Since its invention more than half a century ago by Hanz Kamenzind at Signetics, the famous 555 analog timer (in league with its updated pin-compatible CMOS descendants) has become an iconic design element included in useful standardized function blocks, almost too much for to be counted. The list includes unstable, bistable, monostable, voltage converters, square waves, triangular waves, saw teeth, V-to-Fs, even PWM amplifiers.

Figure 1 illustrates one of these classics: Constant frequency oscillator with continuous cycle, continuously variable from 0% to 100% through single pot P1.

Figure 1 Diode D1 causes temperature dependence in this circuit of timer 555.

It works as the DISCHARGE pin with open drain on 555 ramp C1 down through R1 (upper half of vessel P1) during the T-half of the output cycle, and diode D1 directs C1 increasing current from 555 OUTPUT pin through R2 (lower half) of sweat P1) during the half-life of the positive output T +, which leads to …

t= R1C1 ln (2 / 3V+) / / (1 / 3V+ )) = R1C1 ln (2)
t+ = R2C1 ln (2 / 3V+ – INe) / / (1 / 3V+ ))
Fosc = 1 / (R1C1 ln (2) + R2C1 ln (2 / 3V)+ – INe) / / (1 / 3V+ )))

Unfortunately, the last two equations differ from the usual elegant 555 V + and temperature-independent mathematics for time due to the effects of decreasing diode voltage, Vd = ~ 700mV – 2mV /Fr.C for a typical silicon planar transition diode such as 1N4148. Therefore, the time intervals change in response to variations in both temperature and V + power supply. The magnitude of the changes varies inversely with the nominal V +…

… And may be acceptable for some non-critical applications, but probably not when precision is important.

Figure 2 shows correction: P-channel MOSFET with inverted polarity Q1 for directing the charging current of C1 without significant voltage drop and thus introduces new equations for design of temperature and V +, independent of time …

t+ = R1C1 ln (2 / 3V+) / / (1 / 3V+)) = R1C1 ln (2)
t= R1C1 ln (2 / 3V+) / / (1 / 3V+)) = R1C1 ln (2)
Fosc = 1 / (ln (2) /) (R1 + R2 (C1)) = 1.44 / (P1C1)

Figure 2 Reverse polarity MOSFET Q1 restores timer accuracy.

The variable duty cycle / constant frequency function is now achieved without compromising the inherent accuracy of the 555.

The reason Q1 is connected “inverted” to its drain pin instead of the source pin connected to the positive voltage source (contrary to common p-channel FET practice) is to avoid diverting the FET body diode when the output pin 555 drives the drain pin is low while C1 holds the pin connected to R2 high. An additional advantage is that since the body diode is pointed forward when the output becomes high, we guarantee that the FET output pin will be driven positively relative to the port output and the FET will go into solid saturation during the T + interval.

Interestingly, a version of the chain in Figure 1 is described on page 429 of the third edition of Horowitz and Hill.The art of electronics”“Her art”Shows a Schottky diode instead of a junction type for D1 due to the lower Vd of the first. This is a good idea, as the V + error member is significantly improved, but because the Schottkis temperature coefficient (-2mV /Fr.C) is similar to that of connecting diodes, the temperature error remains almost unchanged:

Stephen Woodward ‘s relationship with EDNThe DI’s column goes back a long way. A total of 64 proposals have been accepted since the publication of his first contribution in 1974.

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