AN856 Datasheet, PDF (1/3 Pages) Silicon Laboratories – LOWEST SUPPLY CURRENT FOR BATTERY-POWERED APPLICATIONS

AN856
SILICON LABS’ TS3004 VS. THE CMOS 555: DETERMINING
LOWEST SUPPLY CURRENT FOR BATTERY-POWERED APPLICATIONS
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 oscillation, timing and pulse generation. But what if you need a timer IC for ultra-
long life, low-frequency battery-powered/portable applications 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 Labs’ TS3004, a single-supply timer IC
fully specified to operate over a supply voltage 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 frequency (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 supply operating voltage and requires a single external
resistor and a capacitor.
First, the output frequencies of both the TS3004 and CMOS555 were set to 1 Hz. To set the TS3004 output
frequency to 1 Hz, the appropriate Frequency Divider Input setting must be selected as well as the necessary
RSET resistance. The equation defining the FOUT frequency is found in the data sheet. As shown in Equation 1,
FOUT is defined by only one external component, the RSET resistor.
FOUT = ------¹----^.-⁰---⁸-----^-----¹---⁰----¹---¹------
8^FDIV2:0  R_SET
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 oscillating 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.
f = ---------------¹-----------------
1.4  C  R_C
Equation 2.
Rev. 1.0 1/15
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AN856