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Avnet Employee
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Registered: ‎06-12-2009

Current Limit Circuit for Voltage Regulator

[ Edited ]

This is a low cost circuit that adds precise current limit functionality to a voltage regulator.

 

Basic Circuit Analysis

The circuit below uses a low cost op-amp to add a precise current limit to a voltage regulator. It utilizes the feedback voltage as a reference for the current measurement.

current limit circuit diagram

The voltage V+ comes from a voltage divider between Vsns and ground. For the time being, we will call the ratio of the divider A+. The voltage V- comes from a voltage divider between Vout and Vfb.  We will call this ratio A-.

  • V+ = A+ * (Vsns)
  • V- = Vfb + A- *(Vout - Vfb) = [A- * Vout] + [(1 - A-) * Vfb]

Vsns is the output voltage plus the voltage across the sense resistor.

  • Vsns = Vout + (Iout * Rsns)
  • V+ = [A+ * Vout] + [A+ * (Iout * Rsns)]

When current limiting, the op-amp will match the inputs so that V+ is equal to V-:

  • V+ = V-
  • [A+ * Vout] + [A+ * (Iout * Rsns)] = [A- * Vout] + [(1 - A-) * Vfb]

If we use the same value for A+ and A- (we will call this A), the Vout terms cancel and we are left with:

  • [A * (Iout * Rsns)] = [(1 - A) * Vfb]
  • (Iout * Rsns) = [(1 - A) * Vfb] / A = (1 - 1/A) * Vfb
  • Iout = (1 - 1/A) * Vfb / Rsns

A is the ratio determined by the values of Ra1 and Ra2. We also know the sum of Ra1 and Ra2 based on the feedback divider specified by the regulator so we can determine the individual values of Ra1 and Ra2

  • A = Ra2/(Ra1+Ra2)
  • Ra = Ra1 + Ra2

The diode prevents the op-amp from sinking current, so the output voltage will not go above the set voltage, but it will drop if the load tries to pull more then the current limit: (1 - 1/A) * Vfb / Rsns


Stabilizing the circuit

The above calculations are based on an ideal DC analysis. In the real world we have to make some adjustments to stabilize the circuit. Here are some details on the functionality of some of the other devices in the circuit:

 

  • Rc and C1 are added to stabilize the op-amp, since the voltage regulator can only respond so quickly. Make sure to apply Rc to both inputs to minimize the effects of the input bias current.
  • Rd reduces the gain of the op-amp and helps prevent the regulator from oscillating. A value close to Rb should be a good starting point.
ILIM_cmp.png