6

Capacitor Selection

The charge pumps require 0.1

µF or greater capacitors for

operation with 3.3V

≤ V

values (by a factor of 2) reduces ripple on the transmitter

capacitors).

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-.

Operation Down to 2.7V

typically ensure inter operability with RS-232 devices.

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 5 shows the response of two 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.

High Data Rates

transmitter output voltages even at high data rates. Figure 6

details a transmitter loopback test circuit, and Figure 7

illustrates the loopback test result at 120Kbps. For this test,

all transmitters were simultaneously driving RS-232 loads in

parallel with 1000pF, at 120Kbps. Figure 8 shows the

loopback results for a single transmitter driving 1000pF and

an RS-232 load at 250Kbps. The static transmitter was also

loaded with an RS-232 receiver.

TIME (20

µs/DIV.)

T1

T2

2V/DIV.

5V/DIV.

SHDN

FIGURE 5. TRANSMITTER OUTPUTS WHEN EXITING

POWERDOWN

C1 - C4 = 0.1

µF

FIGURE 6. TRANSMITTER LOOPBACK TEST CIRCUIT

FIGURE 7. LOOPBACK TEST AT 120Kbps

ISL83384E

+

+

+

+

1000pF

V+

V-

5k

C1+

C1-

C2+

C2-

+

0.1

µF

SHDN

5

µs/DIV.

5V/DIV.

C1 - C4 = 0.1

µF

ISL83384E