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Simple current limiter is programmable via resistor

Available integrated circuits can let you program a current limit, but they generally involve microcontrollers or data converters (or both). As an alternative, the circuit of Figure 1 lets you program a current limit without the intervention of microcontrollers or data converters. It consists of two miniature SOT-5 devices and a few external components. The sense-resistor value determines the maximum current limit.



Figure 1: This circuit limits the RLOAD current to a value determined mainly by R1.

(Click on image to enlarge)

The circuit consists of a charge-pump voltage doubler (U1, MAX1682), a current-sense amplifier (U2, MAX4376), and two n-channel MOSFETs. U1 doubles the supply voltage to provide gate drive for Q2, and U2 amplifies the voltage across the sense resistor (RSENSE) with a gain of 20. This U2 output drives the Q1 gate via the R2/R3 divider, which modulates the current through R1, which in turn sets the Q1 drain voltage and Q2's gate-drive voltage.

U2's output voltage is 20 times the voltage between RS+ and RS-, but has a full-scale limit of 2 V. Figure 2 shows the response of this circuit when you suddenly impose a heavy (low resistance) load. It allows an initial current surge to 10 A, and then (after 20 μs) settles to the desired limit of 7.25 A.



Figure 2: With VSUPPLY and VIN (at U1) equal to 5 V, the Figure 1 circuit limits load current to 7.25 V.

(Click on image to enlarge)

The maximum allowed in-rush current is set by the sense resistor. A 10 mΩ value, for instance, allows a maximum in-rush current of 100 mV/10 mΩ = 10 A. This steady-state limit is determined by the operating characteristics of Q1 and Q2, together with the values chosen for resistors R1-R3.

Figure 3 shows the ILIMIT values obtained by varying R1 alone. To set other values of ILIMIT , you can vary the ratio R2/R3 while holding the R1 value fixed.



Figure 3: With VSUPPLY and VIN (at U1) equal to 5 V, the Figure 1 circuit's steady-state current limit varies with R1 as shown.

(Click on image to enlarge)

About the authors

Perwez Nawabi is an applications engineer at Maxim Integrated Products, Sunnyvale, CA. He graduated from San Jose State University (CA) with a BSEE in 2005. He then joined Maxim as a test engineer, and moved to his current position as an applications engineer two years later.

Budge Ing is an applications manager at Maxim Integrated Products, Sunnyvale, CA.. He received his MSEE from San Jose State University.

1 comment on “Simple current limiter is programmable via resistor

  1. 1o9sjf3
    July 29, 2015

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