Low-Cost Microprocessor Supervisory Circuits with Battery Backup

Transcription

1 ; Rev 2; 11/05 Low-Cost Microprocessor Supervisory General Description The microprocessor (µp) supervisory circuits reduce the complexity and number of components required for power-supply monitoring and battery control functions in µp systems. These devices significantly improve system reliability and accuracy compared to that obtained with separate ICs or discrete components. The are available in 8-pin DIP and SO packages and provide four functions: 1) An active-low reset during power-up, power-down, and brownout conditions. 2) Battery-backup switching for CMOS RAM, CMOS µps, or other low-power logic circuitry. 3) A 1.25 threshold detector for power-fail warning, low-battery detection, or for monitoring a power supply other than. 4) An active-low manual reset input. The and differ only in their supplyvoltage monitor levels. The generates a reset when the supply drops below 4.65, while the generates a reset below 4.4. Computers Controllers Intelligent Instruments Automotive Systems Critical µp Power Monitoring UNREGULATED DC REGULATED R1 Applications Typical Operating Circuit 0.1μF MICROPROCESSOR Features Battery-Backup Power Switching Precision Supply-oltage Monitor 4.65 () 4.4 () 200ms Reset Pulse Width Debounced TTL/CMOS-Compatible Manual Reset Input 200µA Quiescent Current 50nA Quiescent Current in Battery-Backup Mode oltage Monitor for Power-Fail or Low-Battery Warning 8-Pin DIP and SO Packages Guaranteed Assertion to = 1 Ordering Information PART TEMP RANGE PIN-PACKAGE C/D 0 C to +70 C Dice* CPA 0 C to +70 C 8 PDIP CSA 0 C to +70 C 8 SO EPA -40 C to +85 C 8 PDIP ESA -40 C to +85 C 8 SO MJA -55 C to +125 C 8 CERDIP** C/D 0 C to +70 C Dice* CPA 0 C to +70 C 8 PDIP CSA 0 C to +70 C 8 SO EPA -40 C to +85 C 8 PDIP ESA -40 C to +85 C 8 SO MJA -55 C to +125 C 8 CERDIP** *Dice are tested at only. **Contact factory for availability and processing to MIL-STD-883. Devices in PDIP and SO packages are available in both leaded and lead-free packaging. Specify lead free by adding the + symbol at the end of the part number when ordering. Lead free not available for CERDIP package. R2 3.6 LITHIUM BATTERY BATT NMI BUS TOP IEW 1 Pin Configuration 8 BATT MR CMOS RAM MR PUSHBUTTON SWITCH 4 5 DIP/SO Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at , or visit Maxim s website at

2 ABSOLUTE MAXIMUM RATINGS Terminal oltage (with respect to ) to +6. BATT to +6. All Other Inputs (Note 1) to ( CB + 0.3) Input Current...200mA BATT...50mA...20mA Output Current...Short-Circuit Protected for Up to 10s All Other Outputs...20mA Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS Rate-of-Rise BATT,...10/µs Operating Temperature Range C Suffix...0 C to +70 C E Suffix C to +85 C M Suffix C to +125 C Continuous Power Dissipation (T A = +70 C) 8-Pin PDIP (derated 9.09mW/ C above +70 C)...727mW 8-Pin SO (derated 5.88mW/ C above +70 C)...471mW 8-Pin CERDIP (derated 8.00mW/ C above +85 C)...640mW Storage Temperature Range C to +160 C Lead Temperature (soldering, 10s) C Note 1: CB is the greater of and BATT. The input voltage limits on and MR may be exceeded if the current into these pins is limited to less than 10mA. ( = to +5.5 for, = +4.5 to +5.5 for, BATT = 2.8, T A = T MIN to T MAX, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Operating oltage Range (Note 2) 0 5.5, BATT Supply Current (Excluding MAX70_C I SUPPLY µa I OUT ) MAX70_E/M I SUPPLY in Battery-Backup Mode (Excluding I OUT ) BATT Standby Current (Note 3) Output in Battery-Backup Mode Battery Switch Threshold ( - BATT ) =, BATT = T A = T MIN to T MAX > > BATT T A = T MIN to T MAX I OUT = 5mA I OUT = 50mA I OUT = 250µA, < BATT BATT BATT Power-up 20 < RST Power-down -20 Battery Switchover Hysteresis 40 m Threshold RST Threshold Hysteresis 40 m Pulse Width t RST ms Output oltage OH I SOURCE = 800µA I SINK = 3.2mA 0.4 MAX70_C, = 1, falling, OL BATT =, I SINK = 50µA 0.3 MAX70_E/M, = 1.2, falling, BATT =, I SINK = 100µA 0.3 µa µa m

3 ELECTRICAL CHARACTERISTICS (continued) ( = to +5.5 for, = +4.5 to +5.5 for, BATT = 2.8, T A = T MIN to T MAX, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS MR Input Threshold IL Low 0.8 IH High 2.0 MR Pulse Width t MR 150 ns MR to Delay t MD 250 ns MR Pullup Current MR = µa Input Threshold = Input Current na OH I SOURCE = 800µA CC - Output oltage 1.5 OL I SINK = 3.2mA 0.4 Note 2: Either or BATT can go to if the other is greater than 2.. Note 3: - = battery-charging current, + = battery-discharging current. ( =, BATT = 2.8,, unless otherwise noted.) Typical Operating Characteristics OUTPUT OLTAGE vs. LOAD CURRENT = BATT = +2.8 toc OUTPUT OLTAGE vs. LOAD CURRENT = BATT = +2.8 toc02 OUTPUT OLTAGE vs. SUPPLY OLTAGE BATT = toc03 1/div OUT () SLOPE = 5Ω OUT () SLOPE = 80Ω O 2kΩ 330pF 1/div I OUT (ma) I OUT (ma) 500ms/div RESPONSE TIME toc04 POWER-FAIL COMPARATOR RESPONSE TIME toc05 POWER-FAIL COMPARATOR RESPONSE TIME toc /div 1kΩ pF 1kΩ = +3 10kΩ 30pF pF = μs/div 400ns/div 400ns/div 3

4 PIN NAME FUNCTION Pin Description Supply Output for CMOS RAM. When 1 is above the reset threshold, connects to through a p- OUT channel MOSFET switch. When is below the reset threshold, the higher of or BATT is connected to. 2 Supply Input 3 Ground 4 Power-Fail Comparator Input. When is less than 1.25, goes low; otherwise remains high. Connect to or when not used. 5 P ow er - Fai l C om p ar ator O utp ut. It g oes l ow and si nks cur r ent w hen P FI i s l ess than 1.25 ; other w i se P FO r em ai ns hi g h. 6 MR Manual Reset Input. Generates a reset pulse when pulled below 0.8. This active-low input is TTL/CMOS compatible and can be shorted to ground with a switch. It has an internal 250µA pullup current. Leave floating when not used. 7 Reset Output. Remains low while is below the reset threshold (4.65 for the, 4.4 for the ). It remains low for 200ms after rises above the reset threshold (Figure 2) or MR goes from low to high. 8 BATT greater than. When rises 20m above BATT, is switched to. The 40m hysteresis prevents Backup-Battery Input. When falls below the reset threshold, BATT is switched to if BATT is 20m repeated switching if falls slowly. BATT BATTERY-SWITCHOER CIRCUITRY RST BATT = 3. GENERATOR 3. t RST 1.25 MR * 1.25 * DEPENDS ON EXCEPT IN BATTERY-BACKUP MODE, WHERE IS LOW. Figure 1. Block Diagram Figure 2. Timing Diagram 4

5 Detailed Description Output A µp s reset input starts the µp in a known state. Whenever the µp is in an unknown state, it should be held in reset. The assert reset when is low, preventing code-execution errors during power-up, power-down, or brownout conditions. When BATT is 2 or more, is always valid, irrespective of. On power-up, as rises, remains low. When exceeds the reset threshold, an internal timer holds low for a time equal to the reset pulse width (typically 200ms); after this interval, goes high (Figure 2). If a power-fail or brownout condition occurs (i.e., drops below the reset threshold), is asserted. As long as remains below the reset threshold, the internal timer is continually restarted, causing the output to remain low. Thus, a brownout condition that interrupts a previously initiated reset pulse causes an additional 200ms delay from the end of the last interruption. Power-Fail Comparator The input is compared to an internal reference. If is less than 1.25, goes low. The power-fail comparator can be used as an undervoltage detector to signal a failing power supply. In the Typical Operating Circuit, an external voltage-divider at is used to monitor the unregulated DC voltage from which the regulated supply is derived. The voltage-divider can be chosen so the voltage at falls below 1.25 just before the regulator drops out. is then used as an interrupt to prepare the µp for power-down. To conserve power, the power-fail comparator is turned off and is forced low when the enter battery-backup mode. Backup-Battery Switchover In the event of a brownout or power failure, it may be necessary to preserve the contents of RAM. With a backup battery installed at BATT, the / automatically switch RAM to backup power when fails. As long as exceeds the reset threshold, connects to through a 5Ω p-channel MOSFET power switch. Once falls below the reset threshold, goes low and or BATT (whichever is higher) switches to. Note that BATT switches to through an 80Ω switch) only if is below the resetthreshold voltage and BATT is greater than. When exceeds the reset threshold, it is connected to the substrate, regardless of the voltage applied to BATT (Figure 3). During this time, diode D1 (between BATT and the substrate) conducts current from BATT to if BATT ( + 0.6). When the battery-backup mode is activated, BATT connects to. In this mode, the substrate connects to BATT and internal circuitry is powered from the battery (Figure 3). Table 1 shows the status of the / inputs and outputs in battery-backup mode. When is below, but within, 1 of BATT, the internal switchover comparator draws about 30µA. Once Table 1. Input and Output Status in Battery-Backup Mode SIGNAL STATUS Disconnected from. BATT MR BATT Connected to BATT through an internal 80Ω p-channel MOSFET switch. Connected to. Supply current is < 1µA when < ( BATT - 1). Logic-low. Power-fail comparator is disabled. Logic-low. Disabled. S1 S2 D1 D2 SUBSTRATE D3 S3 S4 CONDITION S1/S2 S3/S4 > Reset Threshold Open Closed < Reset Threshold and > BATT Open Closed < Reset Threshold and < BATT Closed Open Figure 3. Battery-Switchover Block Diagram 5

6 drops to more than 1 below BATT, the internal switchover comparator shuts off and the supply current falls to less than 1µA. Manual Reset The manual reset input (MR) allows to be activated by a pushbutton switch. The switch is effectively debounced by the 140ms minimum reset pulse width. Because it is TTL/CMOS compatible, MR can be driven by an external logic line. Applications Information Using a SuperCap as a Backup Power Source SuperCaps are capacitors with extremely high capacitance values (on the order of 0.1 Farad). When using SuperCaps, if exceeds the reset thresholds (4.65 and 4.4, respectively), BATT may not exceed by more than 0.6. Thus, with a 5% tolerance on, BATT should not exceed (min) = Similarly, with a 10% tolerance on, BATT should not exceed 5.1. Figure 4 s SuperCap circuit uses the with a ±5% tolerance voltage supply. In this circuit, the SuperCap rapidly charges to within a diode drop of. However, the diode leakage current with tricklecharge the SuperCap voltage to. If BATT = 5.25 and the power is suddenly removed and then reapplied with = 4.75, BATT - does not exceed the allowable 0.6 difference voltage. Figure 5 s circuit uses the with a ±10% tolerance voltage supply. Note that if = 5.5 and BATT 5.1, the power can be suddenly removed and reapplied with = 4.5, and BATT - will not exceed the allowable 0.6 voltage difference. 100kΩ 0.1F BATT Batteries and Power Supplies as Backup Power Sources Lithium batteries work well as backup batteries because they have very low self-discharge rates and high-energy density. Single lithium batteries with opencircuit voltages of 3. to 3.6 are ideal for use with the. Batteries with an open-circuit voltage less than the minimum reset threshold plus 0.3 can be directly connected to the BATT input with no additional circuitry (see the Typical Operating Circuit). However, batteries with open-circuit voltages greater than the reset threshold plus 0.3 CANNOT be used as backup batteries, since they source current into the substrate through diode D1 (Figure 3) when is close to the reset threshold. TO STATIC RAM TO μp Figure 5. Using a SuperCap as a Backup Power Source with the and a ±10% Supply PART MAXIMUM BACKUP-BATTERY OLTAGE () F BATT TO STATIC RAM TO μp Using the without a Backup Power Source If a backup power source is not used, ground BATT and connect to. A direct connection to eliminates any voltage drop across the internal switch, which would otherwise appear at. Alternatively, use the MAX705 MAX708, which do not have batterybackup capabilities. Figure 4. Using a SuperCap as a Backup Power Source with a and a ±5% Supply SuperCap is a registered trademark of Bankor Industries. 6

7 Ensuring a alid Output Down to = When falls below 1, the output no longer sinks current; it becomes an open circuit. High-impedance CMOS logic inputs can drift to undetermined voltages if left as open circuits. If a pulldown resistor is added to the pin as shown in Figure 6, any stray charge or leakage currents will flow to ground, holding low. Resistor value R1 is not critical. It should be about 100kΩ, which is large enough not to load and small enough to pull to ground. Replacing the Backup Battery The backup battery can be removed while remains valid without triggering a reset. As long as stays above the reset threshold, battery-backup mode cannot BATT Figure 6. alid to Ground Circuit R1 be entered. This is an improvement on switchover ICs that initiate a reset when and BATT are at or near the same voltage level (regardless of the reset threshold voltage). If the voltage on the unconnected BATT pin floats up toward, this condition alone cannot initiate a reset when using the. Adding Hysteresis to the Power-Fail Comparator Hysteresis adds a noise margin to the power-fail comparator and prevents repeated triggering of when IN is near the power-fail comparator trip point. Figure 7 shows how to add hysteresis to the power-fail comparator. Select the ratio of R1 and R2 so that sees 1.25 when IN falls to the desired trip point ( TRIP ). Resistor R3 adds hysteresis. It will typically be an order of magnitude greater than R1 or R2. The current through R1 and R2 should be at least 1µA to ensure that the 25nA (max) input current does not shift the trip point. R3 should be larger than 10kΩ to prevent it from loading down the pin. Capacitor C1 adds additional noise rejection. Monitoring a Negative oltage The power-fail comparator can be used to monitor a negative supply voltage using Figure 8 s circuit. When the negative supply is valid, is low. When the negative supply voltage droops, goes high. This circuit s accuracy is affected by the threshold tolerance, the voltage, and resistors R1 and R2. IN R1 R1 R2 R3 C1* R2 TO μp *OPTIONAL - L TRIP IN H R1+ R2 TRIP = 125. R2 R2 R3 L H = 125. / = R1+ R2 R3 R1 R3 R2 Figure 7. Adding Hysteresis to the Power-Fail Comparator TRIP = R1 R2 NOTE: TRIP IS NEGATIE TRIP - Figure 8. Monitoring a Negative oltage 7

8 Using the Power-Fail Comparator to Assert Reset In addition to asserting reset at the reset threshold voltage, reset can also be asserted at the input threshold voltage. Connect to MR to initiate a reset Table 3. Maxim Microprocessor Supervisory Products PART NOMI NAL THRESHOLD ( ) M I NI M UM PULSE WI DTH ( m s) NOMI NAL WATCH- DOG TI MEOUT PERI OD ( s ) BACKUP- BATTERY SWITCH CE WRITE PROTECT pulse when the monitored supply drops below a userspecified threshold or when falls below the reset threshold. For additional noise rejection, place a capacitor between and. POWER- FAI L COMPARATOR M ANUAL I NPUT WATCH- DOG I NPUT LOW- LI NE OUTPUT ACTI E- HI GH BATT ON OUTPUT MAX690A Yes No Yes No No No No No MAX691A /Adj. 1.6/Adj. Yes Yes Yes No Yes Yes Yes Yes MAX692A Yes No Yes No No No No No MAX693A /Adj. 1.6/Adj. Yes Yes Yes No Yes Yes Yes Yes MAX696 Adj. 35/Adj. 1.6/Adj. Yes No Yes No Yes Yes Yes Yes MAX697 Adj. 35/Adj. 1.6/Adj. No Yes Yes No Yes Yes Yes No MAX /Adj. 200 No No No Yes No No Yes No Yes No Yes Yes No No No No Yes No Yes Yes No No No No MAX No No Yes Yes Yes No No No MAX No No Yes Yes Yes No No No MAX No No Yes Yes No No Yes No MAX No No Yes Yes No No Yes No MAX Yes Yes Yes Yes Yes Yes Yes Yes MAX / /0.60/ 1.2 No No No Yes No No Yes No MAX1259 Yes No Yes No No No No No 8

9 Chip Topography Package Information For the latest package outline information, go to SUBSTRATE MUST BE LEFT UNCONNECTED TRANSISTOR COUNT: 573 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc. REDUTA