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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 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
Temperature Range -40C to +85C -40C to +85C -40C to +85C Package SOT-23-5 SOT-23-5 SOT-23-5
Ordering Information
Part Number MIC2776N-BM5 MIC2776H-BM5 MIC2776L-BM5 Marking UKAA ULAA UMAA
Reset Output Open-Drain, Active-Low /RST Active-High, Complementary RST Active-Low, Complementary /RST
Typical Application
VCORE 1.0V VI/O 2.5V
R1 R2 Power_Good /MR Manual Reset GND MICROPROCESSOR VCORE VI/O VDD IN MIC2776L /RST /RESET GND
IttyBittyTM is a trademark of Micrel, Inc. Micrel, Inc. * 1849 Fortune Drive * San Jose, CA 95131 * USA * tel + 1 (408) 944-0800 * fax + 1 (408) 944-0970 * http://www.micrel.com
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MIC2776
MIC2776
Micrel
Pin Configuration
/MR GND RST
3 2 1
/MR GND /RST
3 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
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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
Electrical Characteristics
Symbol IDD VREF VHYST IIN Parameter Supply Current Condition
VDD = 3.3V; TA = +25C, bold values indicate -40C TA +85C; unless noted Min Typ 3.0 Max Units A VDD = VIN = 3.3V; /MR, RST, /RST open TA = 25C 295
IN, UNDER-VOLTAGE DETECTOR INPUT Under-Voltage Threshold Hysteresis Voltage Input Current TMIN TA TMAX RESET OUTPUTS (/RST, RST) tPROP tRST VOL Propagation Delay Reset Pulse Width RST or /RST Output Voltage Low ISINK = 1.6mA; VDD 1.6V ISINK = 100A; VDD 1.2V, Note 4 VOH RST or /RST Output Voltage High (H and L Version Only) MANUAL RESET INPUTS (/MR) VIH VIL tPROP tMIN IPU IIN
Note 1. Note 2. Note 3. Note 4.
300 3 5
305
mV mv pA
10
nA s
VIN = (VREF(MAX) + 100mV) to VIN = (VREF(MIN) - 100mV) 140
20 280 0.3 0.3 0.8VDD 0.8VDD
ms V V V V
ISOURCE = 500A; VDD 1.5V 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 V/MR < VIL
Input High Voltage Input Low Voltage Propagation Delay Minimum Input Pulse Width Internal Pull-Up Current Input Current, /MR
0.7VDD 0.3VDD 5 33 100 100
V V s ns nA nA
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
Timing Diagram
VDD 0V A A VIN VREF 0V V/MR VOH VOL tRST VRST VOH VOL tRST V/RST VOH VOL tRST tRST >tmin VHYST
Propagation delays not shown for clarity. Note A. The MIC2776 ignores very brief transients. See "Applications Information" for details.
MIC2776
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Functional Diagram
VDD
IPU /MR
R IN
Q
/RST*
Delay VREF
GND
One Shot
RST* S /Q
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 condi-
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 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
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 VTH = VCORE(min) 1.025 = 0.950 = 0.9268V 1.025
Application Information
Programming the Voltage Threshold Referring to the "Typical Application Circuit", the voltage threshold is calculated as follows:
VTH = VREF 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 =
(
R TOTAL VREF ) VTH
)(
R1 = R TOTAL - 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 = 1x 10 -8 A x 2.189 x 105 = VERROR = 2.189 x 10 -3 V =
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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MIC2776
VCORE 1.0V 5% VI/O 2.5V 5%
R1 676k 1% R2 324k 1% MIC2776 /RST
Micrel
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
MICROPROCESSOR VCC MIC2776L /RST
MICROPROCESSOR VCORE VI/O VDD IN /RESET GND
/MR Manual Reset
GND
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 opendrain, 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 MIC2776N VDD /RST R1 IN R2 /MR GND 100k /RESET GND
R1 R2
VDD IN
/RESET GND 100k Rpull-down
/MR Manual Reset
GND
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
MAX. TRANSIENT DURATION (s)
Figure 4. MIC2776L Valid /Reset Below 1.2V
VCC
MICROPROCESSOR VCC R1 MIC2776H RST 100k Rpull-up RESET GND
VDD IN
R2 /MR Manual Reset GND
40 35 30 25 20 15 10 5 0 0 100 200 300
Figure 5. MIC2776H Valid Reset Below 1.2V
RESET COMP. OVERDRIVE, VREF-VIN (mV)
Figure 3. Typical INPUT Transient Response
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MIC2776
MIC2776
Micrel
Package Information
1.90 (0.075) REF 0.95 (0.037) REF
1.75 (0.069) 1.50 (0.059)
3.00 (0.118) 2.60 (0.102)
DIMENSIONS: MM (INCH) 3.02 (0.119) 2.80 (0.110) 1.30 (0.051) 0.90 (0.035) 10 0 0.15 (0.006) 0.00 (0.000) 0.20 (0.008) 0.09 (0.004)
0.50 (0.020) 0.35 (0.014)
0.60 (0.024) 0.10 (0.004)
SOT-23-5 (M5)
MICREL INC.
TEL
1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA
FAX
+ 1 (408) 944-0800
+ 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. (c) 2000 Micrel Incorporated
MIC2776
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September 29, 2000

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