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Achieve High Efficiency with SMPS Light Loads

Sponsored by Texas Instruments: Designers can take two different routes to help suppress the effects of quiescent current in efforts to boost switch-mode power-supply efficiency.
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Switch-mode power supplies (SMPS) like dc-dc converters provide their best efficiency with heavier loads. When a device using a dc-dc converter goes into standby mode, efficiency drops considerable. Virtually the only current flowing at this time is the quiescent current (IQ) (see “Quiescent Current Defined” at the end of the article).

One way to achieve greater efficiency is to use a very low IQ dc-dc IC that automatically adjusts its configuration and operation to boost efficiency during light-load conditions. Another approach for boosting efficiency at light loads is to add an low-dropout regulator (LDO) to power the load. Here’s a summary of both methods.

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One Solution

The goal is to keep the efficiency of the power supply as high as possible to minimize power dissipation and heat as the load current decreases. Assume that a battery and a dc-dc buck step-down converter make up the main power supply. With a normal load of say 400 mA, the efficiency of the supply is typically greater than 80%. However, as the load current drops to 100 mA, the efficiency decreases to a lower level in the range of about 45%.

Available buck-converter ICs can correct this problem. These devices must have a very low IQ. The IC normally operates using pulse width modulation (PWM) at 1.2 MHz. However, the IC senses a drop in load current and switches its mode of operation from PWM to pulse frequency modulation (PFM). In addition, the pulse frequency is reduced, thereby bumping the efficiency back up to 80% or more.

This operation is demonstrated in a Texas Instruments (TI) training video. It discusses how to achieve high light-load efficiency with a TI TPS62743 buck dc-dc converter using PFM. The training shows how to keep output voltage ripple low and ensure a seamless transition between PFM and PWM modes. Finally, you will learn how to attain 360 nA of device quiescent current under full operation with the TPS62743 IC.

Using an LDO for Increased Standby-Mode Efficiency

Battery-operated equipment is particularly sensitive to the effects of IQ. Even when the battery-powered device isn’t being used, it’s still drawing current (IQ) and will eventually drain the battery. Numerous devices use battery voltages of 12 V or more. Yet many of the devices making up the load require low voltages like 3.3 or 5 V. Using an LDO for regulation will decrease efficiency to 10% or less.

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1. The battery voltage is stepped down by the LM5165 synchronous buck dc-dc converter, which provides the inputs to the LDOs that furnish power to the loads.

This problem can be fixed by introducing a buck-type dc-dc switching converter between the battery and the LDO with load. The switching action keeps the efficiency high. Figure 1 shows a battery input of 20 to 60 V to an LM5165 regulator. It efficiently decreases the voltage level to a value compatible with the LDOs. The LDOs deliver the final rail voltages to the product. The IQ is lowered and efficiency is high with this arrangement, even using the LDOs.

One useful solution is to disable your dc-dc converter during light-load operation and regulating output voltage with an LDO only (Fig. 2). A control signal applied to the enable (EN) pin can turn off the dc-dc converter while turning on the LDO.

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2. By combining a switching dc-dc converter with an LDO, one can achieve high efficiency at heavy loads. But this converter can be turned off with the EN signal while turning on the LDO, which is more efficient at the smaller loads.

Quiescent Current Defined

Quiescent current is that small amount of current required to keep an IC or other circuit functional. IQ assumes a no-load condition.

IQ turns out to be the lowest amount of current drawn when the IC isn’t being accessed or used. In other words, it’s the least amount of current that a device requires to stay operational. Thinking about this, you can understand that IQ truly defines the total run-time of the product. In a standby or sleep mode of operation, quiescent current is flowing and therefore determines how long a battery-powered device will function.

In summary, the IQ is just the minimal overhead current that keeps the IC alive to function properly. It doesn’t include any load current that exits the IC. Furthermore, IQ isn’t the no-load input current.

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