MIC1555/1557
IttyBitty
®
RC Timer/Oscillator
IttyBitty is a registered trademark of Micrel, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
August 12
, 2015
Revision 6.1
General Description
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 sim ilar in function to the indus try standard
“555”, without a frequency control (FC) pin or an open-
collector discharge (D) pin. The threshold pin (THR) has
precede nce over the trigg er (TRG) input, e nsuring that the
BiCMOS output is off when TRG is high.
The MIC1555 can 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 micro-
seconds 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 c h ip se lec t/res e t ( CS) input for low po wer shut-
down. 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 and is rated for –40°C to +85°C ambient
temper atur e range. The MIC 155 5/7 is av ai lab le in SOT-23-
5, and thin SOT23-5 5-pin packages. A low profile, ultra-
thin, (UT DFN), vers ion of the MIC1555 ( with chip sele ct) is
also available.
Datasheets and support documentation are available on
Micrel’s webs ite at : www.micrel.com.
Features
• +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
• “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
• 10-pin ultra-thin DFN package (2mm × 2mm × 0.4mm)
• Small SOT-23-5 surface mount package
Applications
• Precision timer
• Pulse generation
• Sequential timing
• Time-dela y generat ion
• Missing pulse detector
• Micropower oscillator to 5MHz
• Charge-pump driver
• LED blinker
• Voltage converter
• Linear sweep generator
• Variable frequency and duty cycle oscillator
Micrel, Inc.
MIC1555/1557
August 12
, 2015 2 Revision 6.1
Typical Ap plication s
Monostable (One-Shot)
Monostable (with Enable)
Astable (Oscillator)
Micrel, Inc.
MIC1555/1557
August 12
, 2015 3 Revision 6.1
Ordering Information
Part Number Top Marking
(1, 2)
Temperature Range Package Package Height (mm)
MIC1555YD5 T12 –40°C to +85°C 5-Pin Thin SOT-23 1.0 (maximum)
MIC1555YMU T14 –40°C to +85°C 10-Pin 2mm × 2mm UTDFN 0.4 (maximum)
MIC1555YM5 T10 –40°C to +85°C 5-Pin SOT-23 0.9 – 1.45
MIC1557YD5 T13 –40°C to +85°C 5-Pin Thin SOT-23 1.0 (maximum)
MIC1557YM5 T11 –40°C to +85°C 5-Pin SOT-23 0.9 – 1.45
Note:
1. Underbars ( ) shown for the top marking may not be to scale.
2. The top marki ng in the YMU package does not have an underbar.
Pin Configuration (MIC1555 SOT-23 and TSOT-23)
5-Pin SOT-23 (M5)
(Top View)
5-Pin TSOT-23 (D5)
(Top View)
Pin Description (M IC1555 SOT-23 and TSOT-23)
Pin Number Pin Name Pin Function
1 VS Supply (Input): +2.7V to + 18V supp ly .
2 GND Ground: Supply return.
3 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).
Micrel, Inc.
MIC1555/1557
August 12
, 2015 4 Revision 6.1
Pin Configuration (MIC1555 UTDFN)
10-Pin UTDFN (MU)
(Top View)
Pin Description (M IC1555 UTDFN )
Pin Number Pin Name Pin Function
1 VS Supply (Input): +2.7 to +18V supply.
2 CS Chip Select/Reset (Input): Active high at >2/3VS. Output off when low at <1/3VS. If chip select
functionality is not desired, CS may be connected directly to VS.
3, 4, 7, 9 NC No Connect. This pin is not internally connected.
5 THR Threshold (Dominant Input): Sets output low. Active high (at ≥ 2/3VS nominal).
6 TRG Trigger (Input): Sets output high. Active low (at ≤2/3VS nominal).
8 GND Ground: Supply return.
10 OUT Output: CMOS totem-pole output.
Pin Configuration (MIC1557 SOT-23 and TSOT-23)
5-Pin SOT-23 (M5)
(Top View)
5-Pin TSOT-23 (D5)
(Top View)
Pin Description (M IC1557 SOT-23 and TSOT-23)
Pin Number Pin Name Pin Function
1 T/T Trigger/Threshold (Input): Internally connected to both threshold and trigger functions. When the
voltage at this pin is ≤2/3VS it will set the output high. When the voltage at this pin is ≥2/3VS it will set
the output low.
2 GND Ground: Supply return.
3 CS Chip Select/Reset (Input): Active high at >2/3VS. Output off when low at <1/3VS. If chip select
functionality is not desired, CS may be connected directly to VS.
4 VS Supply (Input): +2.7 to +18V supply.
5 OUT Output: CMOS totem-pole output.
Micrel, Inc.
MIC1555/1557
August 12
, 2015 5 Revision 6.1
Absolute Maximum Ratings(3)
Supply Voltage (VS) ...................................................... +22V
Threshold Voltage (VTHR, VT/T). .................................... +22V
Trigger Voltage (VTGR, VT/T) .......................................... +22V
Lead Temperature (soldering, 10s) ............................ 300°C
Ambient Storage Temperature .................. –65°C to +150°C
ESD HBM Rating(5) ......................................................... 2kV
ESD MM Rating............................................................ 200V
Operating Ratings(4)
Supply voltage (VS) ....................................... +2.7V to +18V
Ambient Temperature (TA) .......................... –40°C to +85°C
Package Thermal Resistance
SOT23-5 and Thin SOT23-5 (θJA) ................... 250°C/W
2mm × 2mm UTDFN (θJA) ................................. 90°C/W
Electrical Characteristics
TA = 25°C, bold values indicate –40°C ≤ TA ≤ +85°C, unless noted.
Parameter
Condition
Min.
Typ.
Max.
Units
Supply Current VS = 5V 240 300 µA
VS = 15V 350 400
Monostable Timing Accuracy RA = 10kΩ, C =0.1µF, VS = 5V 2 %
RA = 10kΩ, C =0.1µF, VS = 5V 858 1161 µs
Monostable Drift
Overtemperature
VS = 5V, –55°C ≤ TA ≤ +125°C(6) 100
ppm/°C
VS = 10V, –55°C ≤ TA ≤ +125°C
(6)
150
VS = 15V, –55°C ≤ TA ≤ +125°C
(6)
200
Monostable Drift over Supply VS = 5V to 15V(6) 0.5 %/V
Astable Timing Accuracy RA = RB = 10kΩ, C = 0.1µF, VS = 5V 2 %
RA = RB = 10kΩ, C = 0.1µF, VS = 5V 1717 2323 µs
Maximum Astable
Frequency RT = 1kΩ, CT = 47pF, VS = 8 V 5 MHz
Astable Drift
Overtemperature
VS = 5V, –55°C ≤ TA ≤ +125°C(6) 100
ppm/°C
VS = 10V, –55°C ≤ TA ≤ +125°C(6) 150
VS = 15V, –55°C ≤ TA ≤ +125°C(6) 200
Astable Drift over Supply VS = 5V to 15V
(6)
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 on > 2/3 VS 50 67 72 %/VS
off < 1/3 VS 28 33 50
Notes:
3. Exceeding the absolute maximum ratings may damage the device
4. The device is not guarant eed to function outside its operat i ng ratings.
5. Devices are ESD prot ect ed, however handling precautions recommended.
6. Not tested.
Micrel, Inc.
MIC1555/1557
August 12
, 2015 6 Revision 6.1
Electrical Characteristics (Continued)
TA = 25°C, bold values indicate –40°C ≤ TA ≤ +85°C, unless noted.
Parameter Condition Min. Typ. Max. Units
Output Voltage Drop
VS = 15V, ISINK = 20mA 0.3 1.25
V
VS = 5V, ISINK = 3.2mA 0.08 0.5
VS = 15V, ISOURCE = 20mA 14.1 14.7
VS = 5V, ISOURCE = 3.2mA 3.8 4.7
Supply Voltage Functional Operation
(6)
2.7 18 V
Output Rise Time RL = 10MΩ, CL = 10pF, VS = 5V(6) 15 ns
Output Fall Time RL = 10MΩ, CL = 10pF, VS = 5V(6) 15 ns
Micrel, Inc.
MIC1555/1557
August 12
, 2015 7 Revision 6.1
Typical Characteristics
TA = 25°C, VIN = +5V.
Micrel, Inc.
MIC1555/1557
August 12
, 2015 8 Revision 6.1
MIC1555 Functional Block Diagrams
MIC155YM5/MIC155YD5 Block Diagram with External Components (Monostable Configuration)
MIC1555YMU Block Diagram with External Components (Monostable Configuration)
Micrel, Inc.
MIC1555/1557
August 12
, 2015 9 Revision 6.1
MIC1557 Functional Block Diagram
MIC1557YM5/MIC1557YD5 Block Diagram with External Components (Astable Configuration)
Micrel, Inc.
MIC1555/1557
August 12
, 2015 10 Revision 6.1
Functional Description
The MIC1555/7 provides the logic for creating simple RC
timer or oscillator circuits.
The MIC1555 has separate THR (threshold) and TRG
(trigger) c onn ec tio ns for monostab le (on e-s hot) or as ta ble
(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.
For more information, refer to the MIC1555 Functional
Block Diagrams and MIC1557 Functional Block Diagram.
Supply
Voltage supply (VS) is rated for +2.7V to +18V. An
external capacitor is recommended to decouple noise.
Resistive Divider
The res istive voltage divider is constructed of three eq ual
value resistors to produce 1/3VS and 2/3VS voltage for
trigger and threshold reference voltages.
Chip Select/Reset (MIC1555YMU and MIC1557 only)
Chip select/reset (CS) 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 device by setting the flip flop, forcing
the output low. If Chip Select functionality is not desired,
CS may be connected directly to VS.
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
A momentary low signal applied to TRG causes the
output to go high. The external capacitor charges slowly
through the external resistor. When threshold voltage
(VTHR) reaches 2/3VS, the output is switched off,
discharging the capacitor. During power-on, a single
pulse may be generated.
For more information, refer to the MIC1555 Functional
Block Diagrams.
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 extern al resistor . W hen VT/T reaches 2/3VS (thr esh old
voltage), t h e o utpu t is switched of f, slowly disc har g in g 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.
For more information, refer to the MIC1557 Functional
Block Diagram.
Micrel, Inc.
MIC1555/1557
August 12
, 2015 11 Revision 6.1
Application Information
Basic Monostable (One-Shot) Circuit
A monostable oscillator produces a single pulse each
time that it is trigger ed, and is often ref erred t o 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
connecte d to provide astab l e oscillations. The puls e 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
1.1RTCT (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 f unc tion is ass er ted. AC c ou pl ing th e input with
a series ca pacitor and a pull-up resistor, with an RC ti me
constant less than the pulse width, will prevent these
short oscillations. A diode ( DT) in parallel with (RT) resets
the one-shot qu ic kly.
Figure 1. One-Shot Diagram
The period of a monostable circuit is:
t = k2 RC
where:
t = period (s)
k2 = Constant (see Typic a l Charac ter ist ic s )
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. CS is brought out to allow on-off
control of the oscillator.
The MIC1555 may also be used as an astable oscillator
b y t ying t he thr eshold and t rigger pins t ogether, f orm ing 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 j unction ramps up again. The c ontinuing frequ ency 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 sec ond resistor ( RB) in series with an
opposite f acing s witching diode a nd resistor connecte d in
parallel (see Fig ure 2). T he frequenc y is then made up of
two components, the charging time (tA) and the
discharging time (tB) tA = 0.7RACT and tB = 0.7RBCT. The
frequenc y is th e reciprocal of the sum of the two times t A
+ tB, s o the tot al tim e is 1.4 RTCT. T he f irst half -cycle of an
astable, af ter p o wer-on or CS e nab le, is l en gthe ned s i nc e
the capacitor is charging from ground instead of the 1/3
input trigger trip voltage, to 1.1RC, the same as a
monostable pulse.
Micrel, Inc.
MIC1555/1557
August 12
, 2015 12 Revision 6.1
Figure 2. Oscillator Diagram
The MIC1555 or MIC1557 can be used to construct an
oscillator.
The frequency of an astable oscillator is:
RCk 1
f
1
=
where:
f = frequency (Hz)
k1 = Constant (see Typica l Char acter istics)
R = Resistance (Ω)
C = Capacitance (F)
To use the MIC1555 as an oscillator, connect TRG to
THR.
Figure 3. MIC1555 Oscillator Configuration
The MIC1555YMU and MI C155 7 f eatur e a C S inp ut. With
a logic-low signal, CS places the part into a <1µA
shutdown state. If unused, the CS input must be pulled
up.
Figure 4. MIC1557 Oscillator Configuration
Falling-Edge Triggered Monostable Circuit
The MIC155 5 ma y b e trigger ed by an AC-c oupled f alling-
edge, as s hown in Fig ure 5. The RC t ime constant of the
input capacitor and pull-up resistor should be less than
the outpu t pu lse t ime, to preve nt multiple outp ut pulses. A
diode across the timing resistor provides a fast reset at
the end of the positive timing pulse.
Figure 5. Falling Edge Trigger Configuration
Micrel, Inc.
MIC1555/1557
August 12
, 2015 13 Revision 6.1
Rising-Edge Triggered Monosta b le Circuit
The MIC1555 ma y be triggered b y an AC-coupled rising-
edge, as shown in Figure 6. The pulse begins when the
ac-coupled in put rises, and a diode from the output ho lds
the THR input low until TRG discharges to 1/3VS. This
circuit provides a low-going output pulse.
Figure 6. Rising Edge Trigger Configuration
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 res istor s (or a va riabl e res istor ) and t wo dio des
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 boar d spa c e.
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
a 4.55Hz MIC1557 oscillator by 16,384 (4000hex, 214)
using a CD 4020 CMOS d ivider. 4.5Hz m ay be generated
with a 1µF CT and approximately 156kΩ.
Inverting Schmitt Trigger
As shown in 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 t o a fast rise or fall-tim e opposite directi on rail-
to-rail output voltage. This output maybe 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 7. Schmitt Trigger
Charge-Pump Low-Side MOSFET Drivers
A standard MOSFET requires approximately >5V to fully
enhance the gate for minimum RDS(ON). Substituting a
logic-level MOSFET reduces the required gate voltage,
allowing an MIC1557 to be used as an inverting Schmitt
trigger, des cribed abo ve. An MIC15 57 ma y be c onfigur ed
as a voltage quadrupler to boost a 5V input to over 15V
to fully enhance an N-channel MOSFET which ma y have
its drain connected to a higher voltage, through a high-
side load. ATTL 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 ens ures that the FET wi ll t urn off
quickly when the MIC1557 is turned off.
Figure 8. Charge Pump
Micrel, Inc.
MIC1555/1557
August 12
, 2015 14 Revision 6.1
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 b y the c ircuit und er test, m ay be used to drive a
sm all loud speak er or pie zo-electric transd ucer to pro vide
a medium fr equenc y for an ope n or high im pedance st ate
at the probe. A high tone is generated for a high level,
and a lower frequency for a logic low on the probe.
Figure 9. Audible Voltmeter
Micrel, Inc.
MIC1555/1557
August 12
, 2015 18 Revision 6.1
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