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Rev. 1.0 1/15 Copyright © 2015 by Silicon Laboratories AN856 AN856 SILICON L ABS ’ TS3004 VS . THE CMOS 555: D ETERMINING LOWEST S UPPLY C URRENT FOR B ATTERY -P OWERED A PPLICATIONS 1. Introduction The 555 timer is the workhorse of ICs, with close to a billion of them manufactured every year. Introduced in 1972, the 555 is still in widespread use because of its ease of use, reasonable price, and good stability. It can be found in a wide variety of applications for oscilla tion, timing and pulse generation. But what if you ne ed a timer IC for ultra- long life, low-frequency battery-powered/portable applicatio ns where a low supply current is a requirement? Is the CMOS555 timer your best option? In this application note, we will compare the CMOS555 timer to the Silicon La bs’ TS3004, a single-supply timer IC fully specified to operate over a supply volta ge range of 1.55 V to 5.25 V while consuming 1.9 μA supply current. Requiring only a resistor to set the base output freque ncy (or output period) with a 50% duty cycle, the TS3004 timer/oscillator is compact, easy-to-use, and versatile. 2. TS3004 vs. the CMOS555 The TS3004 timer has an output frequency range of 0. 0043 Hz to 300 kHz while maintaining a very low supply current. To demonstrate just how low the supply current is for the TS3004, we will compare the supply current consumption of the TS3004 to a CMOS555 timer side-by-side, with an output frequency of 1 Hz for both timers. For this example, a Texas Instruments LMC555 is used as the CMOS version of the industry-standard 555 series of general-purpose timers. The LMC555 offers 1.5 V s upply operating voltage and requires a single external resistor and a capacitor. First, the output frequencies of bo th the TS3004 and CMOS555 were set to 1 Hz. To set the TS3004 output frequency to 1 Hz, the appropriate Frequency Divider In put setting must be selected as well as the necessary RSET resistance. The equation defining the FOUT frequenc y is found in the data sheet. As shown in Equation 1, FOUT is defined by only one external component, the RSET resistor. Equation 1. The Frequency Divider Input table in the data sheet shows that the correct setting to obtain a 1 Hz output frequency is 101, which is equivalent to 5. By substituting the appropriate value of 5 for FDIV2:0 into Equation [1] and using an output frequency of 1 Hz, the appropriate RSET value of 3.3 M can be calculated. To set the output frequency of the CMOS555 timer to 1 Hz, the oscillati ng frequency is defined by two external components, a “C” capacitor, and an “R C” resistor. The equation defining the output frequency is found in the LMC555 data sheet. Equation 2. FOUT 1.08 10 11 8FDIV2:0 RSET f 1

component instead of the two external components required by the CMOS555. currents were recorded. Table 1 displays the results. compact replacement for crystal and ceramic oscillators. Table 1. Supply Current per Supply Voltage

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