mic2776

mic2776
MIC2776
Micrel, Inc.
MIC2776
Micro-Power Low Voltage Supervisor
General Description
Features
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 supported
by previous supervisor ICs.
The reset output is asserted for no less than 140ms at poweron 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 3.0µA.
• User-adjustable input can monitor supplies as low as
0.3V
• ±1.5% threshold accuracy
• Separate VDD input
• Generates power-on reset pulse (140ms min.)
• Manual reset input
• Choice of active-high, active-low or open-drain activelow reset output
• Inputs can be pulled above VDD (7V abs. max.)
• Open-drain output can be pulled above VDD (7V abs.
max.)
• Ultra-low supply current, 3.0µA typical
• Rejects brief input transients
• IttyBitty™ SOT-23-5 package
Typical Application
Applications
•
•
•
•
•
•
•
•
VCORE 1.0V
VI/O 2.5V
Power_Good
R1
MIC2776L
VDD /RST
IN
Monitoring processor, ASIC, or FPGA core voltage
Computer systems
PDAs/Hand-held PCs
Embedded controllers
Telecommunications systems
Power supplies
Wireless / cellular systems
Networking hardware
MICROPROCESSOR
VCORE
VI/O
/RESET GND
R2
/MR
GND
Manual
Reset
IttyBitty™ is a 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
November 2005
1
MIC2776
MIC2776
Micrel, Inc
Ordering Information
Part Number
Reset Output
Temperature Range
Package
UKAA
Open-Drain, Active-Low / RST
–40ºC to +85ºC
SOT-23-5
ULAA
Active-High, Complementary RST
–40ºC to +85ºC
SOT-23-5
UMAA
Active-Low, Complementary /RST
–40ºC to +85ºC
SOT-23-5
Standard
Marking
Pb-Free
Marking
MIC2776N-BM5
UKAA
MIC2776N-YM5
MIC2776H-BM5
ULAA
MIC2776H-YM5
MIC2776L-BM5
UMAA
MIC2776L-YM5
Pin Configuration
/MR GND /RST
/MR GND RST
3
2
3
1
4
5
IN
VDD
2
1
4
5
IN
VDD
SOT-23-5 (M5)
“L” and “N” Version
SOT-23-5 (M5)
“H” Version
Pin Description
Pin Number
MIC2776H
Pin Number
MIC2776L
MIC2776N
1
Pin Name
Pin Function
RST
Digital (Output): Asserted high whenever VIN falls below the reference voltage. It will remain asserted for no less than 140ms after VIN returns above
the threshold limit.
1
/RST
Digital (Output): Asserted low whenever VIN falls below the reference voltage. It will remain asserted for no less than 140ms after VIN returns above
the threshold limit. (open-drain for “N” version)
2
2
GND
Ground
3
3
/MR
4
4
IN
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 VDD and may be left open if unused.
5
5
VDD
MIC2776
Analog (Input): The voltage on this pin is compared to the internal 300mV
reference. An under-voltage condition will trigger a reset sequence.
Analog (Input): Independent supply input for internal circuitry.
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November 2005
MIC2776
Micrel, Inc.
Absolute Maximum Ratings (Note 1)
Operating Ratings (Note 2)
Supply Voltage (VDD).......................................–0.3V to +7V
Input Voltages (VIN, V/MR) ...............................–0.3V to +7V
RST, (/RST) Current .................................................. 20mA
Storage Temperature (TS) ........................ –65°C to +150°C
ESD Rating, Note 3 .................................................... 1.5kV
Supply Voltage (VDD) .................................. +1.5V to +5.5V
Input Voltages (VIN, V/MR) ............................–0.3V to +6.0V
Output Voltages
V/RST (N version) ......................................–0.3V to +6.0V
V/RST, VRST (H and L versions)........ –0.3V to VDD + 0.3V
Ambient Temperature Range (TA) .............. –40°C to +85°C
Package Thermal Resistance (θJA) ....................... 256°C/W
Electrical Characteristics
VDD = 3.3V; TA = +25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted
Symbol
Parameter
Condition
IDD
Supply Current
VDD = VIN = 3.3V; /MR, RST, /RST open
VREF
Under-Voltage Threshold
IIN
Input Current
IN, UNDER-VOLTAGE DETECTOR INPUT
VHYST
Hysteresis Voltage
RESET OUTPUTS (/RST, RST)
tPROP
Propagation Delay
tRST
Reset Pulse Width
VOL
VOH
RST or /RST Output Voltage Low
RST or /RST Output Voltage High
(H and L Version Only)
MANUAL RESET INPUTS (/MR)
VIH
Input High Voltage
tPROP
Propagation Delay
IPU
Internal Pull-Up Current
VIL
tMIN
IIN
Input Low Voltage
Minimum Input Pulse Width
Input Current, /MR
Min
µA
305
mV
3
mv
5
pA
nA
20
140
ISINK = 1.6mA;
VDD ≥ 1.6V
ISINK = 100µA;
VDD ≥ 1.2V, Note 4
Units
10
µs
280
ms
0.3
V
0.3
V
ISOURCE = 500µA;
VDD ≥ 1.5V
0.8VDD
V
ISOURCE = 10µA;
VDD ≥ 1.2V, Note 4
0.8VDD
V
1.5V ≤ VDD ≤ 5.5V
0.7VDD
V
1.5V ≤ VDD ≤ 5.5V
V/MR < VIL
Reset Occurs, V/MR < VIL
V/MR < VIL
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.
November 2005
300
TMIN ≤ TA ≤ TMAX
VIN = (VREF(MAX) + 100mV) to
VIN = (VREF(MIN) – 100mV)
Max
3.0
295
TA = 25°C
Typ
3
5
0.3VDD
V
µs
33
ns
100
nA
100
nA
MIC2776
MIC2776
Micrel, Inc
Timing Diagram
VDD
0V
A
VHYST
A
VIN
VREF
0V
V/MR
VOH
VOL
VRST
VOH
VOL
V/RST
VOH
VOL
>tmin
tRST
tRST
tRST
tRST
Propagation delays not shown for clarity.
Note A.
MIC2776
The MIC2776 ignores very brief transients.
See “Applications Information” for details.
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MIC2776
Micrel, Inc.
Functional Diagram
VDD
IPU
/MR
IN
Delay
One Shot
R
Q
S
/Q
/RST*
RST*
VREF
GND
MIC2776
* Pinout and polarity vary by device type.
See ordering information table.
Functional Description
The reset outputs are asserted any time /MR is asserted or
if VIN drops below the threshold voltage. The reset outputs
remain asserted for tRST(min) after VIN 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 VIN is within tolerance when /MR is released (returns
high), the reset output will be de-asserted no less than tRST
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
VDD and may be left open if unused.
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.
RST, /RST Reset Output
Typically, the MIC2776 is used to monitor the power supply
of intelligent circuits such as microcontrollers and microprocessors. By connecting the reset output of a MIC2776 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. In addition, asserting /MR, the manual reset input, will
activate the reset function.
November 2005
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MIC2776
MIC2776
Micrel, Inc
Application Information
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
Programming the Voltage Threshold
Referring to the “Typical Application Circuit”, the voltage
threshold is calculated as follows:
VTH = VREF ×
(R1+ R2)
R2
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 values 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 =
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 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
VCORE(min) 0.950
= 0.9268V
VTH =
=
1.025
1.025
(RTOTAL )( VREF )
Solving for R1 and R2 using this value for VTH and the equations 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 calculate 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
VTH
R1 = RTOTAL – R2
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, 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 MIC2776
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 MIC2776’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 resistors,
variations in the devices’ voltage reference, etc., the threshold
will be set slightly below this value. The potential 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% accurate 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 contribution will be very small
thanks to the MIC2776’s very low input bias current.
MIC2776
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.
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November 2005
MIC2776
Micrel, Inc.
VCORE
1.0V 5%
VI/O
2.5V 5%
R1
676k
1%
MIC2776
VDD /RST
IN
R2
324k
1%
/MR
Ensuring Proper Operation at Low Supply
At levels of VDD 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 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 resistor to the /RST output. A value of 100kΩ is recommended 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 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 5.
MICROPROCESSOR
VCORE
VI/O
/RESET GND
GND
Manual
Reset
Figure 1. MIC2776 Example Design
Interfacing to Processors With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bidirectional, 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.
MIC2776N
VDD /RST
MICROPROCESSOR
VCC
MICROPROCESSOR
VCC
VCC
VCC
100k
R1
/RESET GND
R1
R2
IN
R2
/MR
MIC2776L
VDD /RST
IN
/MR
GND
/RESET GND
100k
Rpull-down
GND
Manual
Reset
Figure 2. Interfacing to Bidirectional Reset Pin
Figure 4. MIC2776L Valid /Reset Below 1.2V
Transient Response
The MIC2776 is inherently immune to very short negativegoing “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 generate a reset.
MICROPROCESSOR
VCC
VCC
R1
MIC2776H
VDD RST
IN
RESET
GND
R2
/MR
Typical INPUT
Transient Response
40
100k
Rpull-up
GND
Manual
Reset
35
30
Figure 5. MIC2776H Valid Reset Below 1.2V
25
20
15
10
5
0
0
100
200
300
RESET COMP. OVERDRIVE, VREF–VIN (mV)
Figure 3. Typical INPUT Transient Response
November 2005
7
MIC2776
MIC2776
Micrel, Inc
Package Information
SOT-23-5 (M5)
MICREL INC.
TEL
2180 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
+ 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.
MIC2776
8
November 2005
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