MIC1555 View Datasheet(PDF) - Micrel
Part Name
Description
Manufacturer
MIC1555 Datasheet PDF : 10 Pages
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Micrel, Inc.
MIC1555/1557
Application Information
Basic Monostable (One-Shot) Circuit
A monostable oscillator produces a single pulse each
time that it is triggered, and is often referred to as a
“one-shot”. The pulse width is constant, while the time
between pulses depends on the trigger input. One-shots
are generally used to “stretch” incoming pulses, of
varying widths, to a fixed width. The IttyBitty® MIC1555
is designed for monostable operation, but may also be
connected to provide astable oscillations. The pulse
width is determined by the time it takes to charge a
capacitor from ground to a comparator trip point. If the
capacitor (CT) is charged through a resistor (RT)
connected to the output of an MIC1555, the trip point is
approximately 1.1RCT (the same time as the initial
power-on cycle of an astable circuit.) If the trigger pulse
of an MIC1555 remains low longer than the output pulse
width, short oscillations may be seen in the output of a
one-shot circuit, since the threshold pin has precedence
over the trigger pin. These occur since the output goes
low when the threshold is exceeded, and then goes high
again as the trigger function is asserted. AC coupling the
input with a series capacitor and a pull-up resistor, with
an RC time constant less than the pulse width, will
prevent these short oscillations. A diode (DT) in parallel
with (RT) resets the one-shot quickly.
2.7V to 18V
RT
DT
VS 1
RPU
THR
5
tON = 1.1R T CT
CS
Trigger
1 3VS
CIN
CT
TRG
4
OUT
MIC1555
GND 2
OUT
3
Figure 1. One-Shot Diagram
The period of a monostable circuit is:
t = k2 RC
where:
t = period (s)
k2 = constant [from Typical Characteristics
graph]
R = resistance (Ω)
C = capacitance (F)
Basic Astable (Oscillator) Circuits
An astable oscillator switches between two states, “on”
and “off”, producing a continuous square wave. The
IttyBitty® MIC1557 is optimized for this function, with the
two comparator inputs, threshold and trigger (T/T), tied
together internally. Chip select (CS) is brought out to
allow on-off control of the oscillator.
The MIC1555 may also be used as an astable oscillator
by tying the threshold and trigger pins together, forming
a T/T pin. If a resistor (RT) is connected from the output
to a grounded timing capacitor, (CT) the voltage at their
junction will ramp up from ground when the output goes
high. If the T/T pin is connected to this junction, the
output will switch low when the ramp exceeds 2/3 of the
input voltage. The junction's voltage ramps down toward
ground while the output is low. When the ramp is below
1/3 of the input voltage, the output switches to high, and
the junction ramps up again. The continuing frequency of
an MIC1555/7 astable oscillator depends on the RC time
constant, and is approximately 0.7/RC below 1MHz. At
frequencies above 1MHz the RC multiplier increases as
capacitance is decreased, and propagation delay
becomes dominant. Non-symmetrical oscillator operation
is possible at frequencies up to 5MHz.
If a duty cycle other than 50% is desired, a low-power
signal diode may be connected in series with the timing
resistor (RA), and a second resistor (RB) in series with an
opposite facing diode connected in parallel. The
frequency is then made up of two components, the
charging time (tA) and the discharging time (tB) tA=
0.7RAC and tB= 0.7RBC. The frequency is the reciprocal
of the sum of the two times tA + tB, so the total time is
1.4RTCT. The first half-cycle of an astable, after power-
on or CS enable, is lengthened since the capacitor is
charging from ground instead of the 1/3 input trigger trip
voltage, to 1.1RC, the same as a monostable pulse.
2.7V to 18V
CB
VS 4
RE
T/T
CS
1
CS 3
t
OUT
OUT
5
t = 0.7( RA+RB)CT
MIC1557
RA
GND 2
1k to 1M
RB
CT
100pF to 10µF
Figure 2. Oscillator Diagram
May 2010
7
M9999-052610-D
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