MAX31850 データシートの表示（PDF） - Maxim Integrated

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MAX31850

Cold-Junction Compensated, 1-Wire Thermocouple-to-Digital Converters

Maxim Integrated 

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

VPU

MAX31850

VPU

GND DQ VDD

µP

4.7kΩ

1-Wire BUS

TO OTHER 1-Wire DEVICES

Figure 1. Supplying the Parasite-Powered MAX31850/MAX31851 During Temperature Conversions

MAX31850

VDD (EXTERNAL SUPPLY)

VPU

GND DQ VDD

µP

4.7kΩ

1-Wire BUS

TO OTHER 1-Wire DEVICES

Figure 2. Powering the MAX31850/MAX31851 with an External Supply

Powering the MAX31850/MAX31851

The MAX31850/MAX31851 can be powered by an

external supply on the VDD pin, or they can operate

in “parasite power” mode, which allows the device to

function without a local external supply. Parasite power

is useful for applications that require remote tempera-

ture sensing or those that are very space-constrained.

Figure 1 shows the device’s parasite-power control cir-

cuitry, which “steals” power from the 1-Wire bus through

DQ when the bus is high. The stolen charge powers the

device while the bus is high, and some of the charge is

stored on the internal parasite-power capacitor (CPP) to

provide power when the bus is low. When the device is

used in parasite-power mode, VDD must be connected

to ground.

In parasite-power mode, the 1-Wire bus and CPP can

provide sufficient current to the device for most opera-

tions as long as the specified timing and voltage require-

ments are met (see the DC Electrical Characteristics

and the 1-Wire Timing Characteristics tables). However,

when the device is performing temperature conversions,

the operating current can be as high as 1.5mA. This

current can cause an unacceptable voltage drop across

the weak 1-Wire pullup resistor and is more current than

Maxim Integrated

can be supplied by CPP. To ensure that the device has

sufficient supply current, it is necessary to provide a

strong pullup on the 1-Wire bus whenever temperature

conversions are taking place. This can be accomplished

by using a MOSFET to pull the bus directly to the rail

as shown in Figure 1. The 1-Wire bus must be switched

to the strong pullup within 10Fs (max) after a Convert T

[44h] command is issued, and the bus must be held high

by the pullup for the duration of the conversion (tCONV).

No other activity can take place on the 1-Wire bus while

the pullup is enabled.

The device can also be powered by the conventional

method of connecting an external power supply to VDD,

as shown in Figure 2. The advantage of this method is

that the MOSFET pullup is not required, and the 1-Wire

bus is free to carry other traffic during the temperature

conversion period.

The use of parasite power is not recommended for tem-

peratures above 100NC because the device may not be

able to sustain communications due to the higher leak-

age currents that can exist at these temperatures. For

applications in which such temperatures are likely, it is

strongly recommended that the device be powered by

an external power supply.

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