SNAS558M February 2000 – July 2016 LMC555
The LMC555 is a CMOS version of the industry standard 555 series general-purpose timers. In addition to the standard package (SOIC, VSSSOP, and PDIP) the LMC555 is also available in a chip-sized package (8-bump DSBGA) using TI’s DSBGA package technology. The LMC555 offers the same capability of generating accurate time delays and frequencies as the LM555 but with much lower power dissipation and supply current spikes. When operated as a one-shot, the time delay is precisely controlled by a single external resistor and capacitor. In the astable mode, the oscillation frequency and duty cycle are accurately set by two external resistors and one capacitor. The use of TI’s LMCMOS process extends both the frequency range and the low supply capability. The LMC555 is available in an 8-pin PDIP, SOIC, VSSOP, and 8-bump DSBGA package.
The LMC555 offers the same capability of generating accurate time delays and frequencies as the LM555 but with much lower power dissipation. A power dissipation of less than 0.2 mW can be achieved with a 1.5-V operating supply voltage and less than 1 mW with a 5-V operating supply voltage. The use of TI’s LMCMOS process allows this low supply current and voltage capability. Reduced supply current spikes during output transitions and extremely low reset, trigger and threshold currents also provide low power dissipation advantages with the LMC555.
There are various packages available for use of the LMC555. In addition to the standard package (8-pin SOIC, VSSOP, and PDIP, the LMC555 is also available in a chip-sized package (8-bump DSBGA). The PDIP, SOIC, and VSSOP packages for the LMC555 are pin-for-pin compatible with the 555 series of timers (NE555/SE555/LM555) allowing flexibility in design and unnecessary modifications to PCB schematics and layouts.
The LMC555 can operate in both astable and monostable mode depending on the application requirements.
In this mode of operation, the timer functions as a one-shot (Figure 3). The external capacitor is initially held discharged by internal circuitry. Upon application of a negative trigger pulse of less than 1/3 VS to the Trigger terminal, the flip-flop is set which both releases the short circuit across the capacitor and drives the output high.
The voltage across the capacitor then increases exponentially for a period of tH = 1.1 RAC, which is also the time that the output stays high, at the end of which time the voltage equals 2/3 VS. The comparator then resets the flip-flop which in turn discharges the capacitor and drives the output to its low state. Figure 4 shows the waveforms generated in this mode of operation. Because the charge and the threshold level of the comparator are both directly proportional to supply voltage, the timing internal is independent of supply.
Reset overrides Trigger, which can override threshold. Therefore the trigger pulse must be shorter than the desired tH. The minimum pulse width for the Trigger is 20 ns, and it is 400 ns for the Reset. During the timing cycle when the output is high, the further application of a trigger pulse will not effect the circuit so long as the trigger input is returned high at least 10 µs before the end of the timing interval. However the circuit can be reset during this time by the application of a negative pulse to the reset terminal. The output will then remain in the low state until a trigger pulse is again applied.
When the reset function is not use, it is recommended that it be connected to V+ to avoid any possibility of false triggering. Figure 5 is a nomograph for easy determination of RC values for various time delays.
In monstable operation, the trigger should be driven high before the end of timing cycle.
If the circuit is connected as shown in Figure 6 (Trigger and Threshold terminals connected together) it will trigger itself and free run as a multivibrator. The external capacitor charges through RA + RB and discharges through RB. Thus the duty cycle may be precisely set by the ratio of these two resistors.
In this mode of operation, the capacitor charges and discharges between 1/3 VS and 2/3 VS. As in the triggered mode, the charge and discharge times, and therefore the frequency are independent of the supply voltage.
Figure 7 shows the waveform generated in this mode of operation.
The charge time (output high) is given by
And the discharge time (output low) by:
Thus the total period is:
The frequency of oscillation is:
Figure 8 may be used for quick determination of these RC Values. The duty cycle, as a fraction of total period that the output is low, is: