MIC1555/1557 Micrel MIC1555/1557 IttyBitty™ RC Timer / Oscillator General Description Features The MIC1555 IttyBitty™ CMOS RC timer/oscillator and MIC1557 IttyBitty CMOS RC oscillator are designed to provide rail-to-rail pulses for precise time delay or frequency generation. The devices are similar in function to the industry standard “555”, without a frequency control (FC) pin or an opencollector discharge (D) pin. The threshold pin (TH) has precedence over the trigger (TR) input, ensuring that the BiCMOS output is off when TR is high. • +2.7V to +18V operation • Low current <1µA typical shutdown mode (MIC1557) 200µA typical (TRG and THR low) at 3V supply • Timing from microseconds to hours • TTL compatible inputs and output • “Zero” leakage trigger and threshold inputs • 50% square wave with one Resistor, one Capacitor • Threshold input precedence over trigger input • <15Ω output on resistance • No output cross-conduction current spikes • <0.005%/°C temperature stability • <0.055%/V supply stability • Small SOT-23-5 surface mount package The MIC1555 may be used as an astable (oscillator) or monostable (one-shot) with separate threshold and trigger inputs. In the one-shot mode, the output pulse width is precisely controlled by an external resistor and a capacitor. Time delays may be accurately controlled from microseconds to hours. In the oscillator mode, the output is used to provide precise feedback, with a minimum of one resistor and one capacitor producing a 50% duty cycle square wave. The MIC1557 is designed for astable (oscillator) operation only, with a chip select/reset (CS) input for low power shutdown. One resistor and one capacitor provide a 50% duty cycle square wave. Other duty-cycle ratios may be produced using two diodes and two resistors. The MIC1555/7 is powered from a +2.7V to +18V supply voltage. The MIC1555/7 is available in the SOT-23-5 5-lead package, and is rated for –40°C to +85°C ambient temperature range. Applications • • • • • • • • • • • • Precision timer Pulse generation Sequential timing Time-delay generation Missing pulse detector Micropower oscillator to 5MHz Charge-pump driver LED blinker Voltage converter Linear sweep generator Variable frequency and duty cycle oscillator Isolated feedback for power supplies Ordering Information Part Number Temp. Range Package Marking MIC1555BM5 –40°C to +85°C SOT-23-5 T10 MIC1557BM5 –40°C to +85°C SOT-23-5 T11 10 Typical Applications +5V +5V 100µs 8kHz MIC1555 1 Standby Trigger 4 2 VS OUT MIC1557 3 Enabled Disabled 1k TRG GND 4 Output THR 5 3 2 VS OUT 5 CS GND 1k T/T 1 0.1µF 0.1µF Monostable (One-Shot) December 1997 Output Astable (Oscillator) 10-3 MIC1555/1557 Micrel Pin Configuration OUT GND VS 3 2 CS GND T/T 1 3 T10 Part Number Identification MIC1555BM5 T10 MIC1557BM5 T11 2 1 Part Identification T11 4 5 4 5 TRG THR VS OUT SOT-23-5 (M5) Pin Description Pin Number MIC1555 Pin Number MIC1557 Pin Name 1 4 VS 2 2 GND 3 CS 5 OUT Output: CMOS totem-pole output. 4 TRG Trigger (Input): Sets output high. Active low (at ≤ 2⁄3VS nominal). 5 THR Threshold (Dominant Input): Sets output low. Active high (at ≥ 2⁄3VS nominal). T/T Trigger/Threshold (Input): Internally connected to both threshold and trigger functions. See TRG and THR. 3 1 Pin Function Supply (Input): +2.7 to +18V supply. Ground: Supply return. Chip Select/Reset (Input): Active high at 2/3VS. Output off when low (<VS/3). 10-4 December 1997 MIC1555/1557 Micrel Absolute Maximum Ratings Operating Ratings Supply Voltage (VS) .................................................... +22V Threshold Voltage (VTHR, VT/T) .................................. +22V Trigger Voltage (VTRG, VT/T) ....................................... +22V Lead Temperature (soldering 10sec.) ....................... 300°C Supply Voltage (VS) ..................................... +2.7V to +18V Ambient Temperature Range (TA) ............. –40°C to +85°C Package Thermal Resistance (θJA) ................................................................... 220°C/W (θJC) .................................................................. 130°C/W Electrical Characteristics TA = 25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted. Parameter Condition Supply current Monostable Timing Accuracy Min Typ Max Units VS = 5V 240 300 µA VS = 15V 350 400 µA RA = 10k, C =0.1µF, VS = 5V 2 RA = 10k, C =0.1µF, VS = 5V 858 % 1161 µs VS = 5V, –55 ≤ TA ≤ +125°C, Note 1 100 ppm/°C VS =10V, –55 ≤ TA ≤ +125°C, Note 1 150 ppm/°C VS = 15V, –55 ≤ TA ≤ +125°C, Note 1 200 ppm/°C Monostable Drift over Supply VS = 5V to 15V, Note 1 0.5 %/V Astable Timing Accuracy RA = RB = 10k, C = 0.1µF, VS = 5V 2 % Monostable Drift over Temp RA = RB = 10k, C = 0.1µF, VS = 5V Maximum Astable Frequency RT = 1k, CT = 47pF, Vs = 8V Astable Drift over Temp 1717 2323 µs 5 MHz VS = 5V, –55 ≤ TA ≤ +125°C, Note 1 100 ppm/°C VS =10V, –55 ≤ TA ≤ +125°C, Note 1 150 ppm/°C VS = 15V, –55 ≤ TA ≤ +125°C, Note 1 200 ppm/°C Astable Drift over Supply VS = 5V to 15V, Note 1 0.5 %/V Threshold Voltage VS = 15V 61 67 72 %VS Trigger Voltage VS = 15V 27 32 37 %VS Trigger Current VS = 15V 50 nA Threshold Current VS = 15V 50 nA Chip Select VS = 15V 67 72 %VS Output Voltage Drop VS = 15V, ISINK = 20mA 0.3 1.25 V VS = 5V, ISINK = 3.2mA 0.08 0.5 V Output Voltage Drop 61 VS = 15V, ISOURCE = 20mA 14.1 14.7 V VS = 5V, ISOURCE = 3.2mA 3.8 4.7 V Supply Voltage functional operation, Note 1 2.7 Output Rise Time RL = 10M, CL = 10pF, VS = 5V, Note 1 15 ns Output Fall Time RL = 10M, CL = 10pF, VS = 5V, Note 1 15 ns General Note: Devices are ESD protected, however handling precautions are recommended. Note 1: Not tested. December 1997 10-5 18 V 10 MIC1555/1557 Micrel Typical Characteristics (TA = 25°C) Astable Frequency 100 On Resistance vs. Supply Voltage Pulse Width 100 70 0 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) 20 10 1x107 0 5 10 15 SUPPLY VOLTAGE (V) 20 500 SUPPLY CURRENT (µA) SUPPLY CURRENT (µA) 10 30 Supply Current vs. Supply Voltage 280 260 240 220 200 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) 400 300 200 100 0 0 5 10 15 SUPPLY VOLTAGE (V) 20 k Factors Times RC 2.0 1.9 1.8 k FACTOR 20 40 0 300 1.7 1.6 f=1⁄k1 RC 1.5 1.4 1.3 1.2 t=k2 RC 1.1 1.0 10 100 RC (µs) 1000 MIC1557 Chip Select vs. Supply Voltage 14 CHIP SELECT VOLTAGE (V) 30 50 Supply Current vs. Temperature 70 40 60 PERIOD (µs) On Resistance vs. Temperature 50 1x106 0.0001 1M FREQUENCY (Hz) 60 ON RESISTANCE (Ω) 0.001 1x105 1x107 1x106 1x105 1x104 1x103 1x102 1x101 1x100 1x10-1 1x10-2 0.0001 1M 1x104 0.001 100k 0.01 1x103 0.01 0.1 1x102 10k 10k 1x101 100k 1 1x100 1 1x10-1 RT=1k 0.1 10 CAPACITANCE (µF) 10 ON RESISTANCE (Ω) CAPACITANCE (µF) RT=1k 12 10 ON 8 6 4 2 0 OFF 3 6 9 12 15 SUPPLY VOLTAGE (V) 10-6 18 December 1997 MIC1555/1557 Micrel Functional Diagrams VSUPPLY VSUPPLY VS VS MIC1555 1M MIC1557 CS Bias Bias THR 100µs S R 8kHz S Q Q Output OUT R Output OUT Standby Trigger <100µs TRG T/T GND 1k GND 0.1µF 1k 0.1µF MIC1555 Block Diagram with External Components (Monostable Configuration) MIC1557 Block Diagram with External Components (Astable Configuration) 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 logic high or logic low. 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. December 1997 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 2⁄3 VS, 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. 10-7 10 MIC1555/1557 Micrel 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 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 DT VS 1 CB RPU tON = 1.1RTCT THR VS 4 CS 5 RE Trigger CIN 1⁄3V IN TRG OUT 4 T/T OUT CS 1 3 CS 3 t CT OUT MIC1555 GND 2 OUT 5 t = 0.7(RA+RB)CT MIC1557 RA Figure 1. One-Shot Diagram The period of a monostable circuit is: GND 2 1k to 1M t = k1 R C where: t = period (s) k1 = 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. RB CT 100pF to 10µF Figure 2. Oscillator Diagram The MIC1555 or MIC1557 can be used to construct an oscillator. The frequency of an astable oscillator is: f = 1 k2 R C where: f = frequency (Hz) k2 = constant [from Typical Characteristics graph] R = resistance (Ω) C = capacitance (F) 10-8 December 1997 MIC1555/1557 Micrel To use the MIC1555 as an oscillator, connect TRG to THR. +5V 8kHz MIC1555 1 VS 4 3 OUT Output TRG 2 1k GND 5 THR 0.1µF Figure 3. MIC1555 Oscillator Configuration The MIC1557 features a CS input. When logic-low, CS places the MIC1557 into a <1µA shutdown state. If unused, the MIC1557 CS input on must be pulled up. +5V 8kHz MIC1557 4 1M VS 3 OUT 5 Output CS 2 1k GND T/T 1 0.1µF Figure 4. MIC1557 Oscillator Configuration Falling-Edge Triggered Monostable Circuit The MIC1555 may be triggered by an ac-coupled fallingedge, as shown in figure 5. The RC time constant of the input capacitor and pull-up resistor should be less than the output pulse time, to prevent multiple output pulses. A diode across the timing resistor provides a fast reset at the end of the positive timing pulse. +5V 100µs MIC1555 1M 1 4 Input 2 VS OUT 3 Output 1k TRG GND THR 1N4148 5 0.1µF Accuracy The two comparators in the MIC1555/7 use a resistor voltage divider to set the threshold and trigger trip points to approximately 2⁄3 and 1⁄3 of the input voltage, respectively. Since the charge and discharge rates of an RC circuit are dependent on the applied voltage, the timing remains constant if the input voltage varies. If a duty cycle of exactly 50% (or any other value from 1 to 99%), two resistors (or a variable resistor) and two diodes are needed to vary the charge and discharge times. The forward voltage of diodes varies with temperature, so some change in frequency will be seen with temperature extremes, but the duty cycle should track. For absolute timing accuracy, the MIC1555/7 output could be used to control constant current sources to linearly charge and discharge the capacitor, at the expense of added components and board space. Long Time Delays Timing resistors larger than 1MΩ or capacitors larger than 10µF are not recommended due to leakage current inaccuracies. Time delays greater than 10 seconds are more accurately produced by dividing the output of an oscillator by a chain of flip-flop counter stages. To produce an accurate one-hour delay, for example, divide an 4.55Hz MIC1557 oscillator by 16,384 (4000hex, 214) using a CD4020 CMOS divider. 4.5Hz may be generated with a 1µF CT and approximately 156kΩ. Inverting Schmitt Trigger Refer to figure 7. The trip points of the MIC1555/7 are defined as 1/3 and 2/3VS, which allows either device to be used as a signal conditioning inverter, with hysteresis. A slowly changing input on T/T will be converted to a fast rise or fall-time opposite direction rail-to-rail output voltage. This output may be used to directly drive the gate of a logic-level P-channel MOSFET with a gate pull-up resistor. This is an inverted logic low-side logic level MOSFET driver. A standard N-channel MOSFET may be driven by a second MIC1555/7, powered by 12V to 15V, to level-shift the input. Figure 5. Falling-Edge Trigger Configuration +5V +12V Rising-Edge Triggered Monostable Circuit The MIC1555 may be triggered by an ac-coupled risingedge, as shown in figure 6. The pulse begins when the accoupled input rises, and a diode from the output holds the THR input low until TRG discharges to 1⁄3VS. This circuit provides a low-going output pulse. +5V 4 2 VS OUT GND 3 1k TRG THR ON ≤ 1.6V 4 2 VS THR 5 RL TRG GND OUT 3 Figure 7. Schmitt trigger Output 1N4148 5 Input 0.1µF Figure 6. Rising-Edge Trigger Configuration December 1997 MIC1555 1 100µs MIC1555 1 OFF ≥ 3.3V Charge-Pump Low-Side MOSFET Drivers A standard MOSFET requires approximately 15V to fully enhance the gate for minimum RDS(on). Substituting a logiclevel MOSFET reduces the required gate voltage, allowing an MIC1557 to be used as an inverting Schmitt Trigger, described above. An MIC1557 may be configured as a voltage quadrupler to boost a 5V input to over 15V to fully enhance an N-channel MOSFET which may have its drain 10-9 10 MIC1555/1557 Micrel connected to a higher voltage, through a high-side load. A TTL high signal applied to CS enables a 10kHz oscillator, which quickly develops 15V at the gate of the MOSFET, clamped by a zener diode. A resistor from the gate to ground ensures that the FET will turn off quickly when the MIC1557 is turned off. +12V TTL High = ON N-Channel MOSFET (IRF540) RL Audible Voltmeter If an additional charge or discharge source is connected to the timing capacitor, the frequency may be shifted by turning the source on or off. An MIC1555 oscillator, powered by the circuit under test, may be used to drive a small loudspeaker or piezo-electric transducer to provide a medium frequency for an open or high impedance state at the probe. A high tone is generated for a high level, and a lower frequency for a logic low on the probe. RT 10k +5V MIC1555 MIC1557 4 1N914 3 VS CS T/T C1 100pF to 10µF 10k 5 GND 5 47k 1 2 +5V to +18V 15V OUT RE 100k CT 0.01µF 4 2 THR VS 1 TRG GND OUT 3 10k 100Ω RT 1k to 1M Figure 9. Audible Voltmeter Figure 8. Charge-Pump 10-10 December 1997