470 μF — generates a C1 peak-to-peak ripple
voltage of about 1.4 V at its rated output current
of 100 mA. The circuit in Figure 2 (C1 = 2,200
μF) generates a C1 peak-to-peak voltage of about
2.4 V at its rated output current of 750 mA, and
the circuit in Figure 3 (C1 = 22,000 μF) generates
a C1 peak-to-peak voltage of about 1.6 V at its
rated output of 5 A. At the output of each circuit,
these ripple values are reduced by about 80 dB by
the action of IC1.
When using TTL, all IC input signals must —
unless the IC is fitted with a Schmitt-type
input — have very sharp rising and falling edges
(typical rise and fall times should be less than 40
nS on LS TTL, for example). If rise or fall times
are too long, they may allow the input terminal
to hover in the TTL element’s linear indeterminate zone (see last month’s article) long enough
for the element to burst into wild oscillations
and generate spasmodic output signals that may
disrupt associated circuitry (such as counters
If necessary, slow input signals can be converted into fast ones by feeding them to the IC’s
input terminal via an inverting or non-inverting
Schmitt element, as shown in Figure 4. In practice,
these simple circuits can be cheaply implemented
by using a 74LS14 (or similar) IC that houses six
Schmitt inverter elements. Two of these elements
can be cascaded to make one non-inverting
Schmitt element. All unused elements should be
disabled by connecting their inputs directly to the
0 V rail (see the next paragraph for a deeper
Unused TTL input terminals should never be
allowed to simply float, since this makes them
susceptible to noise pick-up, etc. Instead,
they should be tied to definite logic levels, by
connecting them to VCC via a 1K resistor, by
shorting them directly to the ground rail, or
by connecting them to a TTL input or output
terminal that is already in use.
Figure 5 shows examples of the four options.
The simplest option is to tie the unused input to
VCC via a 1K resistor, as shown in Figure 5(a). This
resistor has to supply only a few microamperes of
current (IIH) to each input, and can thus easily
drive up to 10 unwanted inputs. Alternatively, the
input can be tied directly to ground, as in Figure
5(b), but in that case, an input current of several
hundred microamperes (IIL) may flow to the
ground rail via the input.
If the unwanted input is on a multi-input gate,
FIGURE 1. A 5 V regulated DC supply ( 100 mA maximum output).
FIGURE 2. A 5 V regulated DC supply (750 mA maximum output).
FIGURE 3. A 5 V regulated DC supply ( 5 A maximum output).
FIGURE 4. Slow input signals can be converted into fast ones via (a)
an inverting or (b) non-inverting Schmitt element.
FIGURE 5. Alternative
ways of connecting
unwanted TTL inputs
September 2006 61