting in significant power savings.

Transmitters

The transmitters are proprietary, low dropout, inverting drivers

that translate TTL/CMOS inputs to EIA/TIA-232 output levels.

Coupled with the on-chip

±5.5V supplies, these transmitters

deliver true RS-232 levels over a wide range of single supply

system voltages.

All transmitter outputs disable and assume a high impedance

state when the device enters the powerdown mode (see

Table 2). These outputs may be driven to

±12V when disabled.

The devices guarantee a 250kBps data rate for full load

conditions (3k

transmitter operating at full speed. Under more typical

transmitter easily operates at 900kBps.

Transmitter inputs float if left unconnected, and may cause

performance.

Receivers

All the ISL422XE devices contain standard inverting receivers

that three-state via the EN control line. All the receivers

convert RS-232 signals to CMOS output levels and accept

inputs up to

±25V while presenting the required 3kΩ to 7kΩ

0V). The receivers’ Schmitt trigger input stage uses hysteresis

to increase noise immunity and decrease errors due to slow

input signal transitions.

Receivers driving a powered down UART must be disabled to

prevent current flow through, and possible damage to, the

UART’s protection diodes (see Figures 2 and 3). This can be

accomplished on the ISL422XE by driving the EN input high

whenever the UART powers down. Figure 3 also shows that

the INVALID output can be used to determine when the UART

should be powered down. When the RS-232 cable is

disconnected, INVALID switches low indicating that the UART

is no longer needed. Reconnecting the cable drives INVALID

back high, indicating that the UART should be powered up.

Low Power Operation

These 3V devices require a nominal supply current of

powerdown mode). This is considerably less than the 5mA to

11mA current required by comparable 5V RS-232 devices,

allowing users to reduce system power simply by switching to

this new family.

Powerdown Functionality

The already low current requirement drops significantly when

the device enters powerdown mode. In powerdown, supply

current drops to 150nA, because the on-chip charge pump

transmitter outputs three-state. Receiver outputs are

unaffected by powerdown; refer to Table 2 for details. This

micro-power mode makes the ISL422XE ideal for battery

powered and portable applications.

Software Controlled (Manual) Powerdown

The ISL422XE family provides pins that allow the user to force

the IC into the low power, standby state.

The ISL422XE utilize a two pin approach where the

FORCEON and FORCEOFF inputs determine the IC’s

GND

5k

Ω

-25V

GND

FIGURE 1. INVERTING RECEIVER CONNECTIONS

FIGURE 2. POWER DRAIN THROUGH POWERED DOWN

PERIPHERAL

OLD

POWERED

GND

SHDN = GND

Rx

Tx

CURRENT

FLOW

RS-232 CHIP

DOWN

UART

FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN

ISL422XE

TRANSITION

INVALID

TO

POWERED

DETECTOR

DOWN

UART

WAKE-UP

LOGIC

ISL4221E, ISL4223E