Ultralow Power Comparators Include Reference Design Note 123 Robert Reay With the explosion of battery-powered products has come the need for circuits that draw as little supply current as possible in order to extend battery life. Linear Technology’s new family of micropower comparators with built-in references is designed to meet that need. Drawing only 1μA of supply current per comparator, the LTC ®1440–LTC1445 family provides the perfect solution to battery-powered system monitoring problems. 7 3 IN+ + 8 4 IN – OUT – 5 HYST 6 REF 2.4M 5% The LTC1440–LTC1445 family features 1μA comparators, adjustable hysteresis, TTL/CMOS outputs that sink and source current and a 1μA reference that can drive a bypass capacitor of up to 0.01μF without oscillation. The parts operate from a 2V to 11V single supply or a ±1V to ±5V dual supply. Each comparator’s input voltage range swings from the negative supply rail to within 1.3V of the positive supply. The comparator propagation delay is 12μs with a 10mV overdrive, and the supply current glitches that commonly occur when comparators change logic states have been eliminated. Table 1 summarizes the features of each member of the family. Voltage Reference The internal bandgap voltage reference has an output voltage of 1.182V above V– for the LTC1440–LTC1443 and 1.22V ±1% for the LTC1444 and LTC1445. The reference output is capable of sourcing up to 200μA and sinking 15μA. The reference output can directly drive an external bypass capacitor up to 0.01μF without oscillation. By placing a resistor in series with the V+ LTC1440 51k 5% + 430Ω 5% 1μF V– 2 GND DN123 F01 Test Circuit 8V V+ 5V VREF OUT 2mV/DIV DN123 F01b Figure 1. Reference Settling L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Table 1 PART NUMBER NUMBER OF COMPARATORS LTC1440 1 LTC1441 2 LTC1442 2 Single LTC1443 4 Dual 5.0μA No 1.182V ±1% CMOS LTC1444 4 Single 5.0μA Yes 1.221V ±1% Open Drain LTC1445 4 Single 5.0μA Yes 1.221V ±1% CMOS 08/96/100_conv SUPPLY CURRENT ADJUSTABLE HYSTERESIS Dual 2.5μA Yes 1.182V ±1% CMOS Single 3.5μA No 1.182V ±1% CMOS 3.5μA Yes 1.182V ±1% CMOS SUPPLY REFERENCE COMPARATOR OUTPUT bypass capacitor, ringing at the reference output can be eliminated and a greater capacitance value can be used. The bypass capacitor prevents reference load transients or power supply glitches from disturbing the reference voltage, which helps eliminate false triggering of the comparators when they are connected to the reference. Figure 1 shows the reference voltage settling during a power supply transient. VCC A 1/4 LTC1444 R2 82.5k 1% 1 CELL LITHIUMION BATTERY + 4 R3 392k 1% R4 2.4M 5% 6 + – R7 51k 5% – B 1/4 LTC1444 7 + HYST R3 1M 1% 14 NC NC R5 51k 5% R4 2.4M 5% + 3 1 5 2 7 SW LT1300 ILIM + 8 R6 430Ω 5% C1 1μF 8 REF GND 1 2 DN123 F02 2, 4 Q1 MMFT2955ETI C3 100μF R9 267k 1% R8 732k 1% 11 + C 1/4 LTC1444 10 LTC1444 REF 9 V LTC1442 V– 1 3 SEL C1 0.01μF 4 SHDN PWR GND 6 REF R5 10k 5% Comparators C and D provide the reset input to the microprocessor. As soon as the boost converter output rises above the 4.65V threshold set by R8 and R9, comparator C turns off and R10 starts to charge C4. After 200ms, comparator D turns off and the Reset pin is pulled high by R12. 6 SENSE – 5 HYST microprocessor to warn of a low-battery condition. To protect the battery from overdischarge, the output of comparator B is pulled high by R7 when the battery voltage falls below 2.4V. P-channel Q1 and the LT1300 are turned off, dropping the quiescent current to 20μA. Q1 is needed to prevent the load circuitry from discharging the battery through L1 and D1. VIN C2 100μF OUT B 8 OVERVOLTAGE (5.5V) Figure 2. Undervoltage/Overvoltage Detector D1 1N5817 2 – 4 IN – B L1 10μH SUMIDA CD54-100 3 + + The voltage monitoring circuitry takes advantage of the LTC1444’s open-drain outputs and low supply voltage operation. Comparators A and B, along with R1, R2 and R3, monitor the battery voltage. When the battery voltage drops below 2.6V, comparator A’s output pulls low to generate a nonmaskable interrupt to the V+ 3 IN+ A R2 84.5k 1% Single Cell Lithium-Ion Battery Supply Figure 3 shows a single cell lithium-ion battery to 5V supply with the low-battery warning, low-battery shutdown and reset functions provided by the LTC1444. The LT®1300 micropower step-up DC/DC converter boosts the battery voltage to 5V using L1 and D1. Capacitors C2 and C3 provide input and output filtering. 5 V+ OUT A 1 UNDERVOLTAGE (4.5V) Undervoltage/Overvoltage Detector The LTC1442 can be easily configured as an undervoltage and overvoltage detector as shown in Figure 2. R1, R2 and R3 form a resistive divider from VCC so that comparator A goes low when VCC drops below 4.5V, and comparator B goes low when VCC rises above 5.5V. A 10mV hysteresis band is set by R4 and R5 to prevent oscillations near the trip points. R1 1.1M 5% 7 R1 1.33M 1% 16 – R10 3.37M 5% R11 51k 5% 13 R12 51k 15 5% + D 1/4 LTC1444 12 VCC NMI μP RESET – C4 0.22μF – DN123 F03 C2, C3: AUX TPSD107M010R0100 OR SANYO OS-CON 16SA100M Figure 3. Single Cell to 5V Supply Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn123f_conv LT/GP 0296 155K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1996