DS18B20-PAR
3 of 19
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
capacitor (C
PP
) to provide power when the bus is low.
The 1-Wire bus and C
PP
can provide sufficient parasite power to the DS18B20-PAR for most operations
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
supplied by C
PP
. To assure that the DS18B20-PAR has sufficient supply current, it is necessary to
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
must be held high by the pullup for the duration of the conversion (t
conv
) or data transfer (t
wr
= 10 ms).
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
V
PU
V
PU
4.7K
1-Wire Bus
Micro-
processor
DS18B20-PA
GND
DQ
To Other
1-Wire Devices
