MIC1557BM5 MICREL [Micrel Semiconductor], MIC1557BM5 Datasheet - Page 6

• Manufacturer Part Number: MIC1557BM5
• Manufacturer: MICREL [Micrel Semiconductor]

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 (C
connected to the output of an MIC1555, the trip point is
approximately 1.1RC
cycle of an astable circuit.) If the trigger pulse of an MIC1555
remains low longer than the output pulse width, short oscilla-
tions 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 (D
in parallel with (R
The period of a monostable circuit is:
where:
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 compara-
tor inputs, threshold and trigger (T/T), tied together internally.
Chip select (CS) is brought out to allow on-off control of the
oscillator.
MIC1555/1557
1
⁄
3
V
t = k
t = period (s)
k
R = resistance ( )
C = capacitance (F)
IN
2
Trigger
= constant [from Typical Characteristics graph]
2
RC
R
Figure 1. One-Shot Diagram
C
PU
IN
C
T
T
) resets the one-shot quickly.
T
) is charged through a resistor (R
THR
TRG
T
D
T
(the same time as the initial power-on
5
4
R
T
MIC1555
2.7V to 18V
GND
VS
OUT
CS
1
2
OUT
3
t
ON
= 1.1R
T
C
T
T
T
)
)
6
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 (R
grounded timing capacitor, (C
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
voltage ramps down toward ground while the output is low.
When the ramp is below
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.
multiplier increases as capacitance is decreased, and propa-
gation 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
(R
facing diode connected in parallel. The frequency is then
made up of two components, the charging time (t
discharging time (t
frequency is the reciprocal of the sum of the two times t
so the total time is 1.4R
after power-on or CS enable, is lengthened since the capaci-
tor is charging from ground instead of the 1⁄3 input trigger trip
voltage, to 1.1RC, the same as a monostable pulse.
The MIC1555 or MIC1557 can be used to construct an
oscillator.
The frequency of an astable oscillator is:
where:
A
), and a second resistor (R
f = frequency (Hz)
k
R = resistance ( )
C = capacitance (F)
f
1
= constant [from Typical Characteristics graph]
k RC
T/T
1
1
1
C
100pF to 10µF
T
MIC1557
Figure 2. Oscillator Diagram
1k to 1M
R
R
A
B
B
At frequencies above 1MHz the RC
) t
T
C
) is connected from the output to a
B
T
2.7V to 18V
GND
A
2
C
⁄
= 0.7R
VS
3
T
1
of the input voltage. The junction's
OUT
⁄
. The first half-cycle of an astable,
3
CS
4
2
of the input voltage, the output
T
B
) the voltage at their junction
A
CS
OUT
3
5
) in series with an opposite
C and t
R
t = 0.7(R
E
t
B
= 0.7R
A
+R
B
October 1998
)C
T
A
B
) and the
C. The
Micrel
A
+ t
B
,