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
May 2010 M9999-052610-D
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 similar in function to the industry standard
“555”, without a frequency control (FC) pin or an open-
collector 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 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 chip select/reset (CS) input for low power 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.
The MIC1555/7 is available in the SOT-23-5 5-pin
package, and is rated for –40°C to +85°C ambient
temperature range
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
• 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
Typical Application
Standby
Trigger
VS OUT
MIC1555
13
GND
2THR 5
+5V
Output
TRG
4
100µs
1k
0.1µF
8kHz
Enabled
Disabled
VS
CS
MIC1557
4
3
GND
2
+5V
OUT 5
T/T 1
Output
1k
0.1µF
Monostable (One-Shot)
Astable (Oscillator)
Micrel, Inc. MIC1555/1557
May 2010 2 M9999-052610-D
Ordering Information
Part Number Marking
Standard Pb-Free Standard Pb-Free
Temperature Range Package
MIC1555BM5 MIC1555YM5 T10 T10 –40° to +85°C 5-Pin SOT-23
MIC1557BM5 MIC1557YM5 T11 T11 –40° to +85°C 5-Pin SOT-23
Note:
Under bar ( __ ) may not be to scale.
Pin Configur ation
VS
THRTRG
OUT
T10 Part
Identification
13
GND
2
54
Part
Identification
T/T
OUTVS
CS
T11
13
GND
2
54
MIC1555
5-Pin SOT-23 (M)
MIC1557
5-Pin SOT-23 (M)
Pin Description
Pin Number
MIC1555 Pin Number
MIC1557
Pin Name Pin Name
1 4 VS Supply (Input): +2.7 to +18V supply.
2 2 GND Ground: Supply return.
3 CS
Chip Select/Reset (Input): Active high at >2/3VS. Output off
when low at<1/3VS.
3 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).
1 T/T
Trigger/Threshold (Input): Internally connected to both
threshold and trigger functions. See TRG and THR.
Micrel, Inc. MIC1555/1557
May 2010 3 M9999-052610-D
Absolute Maximum Ratings
Supply Voltage (VS)......................................................+22V
Threshold Voltage (VTHR, VT/T). ....................................+22V
Trigger Voltage (VTGR, VT/T)..........................................+22V
Lead Temperature (soldering, 10sec.)....................... 300°C
Maximum Storage Temperature ................................ 150°C
Operating Ratings
Supply voltage (VS) ....................................... +2.7V to +18V
Ambient Temperature (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 Min Typ Max Units
VS = 5V 240 300 µA Supply current
VS = 15V 350 400 µA
RA = 10k, C =0.1µF, VS = 5V 2 % Monostable Timing Accuracy
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
Monostable Drift over Temp
VS = 15V, –55 ≤ TA ≤ +125°C, Note 1 200 ppm/°C
Monostable Drift over Supply VS = 5V to 15V, Note 1 0.5 %/V
RA = RB = 10k, C = 0.1µF, VS = 5V 2 % Astable Timing Accuracy
RA = RB = 10k, C = 0.1µF, VS = 5V 1717 2323 µs
Maximum Astable Frequency RT = 1k, CT = 47pF, VS = 8V 5 MHz
VS = 5V, –55 ≤ TA ≤ +125°C, Note 1 100 ppm/°C
VS = 10V, –55 ≤ TA ≤ +125°C, Note 1 150 ppm/°C
Astable Drift over Temp
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
on > 2/3 VS 50 67 72 %/VS Chip Select
off > 1/3 VS 28 33 50 %/VS
VS = 15V, ISINK = 20mA 0.3 1.25 V
VS = 5V, ISINK = 3.2mA 0.08 0.5 V
VS = 15V, ISOURCE = 20mA 14.1 14.7 V
Output Voltage Drop
VS = 5V, ISOURCE = 3.2mA 3.8 4.7 V
Supply Voltage functional operation, Note 1 2.7 18 V
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 recommended.
Note 1. Not tested.
Micrel, Inc. MIC1555/1557
May 2010 4 M9999-052610-D
Typical Characteristics (TA = 25°C, VIN = +5)
0
10
20
30
40
50
60
70
-40-20 0 20406080100
(ECNATSISERNO )
TEMPERATURE (°C)
On Resistance
vs. Temperature
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
10 100 1000
ROTCAFk
RC (µs)
k Factors
Times RC
f=1k1RC
t=k2 RC
0
2
4
6
8
10
12
14
36 9 121518
)V(EGATLOVTCELESPIHC
SUPPLY VOLTAGE (V)
MIC1557 Chip Select
vs. Supply Voltage
ON
OFF
Micrel, Inc. MIC1555/1557
May 2010 5 M9999-052610-D
Functional Diagram
S
R
QOUT
GND
Outpu
t
TRG
THR
Standby
Trigger
VS
VSUPPLY
MIC1555
1k
0.1µF
<100µs
Bias
100µs
MIC1555 Block Diagram with External Components
(Monostable Configuration )
S
R
Q
T/T
OUT
VS
Bias
GND
CS
VSUPPLY
MIC1557
Output
1k
0.1µF
8kHz
1M
MIC1557 Block Diagram with External Components
(Astable Configuration)
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.
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
13VS
tON = 1.1R TC
T
2.7V to 18
V
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
CT
100pF to 10µF
5
3
2
4
1
CS
t = 0.7( RA+RB)C
T
t
RB
Figure 2. Oscillator Diagram
Micrel, Inc. MIC1555/1557
May 2010 8 M9999-052610-D
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 [from Typical Characteristics
graph]
R = resistance (Ω)
C = capacitance (F)
To use the MIC1555 as an oscillator, connect TRG to
THR.
VS
TRG
MIC1555
1
4
GND
2
+5V
OUT 3
THR 5
Output
8kHz
1k
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.
VS
CS
MIC1557
4
3
GND
2
+5V
OUT 5
T/T 1
Outpu
t
8kHz
1M 1k
0.1µF
Figure 4. MIC1557 Oscillator Configuration
Falling-Edge Triggered Monostable Circuit
The MIC1555 may be triggered by an ac-coupled falling-
edge, 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.
VS OUT
MIC1555
13
GND
2THR 5
+5V
Outpu
t
TRG
4
100µs
1M
1N41481k
0.1µF
Input
Figure 5. Falling-Edge Trigger Configuration
Rising-Edge Triggered Monostable Circuit
The MIC1555 may be triggered by an ac-coupled rising-
edge, as shown in figure 6. The pulse begins when the
ac-coupled 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.
VS OUT
MIC1555
13
GND
2THR 5
+5V
Output
TRG
4
100µs
1N41481k
0.1µF
Inpu
t
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 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 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
Micrel, Inc. MIC1555/1557
May 2010 9 M9999-052610-D
MOSFET may be driven by a second MIC1555/7,
powered by 12V to 15V, to level-shift the input.
THR
OUT
MIC1555
1
3
GND
2
VS
5
TRG
4
+5V +12V
RL
OFF 3.3V
ON 1.6V
Figure 7. Schmitter Trigger
Charge-Pump Low-Side MOSFET Drivers
A standard MOSFET requires approximately 15V 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, 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 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 ensures that the FET
will turn off quickly when the MIC1557 is turned off.
VS CS
MIC1557
43
GND
2
+5V
OUT
5
T/T
1
TTL High = ON
15V
+12
V
RL
N-Channel
MOSFET
(IRF540)
RE
100k
C1
100pF
to 10µF
RT 1k to 1M
Figure 8. Charge-Pump
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 loud speaker 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.
THR
OUT
MIC1555 1
3
GND
2
VS
5
TRG
4
100
+5V to +18VRT 10k
10k
10k
47k
CT
0.01µF
1N914
0.001µF
Figure 9. Audible Voltmeter
Micrel, Inc. MIC1555/1557
May 2010 10 M9999-052610-D
Package Information
5-Pin SOT-23 (M)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http:/www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its
use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2004 Micrel, Incorporated.
