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Fig.1: this is the standard circuit for a monostable using a 555. Fig.2: the standard circuit for an astable timer, is unchanged when using the ZSCT1555. A new 555 timer – it operates down to 0.9V The ubiquitous 555 timer has for most of its life been manufactured in greater volumes than any other linear IC. Zetex has now produced a new variant of the timer chip which is even more versatile and pulls less power than a CMOS version. By LEO SIMPSON When it was first introduced by Signetics back in the early 70s, the 555 seemed like a solution looking for a problem. Given the job of producing a circuit for it back then, as I worked for “Electronics Australia”, I racked my brains until I came up with a photo-timer (published in May 1973). Now, it seems inconceiv­able that such a versatile device as the 555 could have been regarded in this way. It is now acknowledged by many as one of the most successful ICs, perhaps only equalled in fame by the 741 op amp. The success of the 555 can be attributed to its flexibili­ty, performance and its ability to satisfy the timing require­ments of a huge number of ap76  Silicon Chip plications. Over the years the origi­nal 555 has been supplemented by CMOS versions which operate on much lower current and at lower voltage. The new 555 timer from Zetex takes the performance to the next level – operation from a single cell, guaranteed operation down to 0.9V and bipolar technology. To help with the design, Zetex turned to its long-term associate Hans Camenzind, of Array Design, California. He was responsible for the original 555 produced by Signetics. The new version of the timer, called the ZSCT1555, has the same pin-outs as the original and with the simple adjustment of external components to set the frequency, its function is just the same. As an example, the following equations are used to cal­culate the values of the external components for the familiar monostable and astable circuits as shown in Fig.1 & Fig.2. Monostable: time t = 1.63RAC Astable: frequency f = 0.62/(RA + 2RB)C As already noted, the most significant advantage of the new 555 timer is its guaranteed operation down to 0.9V; better than any CMOS alternatives. This means that it can work with a single cell and still give quite good battery life (ie, down to 0.9V). And even though the ZSCT1555 is a bipolar device, it has a lower current consumption than a CMOS version. Assuming a 5V supply, a typical CMOS 555 device (eg, 7555) draws 170µA while the new Zetex device pulls 140µA, and at 1.5V just 75µA. In addition, the output sink current is better than that of CMOS versions, up to a maximum of 100mA. Output source current is 150µA (maximum). Maximum supply voltage is 6V. Thermal performance is improved too, with a better tempera­ture coeffi- SUPPLY CURRENT (µA) cient and an operating temperature range of -20°C to 100°C. The graph in Fig.3 shows the ZSCT­ 1555 quiescent current versus supply voltage characteristic. As you can see, the current consumption is very low for supply voltages below 1V – ie, below 60µA – and this further extends battery life. Of course, the actual current drain from a timer circuit will depend on the timing components and the loading conditions at the output, pin 3. Single cell boost converter Given that the ZSCT1555 will run from a single cell, it is appropriate that it can also function as the heart of a single cell boost converter. For any portable, battery-powered applica­ tion, extended battery life is not the only consideration. Reduc­ing size and weight is also very important. Until recently it has been usual for portable circuits to operate with up to six cells (9V). With this circuit, only one cell is required. As shown, the circuit is set to deliver a nominal 5V. The ZSCT1555 generates the required 150kHz signal for the PWM circuit while diode D1 allows for very short pulses to be delivered. Inductor L2, transistor Q3 and Schottky diode D3 provide the main boost converter circuit while L1, D2 and Q2 provide an active speed-up to the base drive to Q3. This minimises switching losses. The transistor specified for Q3 has very low saturation voltage, equating to an on-resistance of only 30mΩ at 300mA, which further optimises circuit efficiency. The output voltage is regulated by the circuit involving U3, a shunt regulator, and Q4 which modulates the SUPPLY VOLTAGE (V) Fig.3: very low current drain and low voltage operation are the big advantages of the ZSCT1555 version of the venerable 555 timer. Looking for more information on the ZSCT1555 timer IC? Point your browser to www.zetex.com control input on U2, the ZSCT1555. Acknowledgement: this article is based on an application note pub- lished by Zetex plc, UK. The ZSCT­ 1555 is available from Farnell ComSC ponents. Phone 1300 361 005. Fig.4: capitalising on its low voltage operation, this boost circuit produces 5V from a single cell. April 2001  77