Specifications

40
Variable Output Switcher
This device by far took the most energy in this module. T he complexity in this decision
stems from the need to produce 6, 9, 12, and 15V w hen the input voltage could drift above
or below the output voltage. This device must therefore step up and step down voltages.
The three types of switchers that can regulate above, at, or below the input voltages are
buck-boost, fly-back, and SEPIC
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regulators. A buck-boost regulator has an advantage if
the application needs a negative output voltage. Fly-back regulators can be used in the
DC converter mode or as an AC transformer. Both modes are subject to very high voltage
spikes. SEPIC converters do not invert the power signal and provide a true “power off”
state with no leakage.
The SEPIC converter was selected as the regulator of choice. Finding one that per-
formed with the desired input, performance, sleep, and output specifications turned out to
be very difficult. The solution taken in the end was to use the LTC1871 [39]. This device is
80% efficient but does not satisfy the sleep requirement because its sleep current is 10µA.
To meet power requirements, a power switch was added to the input power line to this
device to completely remove it from calculations.
Battery Voltage Supervisor Circuit
This circuit posed a challenge in finding p arts that operated at less than 7µA. Most
were 15µA parts with desirable features like hys torisis and a wide input voltage range. The
part selected to supervise the battery voltage was the TPS3803-01 [40]. Th is chip has an
internal reference, a difference amplifier, and an active low open drain output. This device
uses an in credible 2µA to operate. However, the input sensing voltage maxes out at 6V.
The device also has a 30mV hystorisis. Both these apparent drawbacks were resolved by
using the TPS3803-01 as a part of a hystorisis generating circuit. This circuit modifies the
input voltage and adds a variable amount of swing to the voltages to trip the supervisor’s
reset signal. The total circuit u ses 4.7µA.
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Single-Ended Primary Inductor Circuit.