How to Design Nearly 100% Duty Cycle for Ultra-Low Power Applications Using WEBENCH

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How to Design Nearly 100% Duty Cycle for Ultra-Low Power Applications Using WEBENCH [Copy link]

Many battery-powered applications require a buck converter to operate at 100% duty cycle, where V _IN is close to V _OUT , to extend battery life when the battery voltage reaches its minimum value.

For example, consider two lithium-manganese dioxide (Li-MnO ₂ ) batteries powering a smart meter. Li-MnO ₂ batteries are disposable, non-rechargeable batteries that are increasingly being used in smart electricity or water meters due to their long life (up to 20 years) and cost-effectiveness over lithium thionyl chloride batteries. Figure 1 shows a system configuration of two Li-MnO 2 batteries connected in series (2s1p), which are then stepped down to power a microcontroller.

查看详情Figure 1: Smart meter power supply architecture

Ultra-low quiescent current (I _Q ) DC/DC converters can help you design applications with up to 20-year battery life. The load profile for smart meter applications is not a continuous load, but a variable load profile. To extend battery life, the system only occasionally draws high current (sending a wireless signal or actuating a valve) and then returns to a very low load state. This type of load profile can achieve low average current consumption in the micro-ampere range. High efficiency at this light load requires ultra-low quiescent current I _Q , especially during the off time. The current consumption during the off time can be much lower than the average current consumption.

Texas Instruments' TPS62840 ultra-low-power step-down converter operates at just 60 nA I _Q and can regulate a 3.3-V rail. The TPS62840 has an extremely low quiescent current I _Q - 150 nA in 100% mode , further extending battery life.

To better assist you in designing and simulating ultra-low power circuits, WEBENCH Power Designer is an online tool that creates custom power designs based on your specifications.

In our example, the average voltage per cell is about 3.0 V. A new cell has an initial voltage of about 3.2 V, and when fully discharged can drop to less than 2 V. Assuming each cell discharges to 1.8 V and the new cell voltage is 3.2 V, enter these parameters into WEBENCH Power Designer (Figure 2).

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Figure 2: Design specifications entered into WEBENCH Power Designer

Using a 3.6 V minimum input voltage in the WEBENCH PowerDesigner search tool yields 51 possible devices, but the TPS62840 is not one of them. Why is that?

WEBENCH focuses on two initial parameters to help you find the best device for your system:

1. V _INMIN > V _OUT is the first parameter that WEBENCH Power Designer looks for in the user input for a buck converter topology. If V _INMIN > V _OUT , then WEBENCH Power Designer selects a buck converter as part of the solution list. If V _INMIN ≤ V _OUT , WEBENCH Power Designer recommends using a buck-boost converter to regulate your V _OUT instead of using a buck converter operating in 100% duty cycle mode. This is because WEBENCH wants to provide you with a solution where your V _OUT will be regulated even if V _INMIN ≤ V _{OUT .}

2. After passing the first check, the recheck verifies that the calculated duty cycle is greater than the maximum duty cycle specified in the buck converter datasheet. For buck converters that can operate in 100% duty cycle mode, 99.9% is used as the threshold. Losses are included when calculating the duty cycle. This increases the duty cycle calculated in WEBENCH Power Designer to be much higher than the ideal V _OUT /V _IN .

After selecting a number of devices, WEBENCH Power Designer performs a detailed design for each device.

Depending on the input parameters used, three different results can be observed:

• The TPS62840 WEBENCH Power Designer model has V _IN from 3.2 V to 6.4 V, I _{OUT_MAX} = 0.75 A, and V _OUT = 3.3 V (Figure 3).

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Figure 3: Fault message when input voltage is too low

Design update failed because minimum V _IN is lower than V _OUT . This design failed the first check in WEBENCH.

• The TPS62840 WEBENCH Power Designer model has V _IN of 3.6 V to 6.4 V, I _{OUT_MAX} = 0.75 A, and V _OUT = 3.3 V (Figure 4).

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Figure 4: Fault message when duty cycle is too high

The design will not update because the duty cycle calculation includes losses such as high-side MOSFET RDSON and inductor DCR. Here, the duty cycle value is greater than 99.9%. This design fails the WEBENCH review.

• Select the design interface on the TPS62840 WEBENCH Power Designer with V _IN of 3.7 V to 6.4 V, I _{OUT_MAX} = 0.75 A, and V _OUT = 3.3 V (Figure 5).

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Figure 5: TPS62840 displayed in WEBENCH Power Designer

The last example shows the TPS62840 because this design passes both checks.

How to use WEBENCH Power Designer more effectively when approaching 100% duty cycle :

• Add a sufficient delta between the input voltage and the output voltage to reduce the duty cycle.

• Reduce output current to reduce losses and reduce duty cycle.

Both solutions enable WEBENCH Power Designer to design with the TPS62840 . In real applications, it is normal and usually acceptable to operate in 100% down mode in order to fully discharge the battery. In 100% mode, the output voltage of the buck converter decreases as the battery voltage decreases. But this still fits the system specifications for most loads.