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Adding a Flexible Current Limit

Aug. 30, 2021
Voltage converters rarely offer an adjustable current limit. However, a high-side N-channel MOSFET static switch driver offers a possible solution to not only limiting current, but monitoring and disconnecting supply lines in a range of systems.

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What you'll learn:

  • The importance of current limiting.
  • A flexible solution for adding current limiting.

In some power-management applications, precise current limiting is required. This is necessary either to protect the energy source—for example, if anintermediate circuit voltage requires overload protection so that it can reliably supply other system parts with energy—or to protect a load that can cause damage due to overcurrent in a faultcondition.

Inthesearchforasuitabledc-dcpoint-of-load (PoL) regulatortomeetthisrequirement, you’ll find very few voltage converters with adjustable current limit on the market. While adjustable current limit is more often found on controller designs withexternalpowerswitches,allintegratedsolutionsseldomlyoffersuchfunctionality. Also, adjustable current limits typically don’t have very high accuracy. On top of that, the current limiters in dc-dc converter ICs usually only limit the inductor current and not the input or output current of the power supply.

Such an integrated current limit is designed to protect “just” the switching regulator itself from destruction in a fault condition. The current limit lies above the nominally specified maximum output current and sometimes has a relatively low accuracy. This is adequate for protecting the switching regulator, but often not adequate for use as an adjustable currentlimiter (Fig. 1).

Add a Switch Driver

A flexible solution to this problem is to add an adjustable current limit via acomponentlike theLTC7003, a high-sideN-channel MOSFET static switch driver. Dependingontheapplication,accuraciesofapproximately15%canbeachieved.Through its adjustable current limit and its current monitoring function, it’s well-suited for adding current limiting to common dc-dc converters.

Figure 2 shows the use of the LTC7003 current limiter for monitoring theoutputcurrentofanADP2370.TheADP2370isastep-downdc-dcconverter.

In general, high-side current-sense amplifiers also can be used to measure a small voltage drop via a current-sense resistor in the power path. They can measurecurrentswithveryhighaccuracy.However,withmostofthem, thepermissiblevoltagedifferencebetweenthetwocurrent-sensingconnections is very small.

When such a general current-sense amplifier is used in a power supply that could suffer from short circuits due to the load, the voltage across the sense resistor can quickly go outside the permissible range. In this case, a driver like the LTC7003, which is permitted for use in a power supply, offers a solution.

Here, the LTC7003 is designed whereby a large voltage difference is permitted at the SENS inputs. Italso offers the possibility ofinterruptingthepowerpaththroughtheoptionalN-channelMOSFETQ1if the set current threshold is reached. Figure 3 shows a solution with an external N-channel MOSFET to interrupt the power path when reaching a set current threshold.

Through the IMON output, a voltage that’s proportional to the current flow through the sense resistor is provided. This voltage is in respect to the system ground and corresponds to the voltage across the sense resistor multiplied by a factor of 20. The voltage lies between 0 and 1.5 V.

That voltage can be used withanadditionalexternalopamptofeedintothefeedbackcircuitofaswitching regulator. In this way, the output voltage of the dc-dc converter can be reducedinproportiontothecurrentlevelsensedbytheLTC7003.Thisoptionis shown in Figure 3 in the gray box.