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MIC2778
Micrel
MIC2778
Voltage Monitor with Adjustable Hysteresis
General Description
The MIC2778 is a voltage monitor--uniquely designed to detect two separate voltage thresholds--combined with a delay generator and logic. It is designed for monitoring the battery supply of portable digital systems, particularly PDAs, pagers, and cellular telephones. High- and low-voltage thresholds can be adjusted independently, allowing for wide hysteresis. Voltage detection thresholds are accurate to 2%. If the battery voltage falls below the low-voltage threshold, the output (RST) is asserted and latched, preventing system operation until the battery is replaced or recharged. Internal logic prevents the output from chattering due to battery recovery or load removal. The output is asserted for 140ms (minimum) when a fresh battery is inserted. For applications not requiring built-in delay, see MIC841. The IC's power supply input is separate from the detector inputs, allowing the MIC2778 to be powered from a downstream supply, such a boost converter. Supply current is extremely low (1A, typical), making it ideal for portable applications. A high-precision 1% grade is available. The MIC2778 is supplied in Micrel's IttyBittyTM 5-lead SOT-23-5 package.
Features
* Optimized for PDAs, cellular telephones, pagers, and other battery-powered devices * Independently adjustable high- and low-voltage thresholds * Internal logic prevents battery-voltage-fluctuation chatter * High 2% voltage threshold accuracy; 1% available * Built in 140ms (minimum) delay deglitches output * Extremely low 1A typical supply current * For applications not requiring built-in delay, see MIC841 * Immune to brief power supply transients * 5-lead SOT-23 package
Applications
* * * * * * PDAs Pagers Cordless phones Consumer electronics Embedded controllers Personal electronics
Ordering Information
Part Number Standard MIC2778-2BM5 MIC2778-1BM5 Marking UFB UFA Pb-Free MIC2778-2YM5 MIC2778-1YM5 Marking UFB* UFA* 2% 1% -40C to +85C -40C to +85C SOT-23-5 SOT-23-5 Accurary Temperature Range Package
* Underscore indicates a Pb-Free part.
Typical Application
MIC5207-3.0BM5
Li-Ion Cell
604k 1% 56k 1% 340k 1%
MIC2778 VDD LTH HTH R ST GND
IN
100k
OUT BYP GND
V OUT 3.0V
470pF
4.7F
EN
V BAT(hi) = 3.6V V BAT(lo) = 3.1V
Cellular Telephone Battery Monitor
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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M9999-011405
MIC2778
Micrel
Pin Configuration
LTH GND HTH
3 2 1
4
5
R ST
VDD
SOT-23-5 (M5)
Pin Description
Pin Number 1 Pin Name HTH Pin Function High-Voltage Theshold (Input): Analog input to a comparator. When the level on this pin initially rises above VREF, the delay generator cycles and the RST remains low for a minimum of 140ms. Ground Low-Voltage Threshold (Input): Analog input to a comparator. This is the voltage monitor input assigned to detect a low voltage condition. When the level on this pin falls below VREF, RST is asserted and the condition is latched until VHTH > VREF.
2 3
GND LTH
4
RST
5
VDD
Reset (Output): Active-low, open-drain output. This output is asserted and latched when VLTH VREF. Power Supply (Input): Independent supply input for internal circuitry.
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MIC2778
Micrel
Absolute Maximum Ratings (Note 1)
Supply Voltage (VDD).......................................-0.3V to +7V Input Voltages (VHI, VLO) .................................-0.3V to +7V RST Output Current (IRST) ......................................... 20mA Storage Temperature (TS) ........................ -65C to +150C ESD Rating, Note 3 ....................................................... 2kV
Operating Ratings (Note 2)
Supply Voltage (VDD) .................................. +1.5V to +5.5V Input Voltages (VRST, VLTH, VHTH)................-0.3V to +6.0V Ambient Temperature Range (TA) .............. -40C to +85C Package Thermal Resistance................................ 256C/W
Electrical Characteristics
Symbol IDD 1.5V VDD 5.5V; TA = +25C, bold values indicate -40C TA +85C; unless noted Parameter Condition Supply Current Input Leakage Current Reference Voltage MIC2778-2 MIC2778-1 tD tRESET VRST Propagation Delay Reset Pulse Width Voltage-Low Reset VLTH = VREF(max) +100mV to VREF(min) - 100mV RST asserted, ISINK = 1.6mA, VDD 1.6V 1.215 1.228 5 140 420 0.3 0.4 RST not asserted Min Typ 1 5 10 1.240 1.265 1.252 Max 2 Units A pA nA V V V s ms V V
ILTH, IHTH VREF
RST asserted, ISINK = 100A, VDD 1.2V,
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 held low down to VDD = 1.2V.
Note 1. Note 2. Note 3. Note 4.
Timing Diagram
VBATT VHTH VLTH 0V tRST A tRST A
VRST
VRSTH VRSTL
Propagation delays not shown for clarity. Note A. The MIC2778 ignores very brief transients. See "Applications Information" for details.
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Micrel
Functional Diagram
V BATT
Low-Voltage Detect
V LO
LTH
RQ
High-Voltage Detect
RST
V HI
HTH
SQ
Delay Line
1.24V Bandgap Reference
MIC2778
GND
Functional Description
The MIC2778 monitors the voltage of a battery and detects when it is discharged below a programmed level. Upon being replaced, or being recharged above a second higher programmed trip point, the output remains low for a minimum of 140ms and then sends a reset signal to a microprocessor or other downstream component. Voltage Low Output The voltage-low output (RST) is an active-low, open-drain output which sinks current when the MIC2778 detects a low input voltage.
Trip Points Battery voltage is monitored by a comparator via a voltage divider network. The divided voltage is compared to an internal reference voltage. When the voltage at the LTH input pin drops below the internal reference voltage, the output pulls low. At this point, the voltage at HTH is assumed to be below the reference voltage. Delay At power-on or when the battery is replaced or recharged, and the voltage at HTH exceeds the reference voltage, the output goes high after a minimum delay of 140ms.
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MIC2778
Micrel of the two remaining resistor values. 1M VBAT(lo) = 3.1V = 1.24 R2 + 344k R2 = 56k R1 = 1M - R2 - R3 R1 = 600k The accuracy of the resistors can be chosen based upon the accuracy required by the system. Input Transients The MIC2778 is inherently immune to very short negativegoing "glitches." Very brief transients may exceed the VBAT(lo) threshold without tripping the output. As shown in Figure 2, the narrower the transient, the deeper the threshold overdrive that will be ignored by the MIC2778. The graph represents the typical allowable transient duration for a given amount of threshold overdrive that will not generate a reset.
140
Applications Information
Output Since the MIC2778 output is an open-drain MOSFET, most applications will require a pull-up resistor. The value of the resistor should not be too large or leakage effects may dominate. 470k is the maximum recommended value. Programming the Thresholds The low-voltage threshold is calculated using: R1 + R2 + R3 VBAT(lo) = VREF R2 + R3 The high-voltage threshold is calculated using: R1 + R2 + R3 VBAT(hi) = VREF R3 where, for both equations: VREF = 1.240V In order to provide the additional criteria needed to solve for the resistor values, the resistors can be selected such that they have a given total value, that is, R1 + R2 + R3 = RTOTAL. A value such as 1M for RTOTAL is a reasonable value because it draws minimum battery current but has no significant effect on accuracy. When working with large resistors, a small amount of leakage current can cause voltage offsets that degrade system accuracy. The maximum recommended total resistance from VBAT to ground is 3M.
V BATT
Input T rans ient R es pons e
MAX. TRANSIENT DURATION (s)
120 100 80 60 40 20 0 1 10 100 1000
R1 R2 R3
604k 1% 56k 1% 340k 1%
MIC2778 VDD LTH HTH R ST GND
470k
RESET COMP. OVERDRIVE, V REF -V LTH (mV)
Figure 2. Input Transient Response 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 MIC2778'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 3.
V BATT
Microprocessor
Figure 1. Example Circuit Once the desired trip points are determined, set the VBAT(hi) threshold first. For example, use a total of 1M = R1 + R2 + R3. For a typical single-cell lithium ion battery, 3.6V is a good "high threshold" because at 3.6V the battery is moderately charged. Solving for R3: 1M VBAT(hi) = 1.24 R3 R3 = 344k Once R3 is determined, the equation for VBAT(lo) can be used to determine R2. A single lithium-ion cell should not be discharged below 2.5V. Many applications limit the drain to 3.1V. Using 3.1V for the VBAT(lo) threshold allows calculation
R1 R2 R3
604k 1% 56k 1% 340k 1%
MIC2778 VDD LTH HTH R ST GND
Supply In
100k
Reset In GND
Figure 3. Interfacing to Bidirectional Reset Pin
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MIC2778
Micrel
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) 2004 Micrel Incorporated M9999-011405
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