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APPLICATION INFORMATION

Operating Features and System Considerations for the PTMA40XX DC/DC Converters

Primary-Secondary Isolation

Undervoltage Lockout

Output Overvoltage Clamp

Overcurrent Protection

Soft-Start Power Up

PTMA403033

PTMA402050

PTMA401120

SLTS259A – FEBRUARY 2006 – REVISED MARCH 2006

These converters incorporate electrical isolation between the input terminals (primary) and the output terminals

(secondary). All converters are tested to a withstand voltage of 1500 Vdc. This complies with UL/cUL 60950 and

EN 60950 and the requirements for functional isolation. It allows the converter to be configured for either a

positive or negative input voltage source. The data sheet Terminal Functions table provides guidance as to the

correct reference that must be used for the external control signals.

The undervoltage lockout (UVLO) is designed to prevent the operation of the converter until the input voltage is

close to the minimum operating voltage. The converter is held off when the input voltage is below the UVLO

threshold, and turns on when the input voltage rises above the threshold. This prevents high start-up current

during normal power up of the converter, and minimizes the current drain from the input source during low input

voltage conditions. The converter meets full specifications when the minimum specified input voltage is reached.

The UVLO circuitry also overrides the operation of the enable or synchronization controls. Only when the input

voltage is above the UVLO threshold does this input become functional.

The module is protected from an overvoltage on the output using an internal clamp. This protects against a break

in the feedback path as well as a ground fault on the external Trim resistor, which would cause the output

voltage to increase.

To protect against load faults, these converters incorporate output overcurrent protection. Applying a load to the

output that exceeds the converter overcurrent threshold (see applicable specification) causes the output voltage

to momentarily fold back, and then shut down. Following shutdown, the module periodically attempts to

automatically recover by initiating a soft-start power up. This is often described as a hiccup mode of operation,

whereby the module continues in the cycle of successive shutdown and power up until the load fault is removed.

Once the fault is removed, the converter automatically recovers and returns to normal operation.

When the converter is first powered, the internal soft-start circuit limits how fast the output voltage can rise. The

soft-start circuit functions after a valid input source is applied and the output is enabled, after the converter output

is enabled using the Enable input, or on a recovery from a load fault. The purpose of the soft-start feature is to

limit the surge of current drawn from the input source when the converter begins to operate. By limiting the rate

at which the output voltage rises, the magnitude of current required to charge up the load circuit capacitance is

significantly reduced.

Figure 13 shows the power-up characteristic of a PTMA403033 converter. The output voltage is 3.3 V. The

soft-start circuit causes a slow, soft rise of the output voltage. The output voltage will begin to rise after a time

delay (typically 85 ms-100 ms) once a valid input voltage is applied. The output then progressively rises to the

voltage set-point. The waveforms were recorded with a resistive load of 3 A.

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