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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 poweron reset generation in a compact 5-pin SOT package. Features 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 subsequently rise back above the threshold boundaries. A reset can be generated at any time by pulling down on the adjustable input. Hysteresis is included to prevent chattering due to noise. Typical supply current is a low 3.5A.
Features
* Monitors two independent power supplies for under-voltage 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, 3.5A typical * Rejects brief input transients * IttyBittyTM 5-pin SOT-23 package
Applications
* Monitoring processor, ASIC, or FPGA core and I/O voltages * Computer systems * PDAs, hand-held PCs * Embedded controllers * Telecommunications systems * Power supplies * Wireless / Cellular systems * Networking hardware
Ordering Information
Part Number Standard Marking Pb-Free MIC2777-XXYM5 Marking UNXX MIC2777-XXBM5 UNXX Junction Tem. Range -40C to +85C Package SOT-23-5
Typical Application
VCORE 1.0V VI/O 1.8V
R1 R2 MIC2777 VDD /RST IN RST
MICROPROCESSOR VCORE
VI/O /RESET
GND
GND
Manual Reset
OTHER LOGIC
Typical Application
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
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MIC2777
MIC2777
Micrel, Inc.
Standard Voltage Options*
Voltage Code 46 44 31 29 28 26 25 23 22 17
Typical Application (VDD) 5.0V 5% 5.0V 10% 3.3V 5% 3.3V 10% 3.0V 5% 2.85V 5% 2.70V 5% 2.5V 5% 2.4V 5% 1.8V 5%
Nominal Threshold Voltage (VTH) 4.68 4.43 3.09 2.93 2.81 2.67 2.53 2.34 2.25 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 Configuration
RST GND /RST
3 2 1
4
5
IN
VDD
SOT-23-5 (M5)
Pin Description
Pin Number 1 Pin Name /RST Pin Function 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. Ground Digital (Output): Asserted high whenever VIN or VDD falls below the reference voltage. It will remain asserted for no less than 140ms after VIN and VDD return within above the threshold limit.
2 3
GND RST
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. 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.
5
VDD
MIC2777
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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) ........................ -65C to +150C 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) .............. -40C to +85C Package Thermal Resistance (JA) ....................... 256C/W
Electrical Characteristics
Note 5; TA = +25C, bold values indicate -40C TA +85C; unless noted Symbol Parameter Condition IDD Supply Current VDD = VIN Note 5; /MR, RST, /RST open VTH-1.5% Min Typ 3.5 Max Units A
VDD VOLTAGE THRESHOLD VHYST VREF IIN Under-Voltage Threshold On VDD Hysteresis Voltage VTH 1 VTH+1.5% V % mV mV pA 10 20 nA s
IN, UNDER-VOLTAGE DECTECTOR INPUT Under-Voltage Threshold Hysteresis Voltage Input Current TMIN TA TMAX VIN = (VREF(MAX) + 100mV) to VIN = (VREF(MIN) - 100mV) or VDD = (VTH + 2.5%) to (VTH - 2.5%), VIN =VDD ISINK = 1.6mA; VDD 1.6V 295 300 3 5 305 VHYST
RST, /RST OUTPUTS tPROP tRST VOL Propagation Delay
Reset Pulse Width RST or /RST Output Voltage Low
140
280 0.3 0.3
ms V V V V
VOH
RST or /RST Output Voltage High
RST asserted, ISOURCE = 10A; VDD 1.2V, Note 4
Note 1. Note 2. Note 3. Note 4. Note 5. Exceeding the absolute maximum rating may damage the device. The device is not guaranteed to function outside its operating rating.
ISOURCE = 500A; VDD 1.5V
ISINK = 100A; VDD 1.2V, Note 4
0.8VDD 0.8VDD
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. VDD equals the nominal "Typical Application (VDD)" as shown in "Standard Voltage Options Table."
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MIC2777
MIC2777
Micrel, Inc.
VDD 0V VIN 0V V/RST VOH VOL VOH VOL
Propagation delays not shown for clarity. Note A. The MIC2777 ignores very brief transients. See "Applications Information" for details.
Timing Diagram
A
A
VHYST VTH A VHYST VREF
(ACTIVE LOW)
tRST
tRST
VRST (ACTIVE HIGH)
MIC2777
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MIC2777
Micrel, Inc.
Functional Diagram
VDD
R VREF IN Delay Line One Shot
Q
/RST
S /Q
RST
VREF MIC2777 GND
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 microprocessors. By connecting the appropriate reset output of a MIC2777 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. 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.
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MIC2777
MIC2777
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 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 VCORE(min) 0.950 = 0.9268V VTH = = 1.025 1.025 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.
Application Information
Programming the Voltage Threshold Referring to the "Typical Application Circuit", the voltage threshold on the IN pin 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 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 resistors, 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 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 MIC2777's very low input bias current.
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MIC2777
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 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 resistor to the /RST output. A value of 100k 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 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 4.
VCC
MICROPROCESSOR VCC
R1 676k 1% R2 324k 1%
MIC2777-23 VDD /RST IN RST GND
/RESET GND
Figure 1. MIC2777 Example Design Transient response The MIC2777 is inherently immune to very short negativegoing "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.
Typical INPUT Transient Response
R1 R2
40 35 30 25 20 15 10 5 0 0
MIC2777-XX VDD /RST IN RST GND
/RESET GND
100k Rpull-down
Figure 3. MIC2777 Valid /RST Below 1.2V
VCC
100 200 300
MICROPROCESSOR VCC
RESET COMP. OVERDRIVE, VREF-VIN(mV)
R1 R2
Figure 2a. Typical INPUT Transient Response
Typical V
100 80 60 40 20 0 0 500 1000 1500 2000 RESET COMP. OVERDRIVE, VREF-VDD(mV)
MIC2777-XX VDD RST IN /RST GND
100k Rpull-up
RESET
GND
Transient Response
DD
Figure 4. MIC2777 Valid RST Below 1.2V
Figure 2b. Typical VDD Transient Response
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MIC2777
MIC2777
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.
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