Demystifying Low Quiescent Current (low Iq): How to Use WEBENCH to Design Nearly 100% Duty Cycle for Ultra-Low-Power Applications

Many battery-powered applications require a buck converter to operate at 100% duty cycle, where VIN is close to VOUT to extend battery life when the battery voltage is at its lowest value.

Many battery powered applications require a buck converter to operate at 100% duty cycle where VINclose to VOUTto extend battery life when the battery voltage is at its lowest value.

For example, suppose there are two lithium-manganese dioxide (Li-MnO2) battery powers the smart meter. Li-MnO2Batteries are disposable, non-rechargeable batteries that are increasingly used in smart electricity or water meters due to their long lifespan (up to 20 years) and their cost-effectiveness over lithium thionyl chloride batteries.

Figure 1 shows the system configuration of two Li-MnO2 batteries connected in series (2s1p) and then stepped down to power the microcontroller.

Demystifying Low Quiescent Current (low Iq): How to Use WEBENCH to Design Nearly 100% Duty Cycle for Ultra-Low-Power Applications
Figure 1: Smart Meter Power Architecture

Ultra-low quiescent current (IQ) DC/DC converters help you design applications with battery life of up to 20 years. The load curve of the smart meter application is not a continuous load, but a variable load curve. To preserve battery life, the system only draws high current occasionally (sending wireless signals or actuating valves) before returning to a very low load state. Such load profiles enable low average current consumption in the microampere range. High efficiency at such light loads requires ultra-low quiescent current IQ, especially during the off-time. The current consumption during the off-time may be much lower than the average current consumption.

Working I of Texas Instruments’ TPS62840 Ultra-Low-Power Buck ConverterQAt only 60 nA, the 3.3-V rail can be regulated. TPS62840 has very low quiescent current I in 100% modeQ – 150 nA, further extending battery life.

To better help you design and simulate ultra-low power circuits, WEBENCH® Power Designer is an online tool that creates custom power designs to your specifications.

In our example, the average voltage per cell is about 3.0 V. The initial voltage of a new battery is about 3.2 V, and the voltage can drop below 2 V after full discharge. Enter these parameters into WEBENCH Power Designer (Figure 2), assuming each cell is discharged to 1.8 V and the new battery is at 3.2 V.

Demystifying Low Quiescent Current (low Iq): How to Use WEBENCH to Design Nearly 100% Duty Cycle for Ultra-Low-Power Applications
Figure 2: Design specifications entered in 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:

VINMIN > VOUTis the first check parameter that WEBENCH Power Designer looks for in user input of a buck converter topology. If VINMIN > VOUT, then WEBENCH Power Designer selects a buck converter as part of the solution list. If VINMIN ≤ VOUTWEBENCH Power Designer will suggest using a buck-boost converter to regulate your VOUT, instead of using a buck converter operating in 100% duty cycle mode. This is because WEBENCH wants to provide you with a solution, even if VINMIN ≤ VOUTyour VOUTwill also be regulated.

After passing the first check, a recheck verifies that the calculated duty cycle is greater than the maximum duty cycle specified in the buck converter datasheet. For a buck converter 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 well above the ideal VOUT/VIN.

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

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

V of TPS62840 WEBENCH Power Designer modelIN3.2 V to 6.4 V, IOUT_MAX = 0.75 A and VOUT = 3.3 V (Figure 3).

Demystifying Low Quiescent Current (low Iq): How to Use WEBENCH to Design Nearly 100% Duty Cycle for Ultra-Low-Power Applications
Figure 3: Fault message when the input voltage is too low

Due to the minimum VINbelow VOUT, the design update failed. This design failed the first inspection by WEBENCH.

V of TPS62840 WEBENCH Power Designer modelIN3.6 V to 6.4 V, IOUT_MAX = 0.75 A and VOUT = 3.3 V (Figure 4).

Demystifying Low Quiescent Current (low Iq): How to Use WEBENCH to Design Nearly 100% Duty Cycle for Ultra-Low-Power Applications
Figure 4: Fault message when duty cycle is too high

The design will not be updated because the calculated duty cycle includes losses such as high-side MOSFET RDSON and Inductor DCR. Here, the duty cycle value is greater than 99.9%. This design failed WEBENCH’s re-inspection.

at VIN3.7 V to 6.4 V, IOUT_MAX = 0.75 A and VOUT = 3.3 V on the TPS62840 WEBENCH Power Designer select the design interface (Figure 5).

Demystifying Low Quiescent Current (low Iq): How to Use WEBENCH to Design Nearly 100% Duty Cycle for Ultra-Low-Power Applications
Figure 5: TPS62840 shown in WEBENCH Power Designer

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

How to use WEBENCH Power Designer more efficiently 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 practical applications, mode operation at 100% is normal and usually acceptable 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 specs for most loads.

The Links:   DMC-40457-NY-LY-B-CKN MIG75Q201H

Author: Yoyokuo