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MIC1555/1557
Micrel
MIC1555/1557
IttyBittyTM RC Timer / Oscillator
General Description
The MIC1555 IttyBittyTM 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. 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 -40C to +85C ambient temperature range.
Features
* +2.7V to +18V operation * Low current <1A typical shutdown mode (MIC1557) 200A typical (TRG and THR low) at 3V supply * Timing from microseconds to hours * "Zero" leakage trigger and threshold inputs * 50% square wave with one Resistor, one Capacitor * Threshold input precedence over trigger input * <15 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
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 MIC1555BM5 MIC1557BM5 Temp. Range -40C to +85C -40C to +85C Package SOT-23-5 SOT-23-5 Marking T10 T11
Typical Applications
+5V MIC1555
1 Standby Trigger 4 2 3
100s VS TRG GND THR
5
+5V 8kHz MIC1557 Output
4 Enabled Disabled 3 2 5
OUT
VS CS GND
OUT
Output 1k
1k 0.1F
T/T
1
0.1F
Monostable (One-Shot)
Astable (Oscillator)
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MIC1555/1557
MIC1555/1557
Micrel
Pin Configuration
OUT GND VS
3 2 1
CS GND T/T
3 2 1
T10
Part Number MIC1555BM5 MIC1557BM5 Identification T10 T11
4 5 4
T11
5
Part Identification
TRG
THR
VS
OUT
SOT-23-5 (M5)
Pin Description
Pin Number MIC1555 1 2 Pin Number MIC1557 4 2 3 3 4 5 1 5 Pin Name VS GND CS OUT TRG THR T/T 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 at <13VS. Output: CMOS totem-pole output. Trigger (Input): Sets output high. Active low (at 2 3VS nominal). Threshold (Dominant Input): Sets output low. Active high (at 2 3VS nominal). Trigger/Threshold (Input): Internally connected to both threshold and trigger functions. See TRG and THR.
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Micrel
Absolute Maximum Ratings
Supply Voltage (VS) .................................................... +22V Threshold Voltage (VTHR, VT/T) .................................. +22V Trigger Voltage (VTRG, VT/T) ....................................... +22V Lead Temperature (soldering 10sec.) ....................... 300C
Operating Ratings
Supply Voltage (VS) ..................................... +2.7V to +18V Ambient Temperature Range (TA) ............. -40C to +85C Package Thermal Resistance (JA) ................................................................... 220C/W (JC) .................................................................. 130C/W
Electrical Characteristics
TA = 25C, bold values indicate -40C TA +85C; unless noted. Parameter Supply current Condition VS = 5V VS = 15V Monostable Timing Accuracy RA = 10k, C =0.1F, VS = 5V RA = 10k, C =0.1F, VS = 5V Monostable Drift over Temp VS = 5V, -55 TA +125C, Note 1 VS =10V, -55 TA +125C, Note 1 VS = 15V, -55 TA +125C, Note 1 Monostable Drift over Supply Astable Timing Accuracy VS = 5V to 15V, Note 1 RA = RB = 10k, C = 0.1F, VS = 5V RA = RB = 10k, C = 0.1F, VS = 5V Maximum Astable Frequency Astable Drift over Temp RT = 1k, CT = 47pF, Vs = 8V VS = 5V, -55 TA +125C, Note 1 VS =10V, -55 TA +125C, Note 1 VS = 15V, -55 TA +125C, Note 1 Astable Drift over Supply Threshold Voltage Trigger Voltage Trigger Current Threshold Current Chip Select VS = 5V to 15V, Note 1 VS = 15V VS = 15V VS = 15V VS = 15V on >
2/3V S
Min
Typ 240 350 2
Max 300 400
Units A A % s ppm/C ppm/C ppm/C %/V % s MHz ppm/C ppm/C ppm/C %/V
858 100 150 200 0.5 2 1717 5 100 150 200 0.5 61 27 67 32
1161
2323
72 37 50 50
%VS %VS nA nA %VS %VS V V V V
50 28
67 33 0.3 0.08
72 50 1.25 0.5
off < 1/3VS Output Voltage Drop VS = 15V, ISINK = 20mA VS = 5V, ISINK = 3.2mA Output Voltage Drop VS = 15V, ISOURCE = 20mA VS = 5V, ISOURCE = 3.2mA Supply Voltage Output Rise Time Output Fall Time functional operation, Note 1 RL = 10M, CL = 10pF, VS = 5V, Note 1 RL = 10M, CL = 10pF, VS = 5V, Note 1
14.1 3.8 2.7
14.7 4.7 18 15 15
V ns ns
General Note: Devices are ESD protected, however handling precautions are recommended. Note 1: Not tested.
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MIC1555/1557
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Micrel
Typical Characteristics (TA = 25C, VIN = +5)
On Resistance vs. Supply Voltage
70 ON RESISTANCE () 60 50 40 30 20 10 0 0 5 10 15 SUPPLY VOLTAGE (V) 20
Astable Frequency
100
CAPACITANCE (F) 100
Pulse Width
RT=1k
RT=1k 1 0.1 0.01 0.001
1x100 1x10-2 1x10-1
CAPACITANCE (F)
10
10 1 10k 0.1 0.01 0.001 1M 100k
100k 10k
1M
1x101 1x102 1x103 1x104 1x105 1x106 1x107
0.0001
1x100
1x101
1x102
1x103
1x104
1x105
1x106
FREQUENCY (Hz)
1x10-1
PERIOD (s)
On Resistance vs. Temperature
70 SUPPLY CURRENT (A) ON RESISTANCE () 60 50 40 30 20 10 0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) 300 280 260 240 220
Supply Current vs. Temperature
500 SUPPLY CURRENT (A) 400 300 200 100 0
1x107
0.0001
Supply Current vs. Supply Voltage
200 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0
5 10 15 SUPPLY VOLTAGE (V)
20
2.0 1.9 1.8 k FACTOR 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 10
k Factors Times RC
f=1k1 RC
t=k2 RC
100 RC (s)
1000
14 CHIP SELECT VOLTAGE (V) 12 10 8 6 4 2 0 3
MIC1557 Chip Select vs. Supply Voltage
ON
OFF 6 9 12 15 SUPPLY VOLTAGE (V) 18
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Functional Diagrams
VSUPPLY VS MIC1555
1M MIC1557 VSUPPLY VS
Bias THR S Q R
Standby Trigger
CS
Bias
100s
S
8kHz Q OUT Output
OUT
Output
R
<100s
TRG
T/T
GND
1k
GND
1k
0.1F
0.1F
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 13VS and 23VS 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 1A. 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 (23VS) is reached, the flip-flop is set, making the output low. THR is dominant over TRG. October 1998 5 Trigger Comparator The trigger comparator is connected to R (reset) on the RS flip-flop. When TRG (trigger) goes below the trigger voltage (13VS), 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 23 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 23VS (threshold voltage), the output is switched off, slowly discharging the capacitor. When VT/T decreases to 13VS (trigger voltage), the output is switched on, causing VT/T to rise again, repeating the cycle.
MIC1555/1557
MIC1555/1557
Micrel
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 23 of the input voltage. The junction's voltage ramps down toward ground while the output is low. When the ramp is below 13 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 13 input trigger trip voltage, to 1.1RC, the same as a monostable pulse.
2.7V to 18V
DT VS
1
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
CB
RPU THR
5
tON = 1.1RTCT CS
VS
4
RE
Trigger
13V IN
CIN CT
TRG
4
OUT
OUT
3
T/T
1
CS
CS
3
t
MIC1555 GND
2
OUT
OUT
5
t = 0.7(RA+RB)CT MIC1557 RA 1k to 1M RB CT 100pF to 10F
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 () 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.
GND
2
Figure 2. Oscillator Diagram The MIC1555 or MIC1557 can be used to construct an oscillator. The frequency of an astable oscillator is: f= 1 k1 RC
where: f = frequency (Hz) k1 = constant [from Typical Characteristics graph] R = resistance () C = capacitance (F) 6 October 1998
MIC1555/1557
MIC1555/1557
To use the MIC1555 as an oscillator, connect TRG to THR.
+5V MIC1555
1 4 2
Micrel
Accuracy The two comparators in the MIC1555/7 use a resistor voltage divider to set the threshold and trigger trip points to approximately 23 and 13 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 1M or capacitors larger than 10F 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 1F CT and approximately 156k. 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.
+5V +12V OFF 3.3V
5 4 2
8kHz
3
VS TRG GND
OUT
Output 1k
THR
5
0.1F
Figure 3. MIC1555 Oscillator Configuration The MIC1557 features a CS input. When logic-low, CS places the MIC1557 into a <1A shutdown state. If unused, the MIC1557 CS input on must be pulled up.
+5V MIC1557
4 5
8kHz VS CS GND T/T
1
OUT
Output 1k 0.1F
1M
3 2
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 1M Input MIC1555
1 4 2 3
100s VS TRG GND THR OUT Output 1N4148
1k
5
0.1F
Figure 5. Falling-Edge Trigger Configuration 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 13VS. This circuit provides a low-going output pulse.
+5V MIC1555
1 4 2
MIC1555 THR TRG GND OUT
3
VS
1
RL
ON 1.6V
100s
3
VS TRG GND
OUT
Output 1N4148 Input
Figure 7. Schmitt trigger 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
1k THR
5
0.1F
Figure 6. Rising-Edge Trigger Configuration
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MIC1555/1557
MIC1555/1557
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 N-Channel MOSFET (IRF540) 15V MIC1557
4 3
Micrel
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 MIC1555 1N914 47k
5
5 4 2 1
RL
TTL High = ON +5V
+5V to +18V
VS
1
CS
10k
THR TRG GND
VS
0.001F OUT
3
T/T C1 100pF to 10F
2
GND
OUT RE 100k
CT 0.01F
10k
100
RT 1k to 1M
Figure 8. Charge-Pump
Figure 9. Audible Voltmeter
Package Information
1.90 (0.075) REF 0.95 (0.037) REF
1.75 (0.069) 1.50 (0.059)
3.00 (0.118) 2.60 (0.102)
DIMENSIONS: MM (INCH) 3.02 (0.119) 2.80 (0.110) 1.30 (0.051) 0.90 (0.035) 10 0 0.15 (0.006) 0.00 (0.000) 0.20 (0.008) 0.09 (0.004)
0.50 (0.020) 0.35 (0.014)
0.60 (0.024) 0.10 (0.004)
SOT-23-5 (M5)
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131
TEL
USA
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. (c) 1998 Micrel Incorporated
MIC1555/1557
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October 1998

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