Datasheet
SLUS599A − JUNE 2004 − REVISED AUGUST 2008
5
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APPLICATION INFORMATION
Overvoltage Protection
An overvoltage condition is commonly created in these situations.
D Unplugging a wall adapter from an AC outlet. Energy stored in the transformer magnetizing inductance is
released and spikes the output voltage.
D Powering an appliance with the wrong voltage adapter (user error)
D Automotive load dump due to ignition, power windows, or starter motor (for example)
D An AC power-line transient
D Power switch contact bounce (causes power supply/distribution inductive kick), (See Figure 2)
Many electronic appliances use a transient voltage suppressor (TVS) for overvoltage protection as shown in
Figure 2. The TVS is typically a metal-oxide varister (MOV) or Transzorb. The former is a non-linear resistor
with a soft turn-on characteristic whereas the latter is a large junction zener diode with a very sharp turn-on
characteristic. These devices have high pulse-power capability and pico-second response time. A TVS clamps
the load voltage to a safe level so the load operates uninterrupted in the presence of power supply
output-voltage spikes. But in the event of a voltage surge, fuse F2 blows and must be replaced to restore
operation.
UDG−04057
V
S
L
S
R
S
F1
AppliancePower Supply
+
TVS LOAD
F2S1
Figure 2. Load Protection Using Transient Voltage Suppressor Clamps
The TPS2400 circuit in Figure 3 protects the load from an overvoltage, not by clamping the load voltage like
a TVS, but by disconnecting the load from the power supply. The circuit responds to an overvoltage in less than
1 µs and rides out a voltage surge without blowing fuse F2. Note that the voltage surge can be of indefinite
duration.
The load can see a voltage spike of up to 1 µs, the amount of time it takes the TPS2400 to disconnect the load
from the power supply. A low-power zener diode D2 can be used to clamp the load voltage to a safe level. In
most cases, diode D2 is not necessary since the load bypass capacitor (not shown) forms a low-pass filter with
resistor R
S
and inductor L
S
to significantly attenuate the spike.