MAX3387 Просмотр технического описания (PDF) - Maxim Integrated

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MAX3387

3V, ±15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones

Maxim Integrated 

3V, ±15kV ESD-Protected, AutoShutdown Plus

RS-232 Transceiver for PDAs and Cell Phones

T_IN

EDGE

DETECT

R_IN

EDGE

DETECT

FORCEON

FORCEOFF

S

30s

TIMER

R

AUTOSHDN

Figure 2c. AutoShutdown Plus Logic

+2.7V

+0.3V

0V

-0.3V

-2.7V

TRANSMITTERS ENABLED, INVALID HIGH

INDETERMINATE

AutoShutdown, TRANSMITTERS DISABLED,

1μA SUPPLY CURRENT, INVALID LOW

INDETERMINATE

TRANSMITTERS ENABLED, INVALID HIGH

Figure 3a. Receiver Thresholds for INVALID

FORCEOFF

FORCEON

AUTOSHDN

POWERDOWN

POWERDOWN IS ONLY AN INTERNAL SIGNAL. IT CONTROLS THE

OPERATIONAL STATUS OF THE TRANSMITTERS AND THE POWER SUPPLIES.

Figure 2d. Power-Down Logic

AutoShutdown Plus Mode

The MAX3387E achieves a1µA supply current with

Maxim’s AutoShutdown Plus feature, which operates

when FORCEOFF is high and a FORCEON is low. When

these devices do not sense a valid signal transition on

any receiver and transmitter input for 30sec, the on-

board charge pumps are shut down, reducing supply

current to 1µA. This occurs if the RS-232 cable is dis-

connected or if the connected peripheral transmitters

are turned off, and if the UART driving the transmitter

inputs is inactive. The system turns on again when a

valid transition is applied to any RS-232 receiver or

transmitter input. As a result, the system saves power

without changes to the existing BIOS or operating sys-

tem.

Figures 2a and 2b show valid and invalid RS-232

receiver voltage levels. INVALID indicates the receiver

input’s condition, and is independent of FORCEON and

FORCEOFF states. Figure 2 and Table 1 summarize the

MAX3387E’s operating modes. FORCEON and FORCE-

OFF override AutoShutdown Plus circuitry. When nei-

ther control is asserted, the IC selects between these

states automatically based on the last receiver or trans-

mitter input edge received.

By connecting FORCEON to INVALID, the MAX3387E

shuts down when no valid receiver level and no receiver or

transmitter edge is detected for 30sec, and wakes up

when a valid receiver level or receiver or transmitter

edge is detected.

By connecting FORCEON and FORCEOFF to INVALID,

the MAX3387E shuts down when no valid receiver level

is detected.

A mouse or other system with AutoShutdown Plus may

need time to wake up. Figure 4 shows a circuit that

forces the transmitters on for 100ms, allowing enough

time for the other system to realize that the MAX3387E

is awake. If the other system outputs valid RS-232 sig-

nal transitions within that time, the RS-232 ports on both

systems remain enabled.

VL Logic Supply Input

Unlike other RS-232 interface devices where the receiv-

er outputs swing between 0V and VCC, the MAX3387E

features a separate logic supply input (VL) that sets

VOH for the receiver outputs and sets thresholds for the

receiver inputs. This feature allows a great deal of flexi-

bility in interfacing to many different types of systems

with different logic levels. Connect this input to the host

logic supply (1.8V ≤ VL ≤ VCC). Also, see the Typical

PDA/Cell-Phone Application section.

8 _______________________________________________________________________________________

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