DS18B20-PAR

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PARASITE POWER

The DS18B20-PAR’s parasite power circuit allows the DS18B20-PAR to operate without a local external

power supply. This ability is especially useful for applications that require remote temperature sensing or

that are very space constrained. Figure 1 shows the DS18B20-PAR’s parasite-power control circuitry,

which “steals” power from the 1-Wire bus via the DQ pin when the bus is high. The stolen charge

powers the DS18B20-PAR while the bus is high, and some of the charge is stored on the parasite power

as long as the specified timing and voltage requirements are met (refer to the DC ELECTRICAL

CHARACTERISTICS and the AC ELECTRICAL CHARACTERISTICS sections of this data sheet).

However, when the DS18B20-PAR is performing temperature conversions or copying data from the

scratchpad memory to EEPROM, the operating current can be as high as 1.5 mA. This current can cause

an unacceptable voltage drop across the weak 1-Wire pullup resistor and is more current than can be

provide a strong pullup on the 1-Wire bus whenever temperature conversions are taking place or data is

being copied from the scratchpad to EEPROM. This can be accomplished by using a MOSFET to pull

the bus directly to the rail as shown in Figure 2. The 1-Wire bus must be switched to the strong pullup

within 10

μs (max) after a Convert T [44h] or Copy Scratchpad [48h] command is issued, and the bus

No other activity can take place on the 1-Wire bus while the pullup is enabled.

SUPPLYING THE DS18B20-PAR DURING TEMPERATURE CONVERSIONS

Figure 2

OPERATION – MEASURING TEMPERATURE

The core functionality of the DS18B20-PAR is its direct-to-digital temperature sensor. The resolution of

the temperature sensor is user-configurable to 9, 10, 11, or 12 bits, which corresponds to increments of

0.5

°C, 0.25°C, 0.125°C, and 0.0625°C, respectively. The default resolution at power-up is 12-bit.

The DS18B20-PAR powers-up in a low-power idle state; to initiate a temperature measurement and A-to-

D conversion, the master must issue a Convert T [44h] command.

Following the conversion, the

resulting thermal data is stored in the 2-byte temperature register in the scratchpad memory and the

DS18B20-PAR returns to its idle state.

The DS18B20-PAR output data is calibrated in degrees

centigrade; for Fahrenheit applications, a lookup table or conversion routine must be used.

The

temperature data is stored as a 16-bit sign-extended two’s complement number in the temperature register

(see Figure 3). The sign bits (S) indicate if the temperature is positive or negative: for positive numbers S

= 0 and for negative numbers S = 1. If the DS18B20-PAR is configured for 12-bit resolution, all bits in

the temperature register will contain valid data. For 11-bit resolution, bit 0 is undefined. For 10-bit

4.7K

1-Wire Bus

Micro-

processor

DS18B20-PAR

GND

DQ

To Other

1-Wire Devices