Ultra Low-Power Comparators

  • 578 views
Uploaded on

To introduce the MAX921 comparator and its applications

To introduce the MAX921 comparator and its applications

More in: Technology , Business
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Be the first to comment
    Be the first to like this
No Downloads

Views

Total Views
578
On Slideshare
0
From Embeds
0
Number of Embeds
0

Actions

Shares
Downloads
0
Comments
0
Likes
0

Embeds 0

No embeds

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
    No notes for slide
  • Welcome to the training module on Ultra Low-Power Comparators. This training module introduces the MAX921 comparator and its applications.
  • A comparator is a device that accepts two analog inputs, compares the inputs, and produces a binary output that is a function of the higher input. If the non-inverting (+) input is greater than the inverting (-) input, then the output goes high. If the inverting (-) input is greater than the non-inverting (+) input, then the output goes low. The output signal remains constant as the differential input voltage changes. When described this way, the comparator resembles a 1-bit ADC. When the voltage difference is near zero volts, it can cause undesirable changes in the comparator's output state. To prevent this output oscillation, a small hysteresis of a few millivolts is integrated into many modern comparators. The difference between the higher-level trip value (V TRIP+ ) and the lower-level trip value (V TRIP- ) equals the hysteresis voltage (VHYST).
  • The MAX921 single micro power, low voltage comparators feature the low power consumption. It draws less than 4 µ A supply current and include an internal 1.182V voltage reference, programmable hysteresis, and TLL/CMOS outputs that sink and source current. Ideal for 3V or 5V single-supply applications, the MAX921 operate from a single +2.5V to +11V supply. The comparator's input voltage range swings from the negative supply rail to within 1.3V of the positive supply. The MAX921’s unique output stage continuously sources as much as 40mA. By eliminating power-supply glitches that commonly occur when comparators change logic states, the MAX921 minimize parasitic feedback, which makes them easier to use.
  • As mentioned before the MAX921 operates from a single +2.5V to +11V power supply. It has a separate ground for the output driver, allowing operation with dual supplies ranging from ±1.25V to ±5.5V. Connect V- to GND when operating the MAX921 from a single supply. For proper comparator operation, the input signal can swing from the negative supply (V-) to within one volt of the positive supply. The guaranteed common-mode input voltage range extends from V- to (V+ - 1.3V). The guaranteed minimum operating voltage is 2.5V. As the total supply voltage is reduced below 2.5V, the performance degrades and the supply current falls.
  • To add hysteresis to the MAX921, connect resistor R1 between REF pin & HYST pin, and connect resistor R2 between HYST pin and V-. If no hysteresis is required, connect HYS pinT to REF pin. When hysteresis is added, the upper threshold increases by the same amount that the lower threshold decreases. The hysteresis band, the difference between the upper and lower thresholds, VHB, is approximately equal to twice the voltage between REF and HYST. The HYST input can be adjusted to a maximum voltage of REF and to a minimum voltage of (REF – 50mV).
  • Because comparators have only two output states, their outputs are near zero or near the supply voltage. Therefore, the MAX921 output swings from V+ to GND, and TTL compatibility is assured by using a +5V supply. One criteria for selecting a comparator is the time its output takes to alter its state after a signal has been applied at its input. This propagation time must account for propagation delay through the component and rise/fall times in the output driver as well. In the propagation delay chart, we can see that the propagation delay is around 3μs with 100mV of overdrive.
  • Here is a simple design of a memory backup supply using the MAX921 comparator and reference circuit in conjunction with a JFET and some discrete components. This circuit provides an output of 5V at 1mA, with an input range of 8V to 32V. The JFET series-pass element (Q1) operates as a switched current source in a switched linear regulator. At start-up, C1 is fully discharged and Q1 acts as a current source. VOUT rises linearly as C1 charges, activating the MAX921 at 2V and continuing toward the 5V threshold set by R3 and R4. As VOUT reaches its threshold, the comparator output goes low, turning off Q1 by reverse-biasing its gate-source junction. As the load current discharges C1 below the output threshold, Q1 turns back on and completes one switching cycle.
  • Measuring battery life for a portable system is a time-consuming task, and the methods that accelerate battery discharge don't provide reliable results. This diagram shows an inexpensive clock tracks the operating time for a battery-powered portable system using the MAX921. When the battery voltage (or a selected output) drops below the discharge threshold set by R1, the comparator-output swings low and turns off Q1, removing power to the clock and the stopped clock retains the elapsed operating time.
  • This circuit provides a remote output for smoke detectors by sensing the supply-current increase associated with activation of the detector's internal sounder. Typical current values are 50µA during monitor mode, and greater than 3mA when the sounder is active. The circuit is powered by the smoke detector's battery, which has minimal impact on the detector itself. The circuit output is an open-drain FET that can drive the home-security system or trigger an RF module that communicates back to the residence.
  • Thank you for taking the time to view this presentation on “ Ultra Low-Power Comparators” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information, you may either visit the MAXIM site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.

Transcript

  • 1. Ultra Low-Power Comparators
    • Source: MAXIM
  • 2. Introduction
    • Purpose
      • To introduce the MAX921 comparator and its applications
    • Outline
      • Overview of the comparator
      • Basic operations of the MAX921
      • Applications
    • Content
      • 11 pages
  • 3. Comparator Basics
    • A comparator is a special type of op amp that is used to compare the voltages of the two inputs.
    • A basic comparator circuit is operated without a feedback loop.
    • A comparator normally changes its output state when the voltage between its inputs crosses through approximately zero volts.
    • When the voltage difference is near zero volts, it can cause undesirable changes in the comparator's output state.
    • To prevent this output oscillation, a small hysteresis of a few millivolts is integrated into many modern comparators.
    _ + V IN+ V IN- OUT
  • 4. MAX921 Comparator Introduction
    • Ultra-Low 4µA Max Quiescent Current Over Extended Temp. Range
    • Power Supplies: +2.5V to +11V
    • Input Voltage Range Includes Negative Supply
    • Internal 1.182V ±1% Bandgap Reference
    • Adjustable Hysteresis
    • TTL/CMOS-Compatible Outputs
    • 12µs Propagation Delay
    • No Switching Crowbar Current
    • 40mA Continuous Source Current
    • Target applications
      • Battery-Powered Applications
      • Oscillator Circuits
      • Threshold Detectors
  • 5. Power Supply and Input Signal Ranges
    • Connect V- to GND when operating the MAX921 from a single supply.
    • This comparator operates from a single +2.5V to +11V power supply.
    • The guaranteed minimum operating voltage is 2.5V.
    • At low supply voltages, the comparators’ output drive is reduced and the propagation delay increases.
    • The input signal should swing from V - to V + - 1V.
  • 6. Programming Hysteresis Adding hysteresis to the MAX921 Threshold Hysteresis Band
  • 7. Comparator Output
    • The MAX921 output swings from V+ to GND, and TTL compatibility is assured by using a +5V supply.
    • With 100mV of overdrive, propagation delay is typically 3μs.
  • 8. Application 1 – Memory-Backup Supply
  • 9. Application 2 – Measuring Battery Drain
  • 10. Application 3 – Remote Alarm for Smoke Detector
  • 11. Additional Resource
    • For ordering the MAX921, please click the part list or
    • Call our sales hotline
    • For additional inquires contact our technical service hotline
    • For more product information go to
      • http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1218