Datasheet

t-Time=5ms/div

(5V/div)

(2V/div)

(2 A/div)

PTN78020W

PTN78020H

www.ti.com

SLTS228C – DECEMBER 2004– REVISED AUGUST 2011

Power-Up Characteristics

When configured per the standard application, the PTN78020 power module produces a regulated output voltage

following the application of a valid input source voltage. During power up, internal soft-start circuitry slows the

rate that the output voltage rises, thereby limiting the amount of in-rush current that can be drawn from the input

source. The soft-start circuitry introduces a short time delay (typically 5 ms – 10 ms) into the power-up

characteristic. This is from the point that a valid input source is recognized. Figure 26 shows the power-up

waveforms for a PTN78020W, operating from a 12-V input and with the output voltage adjusted to 5 V. The

waveforms were measured with a 1.5-A resistive load.

Figure 26. Power-Up Waveforms

Current-Limit Protection

The PTN78020 modules protect against load faults with a continuous current limit characteristic. Under a load

fault condition, the output current cannot exceed the current limit value. Attempting to draw current that exceeds

the current-limit value causes the module to progressively reduce its output voltage. Current is continuously

supplied to the fault until it is removed. On removal of the fault, the output voltage promptly recovers. When

limiting output current, the regulator experiences higher power dissipation, which increases its temperature. If the

temperature increase is excessive, the module overtemperature protection begins to periodically turn the output

voltage completely off.

Overtemperature Protection

A thermal shutdown mechanism protects the module's internal circuitry against excessively high temperatures. A

rise in temperature may be the result of a drop in airflow, a high ambient temperature, or a sustained current limit

condition. If the junction temperature of the internal control IC rises excessively, the module turns itself off,

reducing the output voltage to zero. The module instantly restarts when the sensed temperature decreases by a

few degrees.

Overtemperature protection is a last resort mechanism to prevent damage to the module. It should not be relied

on as permanent protection against thermal stress. Always operate the module within its temperature derated

limits, for the worst-case operating conditions of output current, ambient temperature, and airflow. Operating the

module above these limits, albeit below the thermal shutdown temperature, reduces the long-term reliability of

the module.

Product Folder Link(s): PTN78020W PTN78020H