Scalable latch requires no capacitors or clock

Fairchild BF256C

Robert Most

EDN

Circuits that latch one-of-N switches usually use a digital approach. Such circuits are often useful in human-interface situations, such as audio-mixing consoles, video-feed selection, or current-loop-control redundancy systems. In high-precision analog systems, such as high-fidelity audio or video, elimination of clocking circuits, whenever possible, reduces the chances of coupling noise back into the signals of interest. The circuit in Figure 1 is a scalable one-of-N latch that has the advantages of no clock and no capacitors, and it has an intrinsic "active-channel" indication.

This one-of-N latching circuit uses no clock or capacitors. The LEDs double as channel indicators.
Figure 1. This one-of-N latching circuit uses no clock or capacitors. The LEDs double as channel indicators.

The operating principle of this circuit is based on current steering. A current sink comprising an n-channel JFET, the BF256C, provides approximately 5-mA current draw, which is approximately the hold current of any one of the small-signal SCRs (silicon-controlled rectifiers). When you select a channel by momentarily depressing the corresponding switch, the associated SCR turns on, lighting the LED connected to its cathode and providing a logic one on the CHANx line. The SCR automatically latches the selected line until you depress another channel switch. When you actuate any other channel switch, the corresponding SCR latches, releasing the previous channel. This latch behavior is the result of the inability of the current sink to draw enough current to sustain more than one SCR at a time.

The circuit needs blocking diodes to isolate the cathodes of the SCRs. If you use an LED, as in Figure 1, it also doubles as an "active-channel" indicator. You can the use the CHANx line to select an analog switch, a mechanical relay, or another device. Scalability is straightforward: Additional channel sections require only the momentary switch, a 1.5-kΩ resistor, an SCR, and a diode. Because this method uses a current-based scheme, the use of long lines is not an issue. Channel sections can be across control rooms. Upon initial power-up, all channels are in the off-state. In addition, Figure 1 shows an optional reset-all pushbutton. Depressing the reset-all switch steers the entire current sink's capacity through this switch, thus delatching any SCR that was active.

A current mirror replaces the JFET in Figure 1 and allows the choice of single or dual channels.
Figure 2. A current mirror replaces the JFET in Figure 1 and
allows the choice of single or dual channels.

You can substitute other n-channel JFETs, but you must accordingly scale the drain current of the chosen JFET by varying the value of R1. You can easily modify this circuit to allow more than one SCR to be active at a given time. For a multiple-active-channel system, you must set the current level to activate only the number of SCRs desired and no more. For example, a system with two active channels would require a current level of 8 to 10 mA in the current sink. Figure 2 shows a possible alternative current sink, replacing the JFET and R1, for this arrangement. If you use indicating LEDs in a multiple-channel arrangement, you should take proper precautions with maximum LED current.

Materials on the topic

  1. Datasheet Fairchild BF256C
  2. Datasheet Littelfuse TCR22

EDN

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