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MIC2776
Micrel, Inc.
MIC2776
Micro-Power Low Voltage Supervisor
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 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 3.0A.
Features
* 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, 3.0A 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
Typical Application
VCORE 1.0V
VI/O 2.5V
R1 R2
MIC2776L VDD /RST IN /MR GND
MICROPROCESSOR VCORE VI/O
/RESET GND
Power_Good
Manual Reset
IttyBittyTM 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
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MIC2776
MIC2776
Micrel, Inc
Ordering Information
Part Number Standard MIC2776N-BM5 MIC2776H-BM5 MIC2776L-BM5 Marking UKAA ULAA UMAA Pb-Free MIC2776N-YM5 MIC2776H-YM5 MIC2776L-YM5 Marking UKAA ULAA UMAA Reset Output Open-Drain, Active-Low / RST Active-High, Complementary RST Active-Low, Complementary /RST Temperature Range -40C to +85C -40C to +85C -40C to +85C Package SOT-23-5 SOT-23-5 SOT-23-5
Pin Configuration
/MR GND RST
3
/MR GND /RST
3
2
1
2
1
4
5
4
5
IN
VDD
IN
VDD
SOT-23-5 (M5) "H" Version
SOT-23-5 (M5) "L" and "N" Version
Pin Description
Pin Number MIC2776H 1 Pin Number MIC2776L MIC2776N 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. 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) Ground 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. 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.
1
/RST
2 3
2 3
GND /MR
4 5
4 5
IN VDD
MIC2776
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MIC2776
Micrel, Inc.
Absolute Maximum Ratings (Note 1)
Supply Voltage (VDD).......................................-0.3V to +7V Input Voltages (VIN, V/MR) ...............................-0.3V to +7V RST, (/RST) Current .................................................. 20mA Storage Temperature (TS) ........................ -65C to +150C ESD Rating, Note 3 .................................................... 1.5kV
Operating Ratings (Note 2)
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) .............. -40C to +85C Package Thermal Resistance (JA) ....................... 256C/W
Min Typ 3.0 295 300 3 5 TMIN TA TMAX VIN = (VREF(MAX) + 100mV) to VIN = (VREF(MIN) - 100mV) ISINK = 1.6mA; VDD 1.6V 20 140 280 0.3 0.3 0.8VDD 0.8VDD 10 305 Max Units A mV mv pA nA s ms V V V V
Electrical Characteristics
Symbol Parameter Condition IDD Supply Current Under-Voltage Threshold Hysteresis Voltage Input Current
VDD = 3.3V; TA = +25C, bold values indicate -40C TA +85C; unless noted VDD = VIN = 3.3V; /MR, RST, /RST open TA = 25C
IN, UNDER-VOLTAGE DETECTOR INPUT
VREF IIN
VHYST
RESET OUTPUTS (/RST, RST) tPROP tRST VOL Propagation Delay Reset Pulse Width RST or /RST Output Voltage Low
VOH
RST or /RST Output Voltage High (H and L Version Only)
MANUAL RESET INPUTS (/MR) VIH VIL tPROP tMIN IPU IIN Input High Voltage Input Low Voltage Propagation Delay Minimum Input Pulse Width Internal Pull-Up Current Input Current, /MR
ISOURCE = 10A; VDD 1.2V, Note 4 1.5V VDD 5.5V 1.5V VDD 5.5V V/MR < VIL Reset Occurs, V/MR < VIL
ISOURCE = 500A; VDD 1.5V
ISINK = 100A; VDD 1.2V, Note 4
0.7VDD 5 33 100 100
V 0.3VDD V s ns nA nA
V/MR < VIL
Note 1. Note 2. Note 3. Note 4.
Exceeding the absolute maximum rating may damage the device. The device is not guaranteed to function outside its operating rating. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. VDD operating range is 1.5V to 5.5V. Output is guaranteed to be asserted down to VDD = 1.2V.
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MIC2776
MIC2776
Micrel, Inc
Timing Diagram
VDD 0V
A A VHYST VREF
VIN 0V V/MR
>tmin
VOH VOL
VOH VOL
tRST
tRST
tRST
VRST
tRST
V/RST
VOH VOL
Propagation delays not shown for clarity. Note A. The MIC2776 ignores very brief transients. See "Applications Information" for details.
MIC2776
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November 2005
MIC2776
Micrel, Inc.
VDD
IPU
Functional Diagram
/MR
IN
Delay
One Shot
R
Q
/RST*
S
/Q
RST*
VREF GND
MIC2776
* Pinout and polarity vary by device type. See ordering information table.
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. 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 C or P, 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.
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.
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MIC2776
MIC2776
Micrel, Inc 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 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
Application Information
Programming the Voltage Threshold Referring to the "Typical Application Circuit", the voltage threshold is calculated as follows: VTH = V REF x
(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 1M 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 3M. Applying this criteria and rearranging the VTH expression to solve for the resistor values gives: R2 =
(RTOTAL )( VREF )
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.
Solving for R1 and R2 using this value for VTH 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, 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) x (R1||R2) = VERROR = 1 x 10-8A x 2.189 x105 = VERROR = 2.189 x 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.
MIC2776
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November 2005
MIC2776
VCORE 1.0V 5%
VI/O 2.5V 5% MICROPROCESSOR VCORE VI/O
Micrel, Inc. 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 100k 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 100k is typical for this application as well. See Figure 5.
VCC
MIC2776L VDD /RST IN /MR GND
R1 676k 1% R2 324k 1%
MIC2776 VDD /RST IN /MR GND
/RESET 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.
VCC
MICROPROCESSOR VCC
MICROPROCESSOR
VCC
R1 R2
MIC2776N VDD /RST R1 IN R2 /MR GND
100k
/RESET GND
/RESET GND
100k Rpull-down
Manual Reset
Figure 2. Interfacing to Bidirectional Reset Pin 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.
Typical INPUT Transient Response
Figure 4. MIC2776L Valid /Reset Below 1.2V
VCC
MIC2776H VDD RST IN /MR GND
MICROPROCESSOR
VCC
R1 R2
100k Rpull-up
RESET GND
40 35 30 25 20 15 10 5 0 0
Manual Reset
Figure 5. MIC2776H Valid Reset Below 1.2V
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
+ 1 (408) 944-0800 FAX + 1 (408) 474-1000 WEB http://www.micrel.com
2180 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
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. (c) 2000 Micrel, Inc.
MIC2776
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November 2005

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