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MIC1555BM5 Datasheet(PDF) 6 Page - Micrel Semiconductor |
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MIC1555BM5 Datasheet(HTML) 6 Page - Micrel Semiconductor |
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6 / 10 page  Micrel, Inc. MIC1555/1557 May 2010 6 M9999-052610-D Functional Description Refer to the block diagrams. The MIC1555/7 provides the logic for creating simple RC timer or oscillator circuits. The MIC1555 has separate THR (threshold) and TRG (trigger) connections for monostable (one-shot) or astable (oscillator) operation. The MIC1557 has a single T/T (threshold and trigger) connection for astable (oscillator) operation only. The MIC1557 includes a CS (chip select/reset) control. Supply VS (supply) is rated for +2.7V to +18V. An external capacitor is recommended to decouple noise. Resistive Divider The resistive voltage divider is constructed of three equal value resistors to produce 1/3VS and 2/3VS voltage for trigger and threshold reference voltages. Chip Select/Reset (MIC1557 only) CS (chip select/reset) controls the bias supply to the oscillator’s internal circuitry. CS must be connected to CMOS logic-high or logic-low levels. Floating CS will result in unpredictable operation. When the chip is deselected, the supply current is less than 1µA. Forcing CS low resets the MIC1557 by setting the flip flop, forcing the output low. Threshold Comparator The threshold comparator is connected to S (set) on the RS flip-flop. When the threshold voltage (2/3VS) is reached, the flip-flop is set, making the output low. THR is dominant over TRG. Trigger Comparator The trigger comparator is connected to R (reset) on the RS flip-flop. When TRG (trigger) goes below the trigger voltage (1/3VS), the flip-flop resets, making the output high. Flip-Flop and Output A reset signal causes Q to go low, turning on the P- channel MOSFET and turning off the N-channel MOSFET. This makes the output rise to nearly VS. A set signal causes Q to go high, turning off the P- channel MOSFET, and turning on the N-channel MOSFET, grounding OUT. Basic Monostable Operation Refer to the MIC1555 functional diagram. A momentary low signal applied to TRG causes the output to go high. The external capacitor charges slowly through the external resistor. When VTHR (threshold voltage) reaches 2/3VS, the output is switched off, discharging the capacitor. During power-on, a single pulse may be generated. Basic Astable Operation Refer to the MIC1557 functional diagram. The MIC1557 starts with T/T low, causing the output to go high. The external capacitor charges slowly through the external resistor. When VT/T reaches 2/3VS (threshold voltage), the output is switched off, slowly discharging the capacitor. When VT/T decreases to 1/3VS (trigger voltage), the output is switched on, causing VT/T to rise again, repeating the cycle. |
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