6

Capacitor Selection

The charge pumps operate with 0.1

µF (or greater) capacitors

for 3.0V

factor of 2) reduces ripple on the transmitter outputs and

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 transmitter outputs

when exiting powerdown mode. As they activate, the two

transmitter outputs properly go to opposite RS-562/232

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

The ISL8563 maintains the RS-232

transmitter output voltages even at high data rates. Figure 7

details a transmitter loopback test circuit, and Figure 8

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 9 shows the

loopback results for a single transmitter driving 1000pF and

an RS-232 load at 250kbps. The static transmitters were

also loaded with an RS-232 receiver.

FIGURE 4. POWER DRAIN THROUGH POWERED DOWN

PERIPHERAL

OLD

POWERED

GND

SHDN =GND

Rx

Tx

CURRENT

FLOW

RS-562/232 CHIP

DOWN

UART

FIGURE 5. DISABLED RECEIVERS PREVENT POWER DRAIN

ISL8563

TRANSITION

TO

POWERED

DETECTOR

DOWN

UART

WAKE-UP

LOGIC

FIGURE 7. TRANSMITTER LOOPBACK TEST CIRCUIT

TIME (20

µs/DIV.)

T1

T2

2V/DIV.

5V/DIV.

SHDN

FIGURE 6. TRANSMITTER OUTPUTS WHEN EXITING

POWERDOWN

C1 - C4 = 0.1

µF

ISL8563

+

+

+

+

1000pF

V+

V-

5K

C1+

C1-

C2+

C2-

+

0.1

µF

EN

SHDN

400k

Ω

ISL8563