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Micropower 5V, 100mA Low Dropout Linear Regulator with RESETbar and ENABLEbar
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Linear Regulator
Protection Circuitry
Kieran O’Malley
ON Semiconductor
2000 South County Trail
East Greenwich, RI 02818Most monolithic linear regulators contain basic protection features to safeguard against potentially catastrophic events. These
features include short circuit and overvoltage protection, thermal shutdown, reverse battery and reverse transient protection.
Each of these features effects design performance, system reliability, and cost.
All features may not be necessary or even desirable for a particular system. By gaining a full understanding of each of these
features and their implications, a designer can make the appropriate choices about how and which to include in a system design.
Short Circuit ProtectionLinear regulators can be destroyed if they are forced to
source excessive current. This can happen under short
circuit or excessive load conditions. In a short circuit
condition, not only is the pass transistor sourcing excessive
current, the voltage across it is maximal. (Since VOUT is
ground, the voltage across the transistor is VIN.)
Linear regulators typically use one of two types of short
circuit protection on chip: constant current limit or foldback
current limit.
Figure 1. Constant Current Limit CircuitVIN
Error Amp VOUT
Pass Transistor
IOUT� VBE
In a constant current limit circuit, the maximum current
that a linear regulator can source is limited to a preset value,
IMAX. Figure 1 shows a typical constant current limit circuit.
As IOUT increases, IB in the output device, increases
proportionately as IOUT /β. As IB increases, the base voltage
of transistor Q3 (VBE3 = IB/R) increases, turning on Q3.
Q3’s collector current is steered away from Q1, lowering
Q1’s emitter and Q2’s base voltage. The output current
reaches an equilibrium point where IOUT is held at a
determined maximum value, IMAX.
In a foldback current limiting circuit (Figure 2), the output
voltage remains within specification limits up to IMAX.
Beyond this level, both the output current and voltage
until the current reaches its IFB at VOUT = 0 V (Figure 3). The
foldback current limit circuit is similar in design and
function to the constant current limit circuit except that it
contains feedback elements connected to the output.
Figure 2. Foldback Current Limit CircuitVIN
Error Amp
Source
Figure 3. Curve of Foldback Current for a
Typical 5.0 V, 500 mA Linear Regulator5.0 V
OUT
(V)
