September 29, 2000
1
MIC2776
MIC2776
Micrel
MIC2776
Micro-Power Low Voltage Supervisor
Advance Information
General Description
The MIC2776 is a power supply supervisor which provides
under-voltage monitoring and power-on reset generation in a
compact 5-pin SOT package. Features include an adjustable
under-voltage detector, a delay-generator, a manual reset
input, and a choice of active-high, active-low, or open-drain
active-low reset output. The user-adjustable monitoring input
is compared against a 300mV reference. This low reference
voltage allows monitoring voltages lower than those sup-
ported by previous supervisor ICs.
The reset output is asserted for no less than 140ms at power-
on and any time the input voltage drops below the reference
voltage. It remains asserted for the timeout period after the
input voltage subsequently rises back above the threshold
boundary. A reset can be generated at any time by asserting
the manual reset input, /MR. The reset output will remain
active at least 140ms after the release of /MR. The /MR input
can also be used to daisy-chain the MIC2776 onto existing
power monitoring circuitry or other supervisors. Hysteresis is
included to prevent chattering due to noise. Typical supply
current is a low 3.0
A.
Typical Application
IN
/RST
VDD
/MR
GND
MIC2776L
R1
R2
/RESET
VCORE
GND
Power_Good
V
CORE
1.0V
Manual
Reset
MICROPROCESSOR
VI/O
V
I/O
2.5V
Features
User-adjustable input can monitor supplies as low as
0.3V
1.5% threshold accuracy
Separate V
DD
input
Generates power-on reset pulse (140ms min.)
Manual reset input
Choice of active-high, active-low or open-drain active-
low reset output
Inputs can be pulled above V
DD
(7V abs. max.)
Open-drain output can be pulled above V
DD
(7V abs.
max.)
Ultra-low supply current, 3.0
A typical
Rejects brief input transients
IttyBittyTM SOT-23-5 package
Applications
Monitoring processor, ASIC, or FPGA core voltage
Computer systems
PDAs/Hand-held PCs
Embedded controllers
Telecommunications systems
Power supplies
Wireless / cellular systems
Networking hardware
Micrel, Inc. 1849 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 http://www.micrel.com
IttyBittyTM is a trademark of Micrel, Inc.
Ordering Information
Part Number
Marking
Reset Output
Temperature Range
Package
MIC2776N-BM5
UKAA
Open-Drain, Active-Low /RST
40
C to +85C
SOT-23-5
MIC2776H-BM5
ULAA
Active-High, Complementary RST
40
C to +85C
SOT-23-5
MIC2776L-BM5
UMAA
Active-Low, Complementary /RST
40
C to +85C
SOT-23-5
MIC2776
Micrel
MIC2776
2
September 29, 2000
Pin Configuration
RST
VDD
IN
/MR
1
3
4
5
2
GND
SOT-23-5 (M5)
"H" Version
/RST
VDD
IN
/MR
1
3
4
5
2
GND
SOT-23-5 (M5)
"L" and "N" Version
Pin Description
Pin Number
Pin Number
Pin Name
Pin Function
MIC2776H
MIC2776L
MIC2776N
1
RST
Digital (Output): Asserted high whenever V
IN
falls below the reference
voltage. It will remain asserted for no less than 140ms after V
IN
returns
above the threshold limit.
1
/RST
Digital (Output): Asserted low whenever V
IN
falls below the reference
voltage. It will remain asserted for no less than 140ms after V
IN
returns
above the threshold limit. (open-drain for "N" version)
2
2
GND
Ground
3
3
/MR
Digital (Input): Driving this pin low initiates an immediate and unconditional
reset. Assuming IN is above the threshold when /MR is released (returns
high), the reset output will be de-asserted no less than 140ms later. /MR
may be driven by a logic signal or a mechanical switch. /MR has an internal
pull-up to V
DD
and may be left open if unused.
4
4
IN
Analog (Input): The voltage on this pin is compared to the internal 300mV
reference. An under-voltage condition will trigger a reset sequence.
5
5
VDD
Analog (Input): Independent supply input for internal circuitry.
September 29, 2000
3
MIC2776
MIC2776
Micrel
Absolute Maximum Ratings
(Note 1)
Supply Voltage (V
DD
) ..................................... 0.3V to +7V
Input Voltages (V
IN
, V
/MR
) .............................. 0.3V to +7V
RST, (/RST) Current .................................................. 20mA
Storage Temperature (T
S
) ....................... 65
C to +150C
ESD Rating, Note 3 ................................................... 1.5kV
Operating Ratings
(Note 2)
Supply Voltage (V
DD
) .................................. +1.5V to +5.5V
Input Voltages (V
IN
, V
/MR
) ........................... 0.3V to +6.0V
Output Voltages
V
/RST
(N version) ..................................... 0.3V to +6.0V
V
/RST
, V
RST
(H and L versions) ....... 0.3V to V
DD
+ 0.3V
Ambient Temperature Range (T
A
) ............. 40
C to +85C
Package Thermal Resistance (
JA
) ...................... 256
C/W
Electrical Characteristics
V
DD
= 3.3V; T
A
= +25
C, bold values indicate 40C T
A
+85C; unless noted
Symbol
Parameter
Condition
Min
Typ
Max
Units
I
DD
Supply Current
V
DD
= V
IN
= 3.3V; /MR, RST, /RST open
3.0
A
IN, UNDER-VOLTAGE DETECTOR INPUT
V
REF
Under-Voltage Threshold
T
A
= 25
C
295
300
305
mV
V
HYST
Hysteresis Voltage
3
mv
I
IN
Input Current
5
pA
T
MIN
T
A
T
MAX
10
nA
RESET OUTPUTS (/RST, RST)
t
PROP
Propagation Delay
V
IN
= (V
REF(MAX)
+ 100mV) to
20
s
V
IN
= (V
REF(MIN)
100mV)
t
RST
Reset Pulse Width
140
280
ms
V
OL
RST or /RST Output Voltage Low
I
SINK
= 1.6mA;
0.3
V
V
DD
1.6V
I
SINK
= 100
A;
0.3
V
V
DD
1.2V, Note 4
V
OH
RST or /RST Output Voltage High
I
SOURCE
= 500
A;
0.8V
DD
V
V
DD
1.5V
(H and L Version Only)
I
SOURCE
= 10
A;
0.8V
DD
V
V
DD
1.2V, Note 4
MANUAL RESET INPUTS (/MR)
V
IH
Input High Voltage
1.5V
V
DD
5.5V
0.7V
DD
V
V
IL
Input Low Voltage
1.5V
V
DD
5.5V
0.3V
DD
V
t
PROP
Propagation Delay
V
/MR
< V
IL
5
s
t
MIN
Minimum Input Pulse Width
Reset Occurs, V
/MR
< V
IL
33
ns
I
PU
Internal Pull-Up Current
100
nA
I
IN
Input Current, /MR
V
/MR
< V
IL
100
nA
Note 1.
Exceeding the absolute maximum rating may damage the device.
Note 2.
The device is not guaranteed to function outside its operating rating.
Note 3.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF.
Note 4.
V
DD
operating range is 1.5V to 5.5V. Output is guaranteed to be asserted down to V
DD
= 1.2V.
MIC2776
Micrel
MIC2776
4
September 29, 2000
Timing Diagram
V
OH
V
OL
V
/MR
V
IN
V
DD
0V
0V
V
OH
V
OL
t
RST
t
RST
V
/RST
t
RST
t
RST
V
OH
V
OL
V
RST
>t
min
A
A
V
HYST
V
REF
Propagation delays not shown for clarity.
Note A.
The MIC2776 ignores very brief transients.
See "Applications Information" for details.
September 29, 2000
5
MIC2776
MIC2776
Micrel
Functional Description
IN, Under-Voltage Detector Input
The voltage present at the IN pin is compared to the internal
300mV reference voltage. A reset is triggered if and when V
IN
falls below V
REF
. Typically, a resistor divider is used to scale
the input voltage to be monitored such that V
IN
will fall below
V
REF
as the voltage being monitored falls below the desired
trip-point. Hysteresis is employed to prevent chattering due to
noise.
RST, /RST Reset Output
Typically, the MIC2776 is used to monitor the power supply
of intelligent circuits such as microcontrollers and micropro-
cessors. By connecting the reset output of a MIC2776 to the
reset input of a
C or P, the processor will be properly reset
at power-on and during power-down and brown-out condi-
Functional Diagram
R
S
Q
/Q
/MR
IN
MIC2776
* Pinout and polarity vary by device type.
See ordering information table.
V
DD
/RST*
RST*
V
REF
GND
Delay
One Shot
I
PU
tions. In addition, asserting /MR, the manual reset input, will
activate the reset function.
The reset outputs are asserted any time /MR is asserted or if
V
IN
drops below the threshold voltage. The reset outputs
remain asserted for t
RST
(min) after V
IN
subsequently returns
above the threshold boundary and /MR is released. A reset
pulse is also generated at power-on.
/MR, Manual Reset Input
The ability to initiate a reset via external logic or a manual
switch is provided in addition to the MIC2776's automatic
supervisory functions. Driving the /MR input to a logic low
causes an immediate and unconditional reset to occur.
Assuming V
IN
is within tolerance when /MR is released
(returns high), the reset output will be de-asserted no less
than t
RST
later. /MR may be driven by a logic signal, or
mechanical switch. Typically, a momentary push-button switch
is connected such that /MR is shorted to ground when the
switch contacts close. The switch may be connected directly
between /MR and GND. /MR has an internal 100nA pull-up
current to V
DD
and may be left open if unused.
MIC2776
Micrel
MIC2776
6
September 29, 2000
Application Information
Programming the Voltage Threshold
Referring to the "Typical Application Circuit", the voltage
threshold is calculated as follows:
V
V
R1 R2
R2
TH
REF
=
+
(
)
where V
REF
= 0.300V
In order to provide the additional criteria needed to solve for
the resistor values, the resistors can be selected such that the
two resistors have a given total value, that is, R1 + R2 =
R
TOTAL
. Imposing this condition on the resistor values pro-
vides two equations that can be solved for the two unknown
resistor values. A value such as 1M
for R
TOTAL
is a
reasonable choice since it keeps quiescent current to a
generally acceptable level while not causing any measurable
errors due to input bias currents. The larger the resistors, the
larger the potential errors due to input bias current (I
IN
). The
maximum recommended value of R
TOTAL
is 3M
.
Applying this criteria and rearranging the V
TH
expression to
solve for the resistor values gives:
R2
R
V
V
TOTAL
REF
TH
=
(
)
(
)
R1
R
R2
TOTAL
=
-
Application Example
Figure 1 below illustrates a hypothetical MIC2776 application
in which the MIC2776 is used to monitor the core supply of a
high-performance CPU or DSP. The core supply, V
CORE
, in
this example is 1.0V
5%. The main power rail and I/O
voltage, V
I/O
, is 2.5V
5%. As shown in Figure 1, the MIC2776
is powered by V
I/O
. The minimum value of V
I/O
is 2.5V 5%
= 2.375V; the maximum is 2.5V +5% = 2.625V. This is well
within the MIC2776's power supply range of 1.5V to 5.5V.
Resistors R1 and R2 must be selected to correspond to the
V
CORE
supply of 1.0V. The goal is to insure that the core
supply voltage is adequate to insure proper operation, i.e.,
V
CORE
(1.0V 5%) = 0.950V. Because there is always a
small degree of uncertainty due to the accuracy of the
resistors, variations in the devices' voltage reference, etc.,
the threshold will be set slightly below this value. The poten-
tial variation in the MIC2776's voltage reference is specified
as
1.5%. The resistors chosen will have their own tolerance
specification. This example will assume the use of 1% accu-
rate resistors. The potential worst-case error contribution due
to input bias current can be calculated once the resistor
values are chosen. If the guidelines above regarding the
maximum total value of R1+R2 are followed, this error contri-
bution will be very small thanks to the MIC2776's very low
input bias current.
To summarize, the various potential error sources are:
Variation in V
REF
:
specified at
1.5%
Resistor tolerance:
chosen by designer (typically
1%)
Input bias current, I
IN
:
calculated once resistor values are known, typically
very small
Taking the various potential error sources into account, the
threshold voltage will be set slightly below the minimum
V
CORE
specification of 0.950V so that when the actual thresh-
old voltage is at its maximum, it will not intrude into the normal
operating range of V
CORE
. The target threshold voltage will
be set as follows:
Given that the total tolerance on V
TH
is [V
REF
tolerance] +
[resistor tolerance]
=
1.5% + 1% = 2.5%,
and V
TH(max)
= V
CORE(min)
,
then V
CORE(min)
= V
TH
+ 2.5% V
TH
= 1.025 V
TH
,
therefore, solving for V
TH
results in
V
=
V
1.025
=
0.950
1.025
= 0.9268V
TH
CORE(min)
Solving for R1 and R2 using this value for V
TH
and the
equations above yields:
R1 = 676.3k
673k
R2 = 323.7k
324k
The resulting circuit is shown in Figure 1.
Input Bias Current Effects
Now that the resistor values are known, it is possible to
calculate the maximum potential error due to input bias
current, I
IN
. As shown in the "Electrical Characteristics" table,
the maximum value of I
IN
is 10nA. (Note that the typical value
is a much smaller 5pA!) The magnitude of the offset caused
by I
IN
is given by:
V
I
R1|| R2
ERROR
IN(max)
=
(
)
=
V
1 10
A
2.189 10
=
ERROR
5
=
-8
V
2.189 10
V =
ERROR
3
=
-
V
2.189mV
ERROR
=
The typical error is about three orders of magnitude lower
than this - close to one
microvolt! Generally, the error due
to input bias can be discounted. If it is to be taken into
account, simply adjust the target threshold voltage
downward by this amount and recalculate R1 and R2. The
resulting value will be very close to optimum. If accuracy
is more important than the quiescent current in the
resistors, simply reduce the value of R
TOTAL
to minimize
offset errors.
September 29, 2000
7
MIC2776
MIC2776
Micrel
IN
/RST
VDD
/MR
GND
MIC2776
R1
676k
1%
R2
324k
1%
/RESET
VCORE
GND
V
CORE
1.0V
5%
Manual
Reset
MICROPROCESSOR
VI/O
V
I/O
2.5V
5%
Figure 1. MIC2776 Example Design
Interfacing to Processors With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bidi-
rectional, rather than input only. The Motorola 68HC11 family
is one example. Because the MIC2776N's output is open-
drain, it can be connected directly to the processor's reset pin
using only the pull-up resistor normally required. See Figure 2.
IN
/RST
VDD
/MR
GND
MIC2776N
R1
R2
/RESET
VCC
GND
V
CC
MICROPROCESSOR
100k
Figure 2. Interfacing to Bidirectional Reset Pin
Transient Response
The MIC2776 is inherently immune to very short negative-
going "glitches." Very brief transients may exceed the voltage
threshold without tripping the output.
As shown in Figure 3, the narrower the transient, the deeper
the threshold overdrive that will be ignored by the MIC2776.
The graph represents the typical allowable transient duration
for a given amount of threshold overdrive that will not gener-
ate a reset.
0
5
10
15
20
25
30
35
40
0
100
200
300
MAX. TRANSIENT DURATION (
s)
RESET COMP. OVERDRIVE, V
REF
V
IN
(mV)
Typical INPUT
Transient Response
Figure 3. Typical INPUT Transient Response
Ensuring Proper Operation at Low Supply
At levels of V
DD
below 1.2V, the MIC2776L's /RST output
driver cannot turn on sufficiently to produce a valid logic-low
on the /RST output. In this situation, other circuits driven by
/RST could be allowed to float, causing undesired operation.
(In most cases, however, it is expected that the circuits driven
by the MIC2776L will be similarly inoperative at V
DD
1.2V.)
If a given application requires that /RST be valid below V
DD
= 1.2V, this can be accomplished by adding a pull-down
resistor to the /RST output. A value of 100k
is recom-
mended as this is usually an acceptable compromise of
leakage current and pull-down current. The resistor's value is
not critical, however. See Figure 4.
The statements above also apply to the MIC2776H's RST
output. That is, to ensure valid RST signal levels at V
DD
<
1.2V, a pull-up resistor (as opposed to a pull-down) should be
added to the RST output. A value of 100k
is typical for this
application as well. See Figure 5.
IN
/RST
VDD
/MR
GND
MIC2776L
R1
R2
/RESET
VCC
GND
V
CC
Manual
Reset
MICROPROCESSOR
100k
Rpull-down
Figure 4. MIC2776L Valid /Reset Below 1.2V
IN
RST
VDD
/MR
GND
MIC2776H
R1
R2
RESET
VCC
GND
V
CC
Manual
Reset
MICROPROCESSOR
100k
Rpull-up
Figure 5. MIC2776H Valid Reset Below 1.2V
MIC2776
Micrel
MIC2776
8
September 29, 2000
Package Information
0.20 (0.008)
0.09 (0.004)
0.60 (0.024)
0.10 (0.004)
3.02 (0.119)
2.80 (0.110)
10
0
3.00 (0.118)
2.60 (0.102)
1.75 (0.069)
1.50 (0.059)
0.95 (0.037) REF
1.30 (0.051)
0.90 (0.035)
0.15 (0.006)
0.00 (0.000)
DIMENSIONS:
MM (INCH)
0.50 (0.020)
0.35 (0.014)
1.90 (0.075) REF
SOT-23-5 (M5)
MICREL INC.
1849 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
TEL
+ 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.
2000 Micrel Incorporated
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