How NOT to burn voltage regulator | use current limit protection

How to limit excessive current in voltage regulators | feedback circuits | operational amplifier
differential amplifier
difference amplifier
control loop
control circuit
How to limit excessive load current in voltage regulators | protection circuits

The function of current limiting circuitry is to prevent damage to the IC when an
overload is placed on the output of the regulator (the load impedance is too low).

Without current limiting, the regulator would source excessive load current and
destroy the pass transistor inside the part.

To prevent this occurrence, the current limit circuit will cut down the drive to the pass transistor so that the maximum safe current
level is not exceeded .

The maximum current that a linear regulator can supply to a load is specified on the
data sheet. Many regulators (and most LDO regulators) specify only a single value
of maximum current. This value is guaranteed for any input/output voltage within the
maximum ratings for the part.

For example, the LP2952 is guaranteed to source at least 250 mA without going
into current limiting, as long as the output is in the 1.25V - 29V range and the input
voltage is at least 0.8V above the output.
The load current is sensed by the "I SENSE" resistor, which develops a voltage that
is directly related to the current. This voltage is level shifted (and amplified) by the
differential amplifier.

The voltage at the output of the differential amplifier is a ground-referenced signal
that is proportional to the load current.

This "load current" signal coming from the differential amplifier is applied to the inverting input of the current limit error amplifier,
while the non-inverting input is connected to a reference voltage. The value of this
Note that as long as the load current is below the limit threshold, the output of the
current error amplifier is high (and the voltage error amplifier keeps the regulator in
constant voltage mode).

When the load current reaches the limit threshold, the output of the current error
amplifier drops low and starts sinking current away from the output of the voltage
error amplifier (this puts the regulator in constant current mode).

When current limiting occurs, the regulator output voltage will drop below its nominal
value, which will be sensed by the voltage error amplifier as an undervoltage
condition. The voltage error amplifier will drive its output high in an attempt to raise
the output voltage, but the current error amplifier can sink all of the current coming
from the voltage error amplifier. Like the thermal limiter, the current limiter overrides
the voltage error amplifier to prevent damage to the IC.

reference voltage would be equal to the voltage at the output of the differential
amplifier when the regulator is driving maximum current (at the current limit point).

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