Design Note 123: Ultralow Power Comparators Include Reference

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 LTC1440–LTC1445 family provides the perfect solution to battery-powered system monitoring problems.

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.

Table 1
Part Number Number of Comparators Supply Supply Current Adjustable Hysteresis Reference Comparator Output
LTC1440 1 Dual 2.5μA Yes 1.182V ±1% CMOS
LTC1441 2 Single 3.5μA No 1.182V ±1% CMOS
LTC1442 2 Single 3.5μA Yes 1.182V ±1% CMOS
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

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 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.

Figure 1a. Test Circuit.

Figure 1b. Reference Settling.

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.

Figure 2. Undervoltage/Overvoltage Detector.

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 LT1300 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.

Figure 3. Single Cell to 5V Supply.

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 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.

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.

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Robert Reay