Adding a flexible current limit
Question: Could I easily and precisely limit a current to my load?
Frederik Dostal, Field Applications Engineer
Answer: There are current limiting ICs available.
In some power management applications, precise current limiting is required. This is necessary either to protect the energy source, for example, if an intermediate 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 fault condition.
In the search for a suitable dc-to-dc point of load regulator to meet this requirement, very few voltage converters with adjustable current limit are found on the market. While adjustable current limit is more often found on controller designs with external power switches, all integrated solutions seldomly offer such function. Also, adjustable current limits often do not have very high accuracy. In addition, the current limiters in dc-to-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 current limiter.
Figure 1. System in which the current flow to or from a switching regulator needs to be limited.
A flexible solution to this problem is to add an adjustable current limit via an additional component such as the LTC7003. Depending on the application, accuracies of approximately 15% can be achieved. The LTC7003 is a high-side N-channel MOSFET static switch driver. Through its adjustable current limit and its current monitoring function, it is ideal for adding current limiting to common dc-to-dc converters. Figure 2 shows the use of the LTC7003 limiter for monitoring the output current of an ADP2370. The ADP2370 is a step-down dc-to-dc converter.
Figure 2. Current limiting added via an LTC7003 driver component.
In general, high-side current sense amplifiers can also be used for measuring a small voltage drop via a current sense resistor in the power path. They can measure currents with a very high accuracy. However, with the majority of them, the permissible voltage difference between the two current-sensing connections is very small. When such a general current sense amplifier is used in a power supply in which short circuits can occur due to the load, the voltage across the sense resistor can quickly go outside the permissible range. In this case, a solution such as the LTC7003, which is permitted for use in a power supply, is better. Here, the LTC7003 is designed in such a way that a large voltage difference is permitted at the SENS inputs. The LTC7003 also offers the possibility of interrupting the power path through the optional N-channel MOSFET Q1 if the set current threshold is reached. Figure 3 shows an LTC7003 solution with an external N-channel MOSFET to interrupt the power path when a set current threshold is reached.
Figure 3. Circuit with the LTC7003 for limiting current.
Through the IMONoutput, a voltage that is 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 V and 1.5 V. This voltage can be used with an additional external op amp to feed into the feedback circuit of a switching regulator. In this way, the output voltage of the dc-to-dc converter can be reduced in proportion to the current level sensed by the LTC7003. This option is shown in Figure 3 in a grayed-out circuit.
With its interesting functions, the LTC7003 is suitable for monitoring, limiting, and disconnecting supply lines in numerous different systems.
About the Author: Frederik Dostal studied microelectronics at the University of Erlangen in Germany. Starting work in the power management business in 2001, he has been active in various applications positions including four years in Phoenix, Arizona, where he worked on switch-mode power supplies. He joined © Analog Devices in 2009 and works as a field applications engineer for power management at Analog Devices in München. He can be reached at email@example.com.