7

This automatic powerdown feature provides additional

system power savings without changes to the existing

operating system or hardware.

Utilizing power management circuitry, to power down the

rest of the communication circuitry (e.g., the UART) when

the ICL3217E powers down, produces even greater power

savings. Connecting a transition detector to the V- pin (see

Figure 3) is an easy way for the power management logic to

determine when the ICL3217E enters and exits powerdown.

Capacitor Selection

The charge pumps require 0.1

µF, or greater, capacitors for

3.3V operation. With 0.1

µF capacitors, five percent tolerance

supplies (e.g., 3.14V minimum) deliver greater than

±5V

transmitter swings at full data rate, while ten percent

tolerance supplies (e.g., 2.97V minimum) deliver

±4.95V

transmitter swings. If greater than

±5V transmitter swings

are required with a 10% tolerance 3.3V supply, 0.22

µF

capacitors are recommended (see Table 3). Existing 5V

applications typically utilize either 0.1

µF or 1µF capacitors,

and the ICL32X7E works well with either value. New 5V

designs should use 0.22

µF capacitors for the best results.

For other supply voltages refer to Table 3 for capacitor

values. Do not use values smaller than those listed in

Table 3. Increasing the capacitor values (by a factor of two)

reduces ripple on the transmitter outputs and slightly

When using minimum required capacitor values, make sure

that capacitor values do not degrade excessively with

temperature. If in doubt, use capacitors with a larger nominal

value. The capacitor’s equivalent series resistance (ESR)

usually rises at low temperatures and it influences the

amount of ripple on V+ and V-

.

Power Supply Decoupling

In most circumstances a 0.1

µF bypass capacitor is

adequate. In applications that are particularly sensitive to

Connect the bypass capacitor as close as possible to the IC.

Transmitter Outputs when Exiting

Powerdown

Figure 6 shows the response of two ICL3217E transmitter

outputs when exiting powerdown mode. As they activate, the

two transmitter outputs properly go to opposite RS-232

levels, with no glitching, ringing, nor undesirable transients.

Each transmitter is loaded with 3k

Ω in parallel with 2500pF.

Note that the transmitters enable only when the magnitude

of the supplies exceed approximately 3V.

.

FIGURE 4. DEFINITION OF VALID RS-232 RECEIVER

LEVELS

0.3V

-0.3V

-2.7V

2.7V

INVALID LEVEL - POWERDOWN OCCURS AFTER 30

µs

VALID RS-232 LEVEL - ICL3217E IS ACTIVE

VALID RS-232 LEVEL - ICL3217E IS ACTIVE

INDETERMINATE - POWERDOWN MAY OR

MAY NOT OCCUR

INDETERMINATE - POWERDOWN MAY OR

MAY NOT OCCUR

RECEIVER

INPUTS

TRANSMITTER

OUTPUTS

V+

0

V-

INVALID

REGION

}

FIGURE 5. AUTOMATIC POWERDOWN TIMING DIAGRAM

AUTOPWDN

TABLE 3. REQUIRED CAPACITOR VALUES

3.15 to 3.6

0.1

0.1

3.0 to 3.6

0.22

0.22

4.5 to 5.5

0.1 to 1.0

0.1 to 1.0

3.0 to 5.5

0.22

0.22

TIME (20

µs/DIV.)

T1

T2

2V/DIV

5V/DIV

C1 - C4 = 0.1

µF

FIGURE 6. TRANSMITTER OUTPUTS WHEN EXITING

POWERDOWN (ICL3217E ONLY)

ICL3207E, ICL3217E