Rainbow-electronics MAX1618 Bedienungsanleitung PDF herunterladen (Seite 12)

Slave Addresses

The device address can be set to one of nine different

values by pin-strapping ADD0 and ADD1 so more than

one MAX1618 can reside on the same bus without

address conflicts (Table 6).

The address pin states are checked at POR only, and

the address data stays latched to reduce quiescent

supply current due to the bias current needed for high-

impedance (high-Z) state detection.

The MAX1618 also responds to the SMBus Alert

Response slave address (see the Alert Response

Address section).

POR and UVLO

The MAX1618 has a volatile memory. To prevent ambigu-

ous power-supply conditions from corrupting the data in

the memory and causing erratic behavior, a POR voltage

detector monitors V

and clears the memory if V

falls

below 1.7V (typical, see the Electrical Characteristics

table). When power is first applied and V

rises above

1.75V (typ), the logic blocks begin operating, although

reads and writes at V

levels below 3V are not recom-

mended. A second V

comparator, the ADC UVLO com-

parator, prevents the ADC from converting until there is

sufficient headroom (V

= 2.8V typ).

Power-Up Defaults:

• Interrupt latch is cleared.

• Address select pins are sampled.

• Command byte is set to 01h to facilitate quick

remote Receive Byte queries.

• T

HIGH

and T

LOW

registers are set to max and min

limits, respectively.

• Device is in normal mode. (ALERT acts as a latched

interrupt output.)

Applications Information

Remote Diode Selection

Temperature accuracy depends on having a good-

quality, diode-connected, small-signal transistor.

Accuracy has been experimentally verified for all of the

devices listed in Table 7. The MAX1618 can also direct-

ly measure the die temperature of CPUs and other inte-

grated circuits with on-board temperature sensing

diodes, such as the Intel Pentium II

The transistor must be a small-signal type with a rela-

tively high forward voltage. This ensures that the input

voltage is within the A/D input voltage range. The for-

ward voltage must be greater than 0.25V at 10µA at the

highest expected temperature. The forward voltage

must be less than 0.95V at 100µA at the lowest expect-

ed temperature. The base resistance has to be less

than 100Ω. Tight specification of forward-current gain

(+50 to +150, for example) indicates that the manufac-

turer has good process controls and that the devices

have consistent V

characteristics. Do not use power

transistors.

ADC Noise Filtering

The integrating ADC has inherently good noise rejec-

tion, especially of low-frequency signals such as

60Hz/120Hz power-supply hum. Micropower operation

places constraints on high-frequency noise rejection.

Lay out the PCB carefully with proper external noise fil-

tering for high-accuracy remote measurements in elec-

trically noisy environments.

Filter high-frequency electromagnetic interference

(EMI) at DXP and DXN with an external 2200pF capaci-

tor connected between the two inputs. This capacitor

can be increased to about 3300pF (max), including

cable capacitance. A capacitance higher than 3300pF

MAX1618

Remote Temperature Sensor

with SMBus Serial Interface

12 ______________________________________________________________________________________

0011 001High-ZGND

0011 000

ADDRESS

0101 001GNDHigh-Z

0011 010V

GND

0101 011V

High-Z

0101 010

1001 101High-ZV

1001 100

GNDGND

GNDV

High-ZHigh-Z

1001 110V

ADD0 ADD1

Table 6. Slave Address Decoding

(ADD0 and ADD1)

Note: High-Z means the pin is left unconnected and floating.

Note: Transistors must be diode-connected (short the base to

the collector).

SMBT3904Siemens (Germany)

CMPT3904Central Semiconductor (USA)

MMBT3904Fairchild Semiconductor (USA)

SST3904Rohm Semiconductor (Japan)

FMMT3904CT-NDZetex (England)

MANUFACTURER MODEL NUMBER

Table 7. SOT23 Type Remote-Sensor

Transistor Manufacturers

MMBT3904Motorola (USA)

Pentium II is a registered trademark of Intel Corp.

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 18

Remote Temperature Sensor 1

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