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Power-Up Characteristics

t - Time = 40 ms/div

V

(5 V/Div)

O

I (1 A/Div)

I

V (2 V/Div)

I

Undervoltage Lockout

Current Limit Protection

Overtemperature Protection

PTN04050A

SLTS250 – SEPTEMBER 2005

When configured per the standard application, the PTN04050A 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 time delay (typically 60 ms) into the power-up characteristic.

This is from the point that a valid input source is recognized. Figure 16 shows the power-up waveforms for a

PTN04050A, operating from a 5-V input and with the output voltage adjusted to –12 V. The waveforms were

measured with a 500-mA resistive load.

Figure 16. Power-Up Waveforms

The undervoltage lockout (UVLO) circuit prevents the module from attempting to power up until the input voltage

is above the UVLO threshold. This prevents the module from drawing excessive current from the input source at

power up. Below the UVLO threshold, the module is held off.

The PTN04050 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's overtemperature protection begins to periodically turn the output

voltage completely off.

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.

Note: 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.

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