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ISL83387

+/-15kV ESD Protected, +3V to +5.5V, 1Microamp, 250kbps, RS-232 Transceivers with Enhanced Automatic Powerdown and a Separate Logic Supply

Renesas Electronics 

ISL83387E

RECEIVER

INPUTS

TRANSMITTER

INPUTS

TRANSMITTER

OUTPUTS

INVALID

OUTPUT

tINVL

V+

VCC

0

V-

tINVH

tAUTOPWDN

} INVALID

REGION

tWU

tAUTOPWDN

tWU

FIGURE 7. ENHANCED AUTOMATIC POWERDOWN AND INVALID TIMING DIAGRAMS

RS-232 level, forcing the ISL83387E to power on. See the

“INVALID DRIVING FORCEON AND FORCEOFF” section

of Table 2 for an operational summary. This operational

mode is perfect for handheld devices that communicate with

another computer via a detachable cable. Detaching the

cable allows the internal receiver pull-down resistors to pull

the inputs to GND (an invalid RS-232 level), causing the

30s timer to time-out and drive the IC into powerdown.

Reconnecting the cable restores valid levels, causing the IC

to power back up.

Hybrid Automatic Powerdown Options

For devices which communicate only through a detachable

cable, connecting INVALID to FORCEOFF (with

FORCEON = 0) may be a desirable configuration. While the

cable is attached INVALID and FORCEOFF remain high, so

the enhanced automatic powerdown logic powers down the

RS-232 device whenever there is 30 seconds of inactivity on

the receiver and transmitter inputs. Detaching the cable

allows the receiver inputs to drop to an invalid level (GND),

so INVALID switches low and forces the RS-232 device to

power down. The ISL83387E remains powered down until

the cable is reconnected (INVALID = FORCEOFF = 1) and a

transition occurs on a receiver or transmitter input (see

Figure 5). For immediate power up when the cable is

reattached, connect FORCEON to FORCEOFF through a

network similar to that shown in Figure 3.

Capacitor Selection

The ISL83387E charge pumps require only 0.1F capacitors

for the full operational voltage range. Table 3 lists other

acceptable capacitor values for various supply voltage

ranges. Do not use values smaller than those listed in

Table 3. Increasing the capacitor values (by a factor of 2)

reduces ripple on the transmitter outputs and slightly

reduces power consumption.

TABLE 3. REQUIRED CAPACITOR VALUES

VCC

(V)

C1

C2, C3, C4

(F)

(F)

3.0 to 3.6

0.1

0.1

4.5 to 5.5

0.047

0.33

3.0 to 5.5

0.22

1

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F bypass capacitor is

adequate. In applications that are particularly sensitive to

power supply noise, decouple VCC to ground with a

capacitor of the same value as the charge-pump capacitor C1.

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

Transmitter Outputs when Exiting

Powerdown

Figure 8 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in parallel with 2500pF. Note

that the transmitters enable only when the magnitude of the

supplies exceed approximately 3V.

FN6040 Rev 1.00

July 8, 2005

Page 9 of 13

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