The bipolar NE555 timer IC is widely used in inductorless dc-dc converters, most frequently in doubling and inverting converters. However, another very popular IC, the LM386 audio amplifier, may be a better solution in this application. Note that the results also depend on the specific manufacturer of these multisourced ICs and on the quality of the related components. (We will use only Schottky diodes, to reduce the voltage losses to the minimum.)
Basic Comparison of NE555 and LM386
The full power-supply range of NE555 extends from 4.5 to 16 V, but its use near the maximum supply value with the maximum specified current of 200 mA, and at high frequency, can be a problem. The full power-supply range of LM386N1 is from 4 to 15 V (with a working range of 4 to 12 V) and the full supply range of LM386N4 is 4 to 22 V (working range of 5 to 18 V). Thus, the LM386N4 has an advantage over the NE555 because it can work with higher supply voltage. The quiescent current of NE555 is typically 3 mA (6 mA maximum) and that of the LM386 is typically 4 mA (8 mA maximum); here, the NE555 has a small advantage.
The maximum output current of the NE555 is specified at 200 mA, but the voltage drops over the output transistors are around 2 V at ±100 mA, which makes use of the IC at higher currents questionable. In comparison, the maximum output current of LM386 isn’t specified, but it’s much higher compared to NE555 because the LM386N1 typically provides 0.7-W output with a VCC of 9 V and load RL of 8 Ω, while the LM386N4 provides 1 W (typical) with VCC at 16 V and RL of 32 Ω. (These results are based on the classic formulas for Class AB amplifiers using the maximum peak-to-peak output voltage and peak output current.)
The maximum power dissipation of NE555 in a DIP8 package is only 600 mW, while the comparable specification for the LM386 is 1.25 W; here, the audio amplifier has considerable advantage compared to the timer. The maximum junction temperature of NE555 isn’t explicitly specified in the datasheet; for the LM386, that parameter is 150°C. The junction-case thermal resistance TJC of LM386 is 37⁰C/W, but for NE555 that parameter isn’t specified.
For our tests, the power-supply VCC is 10 V. Since the analysis of these ICs as dc-dc converters will be performed at around 25 kHz (T = 40 μs), which is much lower than their maximum possible operating frequencies, there’s no need to compare switching speeds, slew rates, and related factors. In general, it’s a good idea to use them at below approximately 50 kHz (T = 20 μs).