Rainbow Electronics DS2890 - Manual (Page 7)

DS2890
7 of 28
POWER and CONFIGURATION CONSIDERATIONS
1-Wire devices can typically derive operating power entirely from the 1-Wire bus by storing energy on an
internal capacitor during periods of time when the 1-Wire bus is in a high state. During bus low times the
device continues to operate from the energy stored on the internal capacitor; the capacitor is then
recharged when the bus returns to a high state. This technique of operating entirely from the 1-Wire bus
by powering from energy stored on an internal capacitor during bus low times in known as “parasite
powered” operation. Special consideration is required with the DS2890 in that it supports two power
configurations and two related digital resistor modes: Potentiometer Configuration and Variable Resistor
Configuration. The Potentiometer Configuration requires the presence of a VDD power source to supply
the power needs of the device charge pump which must be turned on to support a terminal-to-terminal
wiper output range. The Variable Resistor configuration is supported with or without a VDD source or
enabled charge pump although resistance range, as described below, is influenced by the charge pump
state.
POTENTIOMETER CONFIGURATION
To configure and operate the DS2890 as a potentiometer as shown in Figure 5 requires the TSOC or flip
chip package, a VDD power source, and the device charge pump turned on (Control Register, bit
CPC=1). The charge pump must be enabled to support RL to RH terminal voltage swings at the WIPER
output and as described previously, a VDD supply must be provided to support the power requirements
of the charge pump.
Figure 5. Potentiometer Configuration
RH
WIPER
RL
VDD
1-WIRE
GND
2.8V to 6.0V
2.8V to 6.0V
Bus
Master
VARIABLE RESISTOR CONFIGURATION
The Variable Resistor configuration as shown in Figure 6 is supported by all package types with or
without an external VDD supply. As described previously, without a VDD source the charge pump must
be disabled.