In the case of a short-circuit or
circuit-breaker event, when the device interrupts current flow, the input inductance
generates a positive voltage spike on the input, and the output inductance generates
a negative voltage spike on the output. The peak amplitude of voltage spikes
(transients) is dependent on the value of inductance in series to the input or
output of the device. Such transients can exceed the absolute maximum ratings of the
device if steps are not taken to address the issue. Typical methods for addressing
transients include:
- Minimize lead length and inductance into and out of the device.
- Use a large PCB GND plane.
- Connect a Schottky diode from the OUT pin ground to absorb negative spikes.
- Connect a low ESR capacitor of 2.2 μF or higher at the OUT pin very close to the
device.
- Connect a ceramic capacitor CIN = 0.1 μF or higher at the IN pin very
close to the device to dampen the rise time of input transients. The capacitor
voltage rating must be at least twice the input supply voltage to be able to
withstand the positive voltage excursion during inductive ringing.
The approximate value of input capacitance can be
estimated with Equation 44.
Equation 44.
VIN is the nominal supply
voltage.
ILOAD is the load current.
LIN equals
the effective inductance seen looking into the source.
CIN is the
capacitance present at the input.
- Some applications can require the addition of a Transient Voltage Suppressor
(TVS) to prevent transients from exceeding the absolute maximum ratings of the
device. In some cases, even if the maximum amplitude of the transients is below
the absolute maximum rating of the device, a TVS can help to absorb the
excessive energy dump and prevent it from creating very fast transient voltages
on the input supply pin of the IC, which can couple to the internal control
circuits and cause unexpected behavior.
The circuit implementation with optional protection components is shown in Figure 9-18.