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Posted on 06 July 2019

System Stability – Intelligence can be Outside the Processor

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Buck Boost DC-to-DC converters that can dynamically limit the inrush current

Portable applications are in constant change. Innovation must be the designer’s motto if they want to be successful in this market. Whether adding new features to re-boost existing devices or developing new projects all together, the consumer electronics industry is in incessant evolution.

By Carmen González, Product Marketing Manager, DC/DC Converters, Texas Instruments

 

Battery driven applications are gaining complexity as they pursue to offer new experiences to the end user. Brighter LED flashes are needed as the number of Megapixels and the quality of the camera increases; new capabilities are incorporated, like imaging projections, communication processors such as GSM modems, application-specific processors, etc. They are all neat, energy-hungry features that are being added to these systems, demanding high levels of output currents. Some of them can be simultaneously implemented in the same application. Hence, increasing the number of high-power rails in the system and, therefore, the design complexity.

The first challenge a designer is faced with is the increased peak-toaverage load current ratio. Power demanding features draw high levels of current from the battery at a given time, from a previous stage where power consumption was minimized. This phenomenon has an immediate consequence: the inherent risk of system stability when a battery is stressed in such a way. You don’t want to miss that important call because your phone suddenly switched off as your call came in while your phone camera flash was flashing!

Batteries undergoing current surges of this kind will decrease the output voltage. For example, consider a battery with 0.5 Ohm impedance and a terminal voltage of 1.8V (Figure 1). Different scenarios need to be taken into consideration to understand what is the real output power of the battery. Increases of battery current deliver higher output power until the voltage at the terminals start reducing. Regardless of the output current demanded, the battery will be able to deliver only so much output power. If the current demanded to the battery is higher than 1.8A, this will not translate into higher output power as can be seen in Figure 1 (A). When the current surge is considerable (higher than 3.5A in this case), a drastic voltage droop at the battery terminals will take place and be drastically reduced to zero, which results in a battery short, as is shown in Figure 1 (B).

Battery output power with 1.8V terminal voltage and no load.

This event forces the DC-to-DC converter that follows the battery into under voltage lockout, which translates into system shut-down. When this happens, the current demanded from the battery is reduced, giving time to the battery to recover the output voltage. This on-off swing continues to take place until the high current demand disappears, or the battery is fully discharged.

To address these concerns, a 4A switch single inductor buck boost converter can assist with this matter. For example, the TPS63020 implements two very unique features which provides this DC-to-DC converter with a touch of intelligence; dynamic input current limit and smart power good.

These two features help protect both the device and the application in an elegant way. How do they work? The average inductor current is limited internally in the IC. At nominal operating conditions, the current limit is constant. If the input power source becomes weak, such as when batteries are almost discharged and they increase output impedance or there is an overload condition due to an additional heavy pulse load connected to the battery, the supply voltage at VIN may drop below 2.3V. If this occurs, the single inductor buck boost converter automatically reduces the current limit as shown in Figure 2.

Average switch current limit.

Limiting the input current of the DC-to-DC converter avoids shorting the battery and gives the battery time to recover its output voltage after a droop. If no recovery is possible, the device smoothly goes into under voltage lockout condition.

When there is an excess of current demanded by the system, the current limit is active and the power good signal goes low. The power good signal is triggered at the earliest possible time and it notifies the load of a potential unregulated output voltage. This event occurs when the input conditions of the DC-to-DC converter no longer allows for the original regulated output voltage value. This notification happens at a much earlier stage than with a traditional power good signal, which goes low only when the output voltage of the converter falls below 95 percent of the nominal value. This extra lapse of time allows the system to react in a safe way, such as allowing sufficient time to safely store data before shut down or reduce processor activity to reduce the load. If the overload condition is only sporadic and goes away, the battery voltage recovers and the TPS63020 goes back into normal operating conditions; the input current is no longer limited and the smart power good signal goes high again.

If the temperature of the device increases above the recommended temperature, the smart power good signal also goes low and notifies the load of a potential over temperature shut down. Under these conditions, the input current limit decreases, reducing the current flowing through the device and allowing the temperature to go back to lower levels. If the temperature continues to increase, the device goes into over temperature shut down.

Summary

By selecting high efficiency buck boost DC-to-DC converters that can dynamically limit the inrush current and give early notification to the load of a potential unregulated output allows the designer to implement the latest features available in the market in battery driven applications in a reliable fashion.

 

References:

Download a datasheet and find other technical documents for the TPS63020: http://www.ti.com/tps63020-ca.
To learn more about these and other converters, visit www.ti.com/dcdc  or the TI E2ETM Community where you can ask questions and solve problems with other engineers: www.ti.com/e2e-ca

 

 

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