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Detailed Description

The ICL3237E operates from a single +3V to +5.5V supply,

requires only four small external 0.1

µF (0.22µF for

and meets all EIA/TIA-232 and V.28 specifications. The

circuit is divided into three sections: The charge pump, the

transmitters, and the receivers.

Charge-Pump

Intersil’s new ICL32XX family utilizes regulated on-chip dual

charge pumps as voltage doublers, and voltage inverters to

generate

±5.5V transmitter supplies from a V

low as 3.0V. This allows these devices to maintain RS-232

compliant output levels over the

±10% tolerance range of

3.3V powered systems. The efficient on-chip power supplies

require only four small, external 0.1

µF capacitors for the

the “Capacitor Selection” section, and Table 3 for capacitor

recommendations for other operating conditions. 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 ICL3237E guarantees a 1Mbps data rate

at 1.7Mbps.

Transmitter inputs incorporate an active positive feedback

resistor that maintains the last driven input state in the

absence of a forcing signal. Unused transmitter inputs may

be left unconnected.

Receivers

The ICL3237E contains standard inverting receivers that

tristate only when the EN control line is driven high.

Additionally, it includes a noninverting (monitor) receiver

regardless of the state of any control lines. All the receivers

convert RS-232 signals to CMOS output levels and accept

inputs up to

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

input impedance (see Figure 1) even if the power is off

hysteresis to increase noise immunity and decrease errors

due to slow input signal transitions.

Monitor receivers remain active even during manual

powerdown and forced receiver disable, making them

extremely useful for Ring Indicator monitoring. Standard

receivers driving powered down peripherals must be

disabled to prevent current flow through the peripheral’s

protection diodes (see Figures 2 and 3). This renders them

useless for wake up functions, but the corresponding monitor

receiver can be dedicated to this task as shown in Figure 3.

Powerdown Functionality

This 3V device requires a nominal supply current of 0.3mA

during normal operation (not in powerdown mode). This is

considerably less than the 5mA to 11mA current required of

5V RS-232 devices. The already low current requirement

drops significantly when the device enters powerdown mode.

In powerdown, supply current drops to 10nA, because the

collapses to GND), and the transmitter outputs tristate. This

micro-power mode makes the ICL3237E ideal for battery

powered and portable applications.

Software Controlled (Manual) Powerdown

On the ICL3237E, 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

function isn’t needed. Note that all the receiver outputs

remain enabled 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 2 and 3). The time required to exit

powerdown, and resume transmission is only 100

µs.

GND

5k

Ω

-25V

GND

FIGURE 1. INVERTING RECEIVER CONNECTIONS

ICL3237E