The circuit shown in Figure 1 steals energy from a PC's parallel port or from any limited-energy source. To use the available energy, such power-conversion applications usually require very high efficiency. For example, a parallel data port supplies as much as 2.6 mA per data line at 2.4 V. When the eight data lines are software-configured to FF hex (all high), that condition yields an input power of 50 mW. The system can then use the four bidirectional control-port lines and the five status-port inputs as an alternative means of communication via the PC's parallel port.
As shown, one Schottky diode per data line protects against possible errors. (Without diodes, the lines would short each other unless all were in the same state.) BAT54C devices contain two diodes per package but exhibit a forward voltage drop of 0.3 V. As an alternative, RB411D or ZHCS500 devices drop only 170 mV but contain only one diode per SOT-23 package.
An input voltage (logic high) of 2.4 V leaves only about 2 V for supplying the step-up circuit. And, most such circuits (operating under low input voltage and low power) offer efficiencies no greater than 85%. However, by shunting the free-wheeling output diode with a synchronous p-channel MOSFET, IC1 yields an efficiency as high as 89% for the conditions of 2-V input, 5-V output, and 10 mA of output current. To gain a few more efficiency points, carefully select the passive components.
The power inductor has a saturation current much higher than the operational current, so its ferrite loss is negligible. A good choice is the 15-µH RCR110D (Sumida), which adds a low series resistance of 36-mΩ maximum with a saturation current greater than 2.88 A. Then, you can reduce loss in the input and output capacitors (C1 and C4) by selecting very low-ESR devices. Values of 470 µF give them a comfortable reservoir capacity. Plus, you can minimize the ESR by oversizing their voltage ratings. For example, the 470-µF type ZLH from Rubycon (despite being an affordable aluminum electrolytic) shows an ESR better than 45 mΩ when rated at 25 V.
For supplementary protection, the 22-Ω resistor, R4, limits input current to 100 mA during startup. Note that R4 is shunted by Q1 as soon as enough voltage is detected by the 100-kΩ/100-kΩ divider (R2/R3). Even in an SC-70 package, Q1 reduces the loss after startup to only 90 µW. A 220-kΩ gate resistor (R1) allows decent reaction times for an added loss of only 26 µW.
Adding two 2.2-µF capacitors (C2 and C3) near the MAX1796 eliminates high-frequency spikes, and X5R dielectric material enables 10-V-rated devices to fit in an 0805 (TDK) package. As shown in Figure 2, all of these measures add crucial amounts of efficiency.
For a worst-case input voltage of 2.1 V, and without diodes, the circuit's efficiency is about 92%. At 3.3 V, the efficiency reaches 94.4%, which is unusual for a low-power application like this one. Using BAT54C protection diodes drops the efficiency to 80% at 2.4 V, yielding a typical output power of 40 mW. If this is insufficient, increase the available power to 42.75 mW by substituting the lower-voltage (RB411D) diodes.