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APPLICATION INFORMATION

Operating Features and System Considerations for the PTB48560x DC/DC Converters

Primary-Secondary Isolation

Undervoltage Lockout

Soft-Start Power Up

UVLO Threshold

t - Time = 5 ms/div

I (0.5 A/div)

I

V (5 V/div)

O

V (20 V/div)

I

Overcurrent Protection

PTB48560A, PTB48560B

PTB48560C

SLTS236A–MARCH 2005–REVISED OCTOBER 2005

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 PEN and NEN enable controls. Only when the input

voltage is above the UVLO threshold do these inputs become functional.

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

soft-start circuit functions whenever the converter output is enabled from the PEN and NEN inputs, or when a

valid input source is first applied with the output enabled. It also functions on a recovery from a load fault,

overtemperature, or input overvoltage condition. 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.

soft-start circuit introduces a short time delay (typically 5-10 ms) before allowing the output to rise. The output

then progressively rises to the voltage set-point. The waveforms were recorded with a resistive load of 2.5 A.

Figure 13. Soft-Start Waveform

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

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