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| MIC2778 Micrel MIC2778 Voltage Monitor with Adjustable Hysteresis Preliminary Information 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 MIC833. 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 MIC833 * 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 MIC2778-2BM5 MIC2778-1BM5 Marking UFB UFA Accuracy 2% 1% Temperature Range -40C to +85C -40C to +85C Package SOT-23-5 SOT-23-5 Typical Application MIC5207-3.0BM5 Li-Ion Cell MIC2778 VDD LTH 56k 1% 340k 1% HTH GND RST IN 100k EN OUT BYP GND 470pF VOUT 3.0V 4.7F 604k 1% VBAT(hi) = 3.6V VBAT(lo) = 3.1V Cellular Telephone Battery Monitor 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 October 1999 1 MIC2778 MIC2778 Micrel Pin Configuration LTH GND HTH 3 2 1 4 5 RST 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. Reset (Output): Active-low, open-drain output. This output is asserted and latched when VLTH 2 3 GND LTH 4 RST 5 VDD MIC2778 2 October 1999 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 1.5V VDD 5.5V; TA = +25C, bold values indicate -40C TA +85C; unless noted Symbol IDD ILTH, IHTH VREF Parameter Supply Current Input Leakage Current Condition RST not asserted Min Typ 1 5 10 Reference Voltage MIC2778-2 MIC2778-1 tD tRESET VRST Propagation Delay Reset Pulse Width Voltage-Low Reset RST asserted, ISINK = 1.6mA, VDD 1.6V RST asserted, ISINK = 100A, VDD 1.2V, 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 held low down to VDD = 1.2V. Max 2 Units A pA nA V 1.240 1.215 1.228 5 140 420 0.3 0.4 1.265 1.252 V V s ms V V VLTH = VREF(max) +100mV to VREF(min) - 100mV Timing Diagram A VHTH VBATT VLTH A 0V VRST VRSTH VRSTL Propagation delays not shown for clarity. Note A. The MIC2778 ignores very brief transients. See "Applications Information" for details. tRST tRST October 1999 3 MIC2778 MIC2778 Micrel Functional Diagram VBATT Low-Voltage Detect LTH RQ HTH VHI High-Voltage Detect Delay Line SQ RST VLO 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. MIC2778 4 October 1999 MIC2778 Micrel 3.1V. Using 3.1V for the VBAT(lo) threshold allows calculation of the two remaining resistor values. 1M VBAT(lo) = 3.1V = 1.24 R2 + 344k 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 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. MAX. TRANSIENT DURATION (s) 140 120 100 80 60 40 20 0 1 10 100 1000 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. VBATT Input Transient Response R1 604k 1% 56k 1% 340k 1% MIC2778 VDD LTH HTH RST GND 470k RESET COMP. OVERDRIVE, VREF-VLTH (mV) R2 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 4. VBATT Microprocessor MIC2778 VDD LTH R2 56k 1% 340k 1% HTH GND GND RST Supply In 100k Reset In R3 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: VBAT(hi) 1M = 1.24 R3 R1 604k 1% 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 Figure 4. Interfacing to Bidirectional Reset Pin October 1999 5 MIC2778 MIC2778 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) MIC2778 6 October 1999 MIC2778 Micrel October 1999 7 MIC2778 MIC2778 Micrel MICREL INC. TEL 1849 FORTUNE DRIVE SAN JOSE, CA 95131 FAX USA + 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) 1999 Micrel Incorporated MIC2778 8 October 1999 |
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