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

µs maximum after issuing a

command that involves copying to the EEPROM or initiates a temperature conversion.