Rainbow Electronics DS1920 - Manual (Page 3)

DS1920
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DS1920 BLOCK DIAGRAM Figure 1
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power
whenever the data contact is high. Data will provide sufficient power as long as the specified timing and
voltage requirements are met (see the section titled “1-Wire Bus System”). The advantage of parasite
power is that no local power source is needed for remote sensing of temperature.
In order for the DS1920 to be able to perform accurate temperature conversions, sufficient power must be
provided over the data line when a temperature conversion is taking place. The DS1920 requires a current
during conversion of up to 1 mA, therefore, the data line will not have sufficient drive due to the 5 k
Ω
pullup resistor. This problem is particularly acute if several DS1920s are on the same data line and
attempting to convert simultaneously.
The way to assure that the DS1920 has sufficient supply current is to provide a strong pullup on the data
line whenever temperature conversion or copying to the EEPROM is taking place. This may be
accomplished by using a MOSFET to connect the data line directly to the power supply as shown in
Figure 2. The data line must be switched over to the strong pullup within 10
µ
s maximum after issuing a
command that involves copying to the EEPROM or initiates a temperature conversion.