November 2005 1 MIC2777
MIC2777 Micrel, Inc.
MIC2777
Dual Micro-Power Low Voltage Supervisor
General Description
The MIC2777 is a dual power supply supervisor that provides
under-voltage monitoring, manual reset capability, and power-
on reset generation in a compact 5-pin SOT package. Fea-
tures include two under-voltage detectors, one fixed and one
adjustable, and both active-high and active-low reset outputs.
One under-voltage detector compares VDD against a fixed
threshold. Ten factory-programmed thresholds are available.
The other under-voltage detector is user-adjustable.
The reset outputs are asserted for no less than 140ms at
power-on and any time VDD or the input voltage drops below
the corresponding reference voltage. They remain asserted
for the timeout period after the input voltage and VDD sub-
sequently rise back above the threshold boundaries. A reset
can be generated at any time by pulling down on the adjust-
able input. Hysteresis is included to prevent chattering due
to noise. Typical supply current is a low 3.5µA.
Typical Application
IN
R1
R2
/RST
VDD
RST
OTHER
LOGIC
MIC2777
MICROPROCESSOR
Manual
Reset
/RESET
VCORE
VCORE
1.0V
VI/O 1.8V
VI/O
GND
GND
Typical Application
Features
• Monitors two independent power supplies for under-volt-
age conditions
• One fixed and one user adjustable input
• 1.5% theshold accuracy
• Choice of factory-programmed thresholds
• User-adjustable input can monitor supplies as low as
0.3V
• Generates 140ms (minimum) power-on RESET pulse
• Manual reset capability
• Both active-high and active-low RESET outputs
• Input may be pulled above VDD (abs. max.)
• /RST output valid down to 1.2V
• Ultra-low supply current, 3.5µA typical
• Rejects brief input transients
• IttyBitty™ 5-pin SOT-23 package
Applications
• Monitoring processor, ASIC, or FPGA core and I/O volt-
ages
• Computer systems
• PDAs, hand-held PCs
• Embedded controllers
• Telecommunications systems
• Power supplies
• Wireless / Cellular systems
• Networking hardware
Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
IttyBitty™ is a trademark of Micrel, Inc.
Ordering Information
Part Number Junction Tem. Range Package
Standard Marking Pb-Free Marking
MIC2777-XXBM5 UNXX MIC2777-XXYM5 UNXX –40ºC to +85ºC SOT-23-5
MIC2777 Micrel, Inc.
MIC2777 2 November 2005
Pin Configuration
/RST
VDDIN
RST
13
4 5
2
GND
SOT-23-5 (M5)
Standard Voltage Options*
Voltage Typical Nominal Threshold
Code Application (VDD) Voltage (VTH)
46 5.0V ±5% 4.68
44 5.0V ±10% 4.43
31 3.3V ±5% 3.09
29 3.3V ±10% 2.93
28 3.0V ±5% 2.81
26 2.85V ±5% 2.67
25 2.70V ±5% 2.53
23 2.5V ±5% 2.34
22 2.4V ±5% 2.25
17 1.8V ±5% 1.69
*There are ten standard versions available with an order increment of 3000 pieces. Samples of standard versions are
normally available from stock. Contact factory for information on non-standard versions. Available in tape-and-reel only.
Pin Description
Pin Number Pin Name Pin Function
1 /RST Digital (Output): Asserted low whenever VIN or VDD falls below the reference
voltage. It will remain asserted for no less than 140ms after VIN and VDD
return above the threshold limits.
2 GND Ground
3 RST Digital (Output): Asserted high whenever VIN or VDD falls below the refer-
ence voltage. It will remain asserted for no less than 140ms after VIN and
VDD return within above the threshold limit.
4 IN Analog (Input): The voltage on this pin is compared to the internal 300mV
reference. An undervoltage condition will trigger a reset sequence. Manual
reset capability can be achieved by adding a switch between this pin and
ground.
5 VDD Analog (Input): Power supply input for internal circuitry and input to the fixed
voltage monitor: The voltage on this pin is compared against the internal
voltage reference. An undervoltage condition will trigger a reset sequence.
November 2005 3 MIC2777
MIC2777 Micrel, Inc.
Absolute Maximum Ratings (Note 1)
Supply Voltage (VDD) .......................................–0.3V to +7V
Input Voltage (VIN) ...........................................–0.3V to +7V
RST, (/RST) Current ................................................... 20mA
Storage Temperature (TS) ........................ –65°C to +150°C
ESD Rating, Note 3 .................................................... 1.5kV
Operating Ratings (Note 2)
Supply Voltage (VDD) .................................. +1.5V to +5.5V
Input Voltage
(VIN) .............................................................–0.3V to +6.0V
Ambient Temperature Range (TA) .............. –40°C to +85°C
Package Thermal Resistance (θJA) ....................... 256°C/W
Electrical Characteristics
Note 5; TA = +25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted
Symbol Parameter Condition Min Typ Max Units
IDD Supply Current VDD = VIN 3.5 µA
Note 5; /MR, RST, /RST open
VDD VOLTAGE THRESHOLD
Under-Voltage Threshold On VDD
VTH-1.5%
VTH
VTH+1.5%
V
VHYST Hysteresis Voltage 1 %
IN, UNDER-VOLTAGE DECTECTOR INPUT
VREF Under-Voltage Threshold 295 300 305 mV
VHYST Hysteresis Voltage 3 mV
IIN Input Current 5 pA
TMIN ≤ TA ≤ TMAX 10 nA
RST, /RST OUTPUTS
tPROP Propagation Delay VIN = (VREF(MAX) + 100mV) to 20 µs
VIN = (VREF(MIN) – 100mV) or
VDD = (VTH + 2.5
%
) to (VTH – 2.5
%
), VIN =VDD
tRST Reset Pulse Width 140 280 ms
VOL RST or /RST Output Voltage Low ISINK = 1.6mA; 0.3 V
VDD ≥ 1.6V
ISINK = 100µA; 0.3 V
VDD ≥ 1.2V, Note 4
VOH RST or /RST Output Voltage High ISOURCE = 500µA; 0.8VDD V
VDD ≥ 1.5V
RST asserted, ISOURCE = 10µA; 0.8VDD V
VDD ≥ 1.2V, Note 4
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. VDD operating range is 1.5V to 5.5V. Output is guaranteed to be asserted down to VDD = 1.2V.
Note 5. VDD equals the nominal “Typical Application (VDD)” as shown in “Standard Voltage Options Table.”
MIC2777 Micrel, Inc.
MIC2777 4 November 2005
Timing Diagram
VTH
VIN
0V
VHYST
VDD
0V
V/RST
(ACTIVE LOW)
VOH
VOL
VRST
(ACTIVE HIGH)
VOH
VOL
tRST tRST
A
AA
VREF
VHYST
Propagation delays not shown for clarity.
Note A. The MIC2777 ignores very brief transients.
See “Applications Information” for details.
November 2005 5 MIC2777
MIC2777 Micrel, Inc.
Functional Diagram
GND
VDD
IN
MIC2777
VREF
VREF
RST
R
S
Q
/Q
/RST
Delay
Line
One Shot
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 VIN
falls below VREF. Typically, a resistor divider is used to scale
the input voltage to be monitored such that VIN will fall below
VREF as the voltage being monitored falls below the desired
trip-point. Hysteresis is employed to prevent chattering due
to noise. The comparator on the IN input is relatively immune
to very brief negative-going transients.
VDD Input
The VDD pin is both the power supply terminal and a monitored
input voltage. The voltage at this pin is continually compared
against the internal reference. The trip-point at which a reset
occurs is factory programmed. A reset is triggered if and
when VDD falls below the trip-point. Hysteresis is employed
to prevent chattering due to noise. The comparator on the
VDD input is relatively immune to very brief negative-going
transients.
RST, /RST Reset Output
Typically, the MIC2777 is used to monitor the power supplies
of intelligent circuits such as microcontrollers and micropro-
cessors. By connecting the appropriate reset output of a
MIC2777 to the reset input of a µC or µP, the processor will
be properly reset at power-on and during power-down and
brown-out conditions.
The reset outputs are asserted any time VDD or VIN drops
below the corresponding threshold voltage. The reset outputs
remain asserted for tRST(min) after VIN and/or VDD subsequent
return above the threshold boundaries and/or /MR is released.
A reset pulse is also generated at power-on.
Manual Reset
The ability to initiate a reset via external logic or a manual
switch is provided in addition to the MIC2777’s automatic
supervisory functions. Typically, a momentary push-button
switch is connected such that IN is shorted to ground when
the switch contacts close. Assuming VDD and VIN are within
tolerance when the switch is released, the reset outputs will
be de-asserted no less than 140ms later. IN can also be driven
by an open-drain or open-collector logic signal.
MIC2777 Micrel, Inc.
MIC2777 6 November 2005
Application Information
Programming the Voltage Threshold
Referring to the “Typical Application Circuit”, the voltage
threshold on the IN pin is calculated as follows:
V V R1 R2
R2
TH REF
= × +
( )
where VREF = 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 = RTOTAL. Imposing this condition on the resistor val-
ues provides two equations that can be solved for the two
unknown resistor values. A value such as 1MΩ for RTOTAL
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 (IIN). The
maximum recommended value of RTOTAL is 3MΩ.
Applying this criteria and rearranging the VTH expression to
solve for the resistor values gives:
R2 R V
V
TOTAL REF
TH
=
( )
( )
R1 = RTOTAL – R2
Application Example
Figure 1 illustrates a hypothetical MIC2777-23 application
in which the MIC2777-23 is used to monitor the core and
I/O supplies of a high-performance CPU or DSP. The core
supply, VCORE, in this example is 1.0V ±5%. The main power
rail and I/O voltage, VI/O, is 2.5V ±5%. As shown in Figure 1,
the MIC2777 is powered by VI/O. The minimum value of VI/O
is 2.5V –5% = 2.375V; the maximum is 2.5V +5% = 2.625V.
This is well within the MIC2777’s power supply range of
1.5V to 5.5V.
Resistors R1 and R2 must be selected to correspond to the
VCORE supply of 1.0V. The goal is to insure that the core
supply voltage is adequate to insure proper operation, i.e.,
VCORE ≥ (1.0V –5%) = 0.950V. Because there is always a
small degree of uncertainty due to the accuracy of the resis-
tors, variations in the devices’ voltage reference, etc., the
threshold will be set slightly below this value. The potential
variation in the MIC2777’s input voltage reference (VREF) is
specified as ±1.5%. The resistors chosen will have their own
tolerance specification. This example will assume the use of
1% accurate resistors. The potential worst-case error contri-
bution 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
contribution will be very small thanks to the MIC2777’s very
low input bias current.
To summarize, the various potential error sources are:
• Variation in VREF: specified at ±1.5%
• Resistor tolerance: chosen by designer (typically ≤
±1%)
• Input bias current, IIN: 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 VCORE
specification of 0.950V so that when the actual threshold
voltage is at its maximum, it will not intrude into the normal
operating range of VCORE. The target threshold voltage will
be set as follows:
Given that the total tolerance on VTH for the IN pin is [VREF
tolerance] + [resistor tolerance]
= ±1.5% + ±1% = ±2.5%,
and VTH(max) = VCORE(min),
then VCORE(min) = VTH + 2.5% VTH = 1.025 VTH,
therefore, solving for VTH results in
V = V
1.025 =0.950
1.025 = 0.9268V
TH
CORE(min)
Solving for R1 and R2 using this value for VTH and the equa-
tions above yields:
R1 = 676.3kΩ ≈ 673kΩ
R2 = 323.7kΩ ≈ 324kΩ
The resulting circuit is shown in Figure 1.
Input Bias Current Effects
Now that the resistor values are known, it is possible to cal-
culate the maximum potential error due to input bias current,
IIN. As shown in the “Electrical Characteristics” table, the
maximum value of IIN is 10nA. (Note that the typical value
is a much smaller 5pA) The magnitude of the offset caused
by IIN is given by:
VERROR = IIN(max) × (R1||R2) =
VERROR = ±1 × 10-8A × 2.189 ×105Ω =
VERROR = ±2.189 × 10-3V =
VERROR = ±2.189mV
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 RTOTAL to minimize
offset errors.
November 2005 7 MIC2777
MIC2777 Micrel, Inc.
IN
/RST
RST
VDD
GND
MIC2777-23
R1
676k
1%
R2
324k
1%
/RESET
VCORE
GND
VCORE
1.0V 5%
MICROPROCESSOR
VI/O
VI/O
2.5V 5%
Figure 1. MIC2777 Example Design
Transient response
The MIC2777 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the voltage
thresholds without tripping the output.
As shown in Figure 2a and 2b, in general the narrower the
transient, the deeper the threshold overdrive that will be
ignored by the MIC2777. The graph represents the typical
allowable transient duration for a given amount of threshold
overdrive that will not generate a reset.
Figure 2a. Typical INPUT Transient Response
0
20
40
60
80
100
0 500 1000 1500 2000
RESET COMP. OVERDRIVE, VREF–VDD(mV)
Typical VDD
Transient Response
Figure 2b. Typical VDD Transient Response
Ensuring Proper Operation at Low Supply
At levels of VDD below 1.2V, the MIC2777’s /RST output
cannot turn on sufficiently to produce a valid logic-low on
/RST. 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
MIC2777 will be similarly inoperative at VDD ≤ 1.2V.)
If a given application requires that /RST be valid below VDD
= 1.2V, this can be accomplished by adding a pull-down re-
sistor to the /RST output. A value of 100kΩ is recommended
as this is usually an acceptable compromise of quiescent
current and pull-down current. The resistor’s value is not
critical, however. See Figure 3.
The statements above also apply to the MIC2777’s RST out-
put. That is, to ensure valid RST signal levels at VDD<1.2V,
a pull-up resistor (as opposed to a pull-down) should be
added to the RST output. A value of 100kΩ is typical for this
application as well. See Figure 4.
IN
/RST
RST
VDD
GND
MIC2777-XX
R1
R2
/RESET
VCC
GND
VCC
MICROPROCESSOR
100k
Rpull-down
Figure 3. MIC2777 Valid /RST Below 1.2V
IN
RST
/RST
VDD
GND
MIC2777-XX
R1
R2
RESET
VCC
GND
VCC
MICROPROCESSOR
100k
Rpull-up
Figure 4. MIC2777 Valid RST Below 1.2V
MIC2777 Micrel, Inc.
MIC2777 8 November 2005
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
This 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.
© 2000 Micrel, Inc.
Package Information
SOT-23-5 (M5)