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MIC1555YM5 Datasheet(PDF) 7 Page - Micrel Semiconductor |
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MIC1555YM5 Datasheet(HTML) 7 Page - Micrel Semiconductor |
7 / 10 page Micrel, Inc. MIC1555/1557 May 2010 7 M9999-052610-D 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. CS OUT THR TRG VS GND DT RT OUT CIN RPU CT MIC1555 Trigger 1 3 VS tON = 1.1R T CT 2.7V to 18V 1 3 2 4 5 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. RA 1k to 1M CS OUT T/T VS GND OUT MIC1557 2.7V to 18V CB RE C T 100pF to 10µF 5 3 2 4 1 CS t = 0.7( RA+RB)CT t RB Figure 2. Oscillator Diagram |
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