What is a latch-up problem in CMOS

Keyword: latchup effect

UPDATE: pull-up, pull-down resistance, open collector, wired OR, latch-up effect

In this revised electronics mini-course, we deal with pull-up and pull-down resistances in the use of digital circuits. It's about the old TTL and the modern CMOS input stages. Because the brief review of the TTL era also includes the then modern, faster and reduced-power low-power Schottky version (LS-TTL), what is next to the Schottky diode, the Schottky transistor is also briefly explained.

After that, the focus is on CMOS. Eliminating and correcting interference problems are an issue. Further contents are unused logic inputs, open collector outputs and the wired-OR link on a practical example with light sensors.

The topic of what happens when a CMOS circuit is connected to active input signals and the operating voltage is missing, e.g. due to a local interruption of the power supply or simply by switching off, is completely new. This is presented using a practical, comprehensible experiment, e.g. with a plug-in board.

Greetings to you
ELKO Thomas


New CMOS, electrostatic influence, latch-up effect, LSTTL, pull-down resistance, pull-up resistance, Schmitt trigger, Schottky diodes, Schottky transistors, TTL, unused logic inputs (CMOS), wired-OR


A TRANSIENT PULSE CONVERTER? what could that be? It is a circuit that uses a square-wave signal generator to test digital circuits with pulses. But let's start with what a transient is. There are different explanations. The most common idea is that of the steep edge of a pulse, but by no means the slow charging or discharging of a capacitor. But why not, because transient simply has to do with walking through. A voltage, starting from one fixed voltage value (e.g. GND) to another (e.g. + 5V) or in the opposite direction, is a transient voltage. The Latin language derives the term transient from transire, and this means something like to traverse or to pull through. This also applies to Figure 1. The capacitor is charged and discharged with a time constant of 1s. The rising and falling edges of Ua are transients. Likewise, of course, the steep edges of the square-wave voltage at Ue. In principle, the TRANSIENT-PULSE-CONVERTER is nothing more than generating a pulse from a rising or falling voltage edge (transient) and for this you need, for example, a monoflop, as shown in Figure 2. The pulse itself naturally consists of two voltage edges (transients).

Figure 3 shows two types of monoflop. To make it easier to understand, these two monoflops are implemented quasi-discretely with HCMOS gates and HCMOS inverters and explained accordingly in the mini course. It has to do with the fact that one circuit is called a monoflop and the other is called a one-shot, although both do (almost) the same thing. In a figurative sense, they give off a shot (impulse) at the push of a button (trigger, shutter release). Nevertheless, there is a functional difference between the two circuits which, depending on the application, can have an important meaning.

The circuit of the TRANSIENT-PULSE-CONVERTER, with some additional functions, is the product of the time when the TTL logic was of great importance. This circuit is described and can be reproduced. The LS-TTL-ICs are all still available, evaluated in three electronics distributors. Figure 3 shows the one-shot with HCMOS ICs. The one-shot in HCMOS is suitable for replacing the TTL monoflop 74LS221 with the advantage that the minimum pulse time that can be set is 20 ns instead of 40 ns. It is up to the interested reader to modernize the entire circuit in HCMOS instead of just recreating it.

Figure 4 shows how easy a logic level conversion can be from the output TTL or HCMOS logic level to the even more modern LVCMOS logic (+ Ub = 3.3 VDC). The TTL output even works with just one resistor (part 4.1). This internal TTL circuit of an inverter (7404) offers modern trainees and students a brief insight into part of everyday digital electronics in the 1970s. The age of CMOS began in 1980.

Have fun and greetings from
ELKO Thomas

New CERMET trim pot, CMOS, HCMOS, latchup effect, LSTTL, LVCMOS, monoflop, one-shot, transient, TTL

EXOR logic with IC or transistors (BJT) - monitoring of motorcycle lamps

The trigger for this new electronics mini-course for the section "DIGITAL CIRCUITS: BASICS, SMALL APPLICATIONS" was the desire of a motorcyclist to see on the display board with just one additional LED whether the dipped beam of the headlight and / and the left or right indicator lamp is active. This works quite easily with the use of an exclusive-or circuit, also called EXOR or simply XOR. This prompted me to focus on the property of the XOR gate first and then I noticed that the integrated AND, NAND, OR and NOR gates have ICs with multiple inputs, but not with XOR and XNOR Gate. There are only gates with two inputs. The reason is that an XOR or XNOR with only three inputs does not work 100% correctly. This is discussed in the introduction.

In the next step we deal with the task and the XOR logic. It is shown briefly how an XOR gate is implemented with four NAND gates and how an XOR gate can also be useful as a controllable inverter. Two circuit variants are presented in the following chapters. One variant with the use of a CMOS IC with four XOR gates, two of which are used. The alternative variant is a discrete XOR circuit with four transistors (BJT). This circuit is especially suitable for higher operating voltages if you don't want to use voltage stabilization.

There are significant faults in cars or motorbikes that are associated with overvoltages. To do this, we examine a test application note from SGS-Thomson and use this as a reference to implement a protective environment for the two XOR circuits. With the CMOS variant, of course, the latchup effect is an issue.

At the very end there is a note that there are legal provisions for self-assembly on mopeds, motorcycles or cars, which you should inform yourself about as a precaution ...

Greetings to you
ELKO Thomas

New exor, latch-up effect, pull-up resistance, controllable inverter, tvs diode, xnor, xor