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The System Advantages of Low-Power Microcontrollers in Line-Powered

Posted: 31 Jan 2005 ?? ?Print Version ?Bookmark and Share

Keywords:microcontroller? low-voltage components?

It is ironic that the challenge in designing line power applications is not high voltages or dangerous currents. Rather, it is the process of creating a simple, efficient power supply for low-voltage components. Often, delivering a minimal 50 mw to the control section can dissipate over 2!3 watts in the power supply. This added heat dissipation both increases the cost of the power supply and introduces significant heating in the case. That is why it is important, when designing low-cost line-powered control applications, to utilize every possible power-saving feature and technique.

Microcontroller
Reducing power in a design starts with the selection of the microcontroller. While older CMOS microcontrollers claim to be low power, only a new low-power microcontroller designed for battery operation can provide effective power management. These new microcontrollers have features that can significantly reduce their current consumption in a design, including:

1. Newer low-power microcontrollers have been optimized specifically for lower current consumption, so they consume less current than older CMOS microcontrollers.

2. Newer microcontrollers have low-frequency clock options, which further reduce their current consumption.

3. Newer microcontrollers can run at lower voltages, which also reduces the their current consumption.

4. Sleep modes in the new microcontrollers cut current consumption dramatically.

Just these four capabilities drop the 1!2 mA current draw of an older CMOS microcontroller, to less than 18 5A by using a newer low-power microcontroller at 3V with a 32 kHz clock, while spending at least 50 percent of its time asleep. And, achieving this current draw is not difficult with these new microcontrollers if their function is to turn on a TRIAC every = cycle and monitor a few pushbuttons.

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TRIAC Drive
TRIACs are a common choice for switching AC power, due to their latching nature and bi-directional switching capability. Unfortunately, most designers forget what the latching nature of a TRIAC means for a design. Because a TRIAC is latched on once it conducts more than its minimal hold current, the bias current to the TRIAC gate can be discontinued, saving considerable current. In fact, a 3 mA bias pulse, 300 5S wide, on the gate of a sensitive gate TRIAC is all that is required to turn on the TRIAC for the full half cycle of the waveform. This means the 3 mA current pulse, averaged over the 60 Hz half cycle, is actually equivalent to a continuous draw of less than 100 5A. So, a narrow pulse drive on the TRIAC can save almost 96 percent (the current traditionally used to control a TRIAC).

The User Interface

Older designs typically use low-current LEDs for indicators in their user interface. However, LEDs can draw up to 1!5 mA per LED. In a design trying to conserve microamps, this not good. The solution is to once again turn to low-power microcontrollers. Specifically, microcontrollers designed with on-chip LCD drivers.

A typical LCD driver will draw less than 30!40 5A, with an additional 100!200 5A to generate the bias voltages for the display. When compared to 1!5 mA for a single LED, the advantages of an LCD become clear. Not only does it draw less current, it also offers designers a flexible and user-friendly display.

A frequently cited drawback to LCD displays is that they have poor readability in low light. The answer is simple, add a backlight to increase the display contrast. One might ask, doesn't that increases current consumption? If an LED backlight is used, then the design would be back to a higher current consumption. However, using an electroluminescent (EL) backlight with an LCD display avoids this issue because EL panels can be driven directly from the 110 VAC supply with only a small current-limiting resistor. So, an EL backlight would have no impact on the low-voltage current consumption.

The other half of a user interface is the pushbutton inputs. Traditionally, this interface consists of one or more pushbuttons with individual resistor pullups that are tied to digital inputs on the microcontroller. While the current draw of the pullup resistors may seem too small an opportunity for improvement, low-power microcontrollers can even help here with their 'weak pullup' feature. These internal weak-pullup resistors offer a comparable current source, without the cost of the external resistors. The weak pullups can also be enabled and disabled in software, which can be used to limit their current consumption to only those times when the microcontroller is actually reading the state of the pushbutton.

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The Power Supply

At the start of this article, it was indicated that low-power microcontrollers significantly reduce current consumption, decrease power dissipation and reduce cost. This savings is compared against a traditional design using: a CMOS microcontroller, a TRIAC, two pushbuttons, and 6 LEDs. In this case, the current draw is approximately 10 mA: 3 mA for the TRIAC, 5 mA per LEDs (assuming only one is lit) and 2 mA for the microcontroller. To create this current at 5V, over 2.4 watts will have to be dissipated in the power supply.

2.4W = (110 VAC!5 VDC) * (10 mA + 10 mA + 3 mA)

10 mA is the power-supply current during the positive half of the cycle, another 10 mA is to charge the bulk capacitor, which supplies current during the negative half cycle, and 3 mA is used to bias the zener diode.

Using the current reduction techniques discussed above, the current consumption for an equivalent design with a low-power microcontroller would be less than 400 5A: 100 5A average for the TRIAC, 240 5A for the LCD, and 18 5A for the microcontroller. To create this current, the power supply will only dissipate 140mw. That is a reduction of 2.25 watts of power, compared to a traditional CMOS-microcontroller based design.

140 mW = (110 VAC!3 VDC) * (400 5A + 400 5A + 500 5A)

Due to this lower dissipation,

Conclusion
So, not only do low-power microcontrollers reduce the current requirements of an offline application, they can significantly improve the design by eliminating heat, reducing cost and improving appeal.

Microchip Featured Component Highlights

  • PIC Microcontrollers

  • A broad family of low power, Flash-based 8-bit microcontrollers with a range of nanoWatt technology features for power-reduction features, including a lower operating current, clock-frequency controls, and a sleep mode. In addition, the weak pullups and LCD-display driver are available options in this family of microcontrollers. Other peripherals including EEPROM data memory, Capture/Compare/PWM timer functions, 10-bit ADC modules, comparators and various hardware serial-communications peripherals. The combination of low-power features and a wealth of peripheral options gives the system designer the versatility to reduce system power consumption, increase reliability and performance, and minimize cost by eliminating external components.

    Summary
    This type of electronic control unit is another example of the role that semiconductor and low-power technologies are playing in enhancing everyday appliances. Microcontroller-based control gives the user unprecedented features and functionality. The feature set and low-power focus also provide the designer with unprecedented options for system design and power reduction.

    Note: The Microchip name and logo, and PIC are registered trademarks of Microchip Technology Inc. in the USA and other countries. All other trademarks mentioned herein are property of their respective companies.

    - Keith Curtis
    Principal Applications Engineer
    Security, Microcontroller and Technology Division, Microchip Technology Inc.




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