Application Notes

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Operating Features of the PT4470 and PT4480

Series of Isolated DC/DC Converters

Under-Voltage Lockout

An Under-Voltage Lock-Out (UVLO) inhibits the

operation of the converter until the input voltage is

above the UVLO threshold (see the applicable data

sheet specification). Below this voltage, the module’s

output is held off, irrespective of the state of the Inhibit

(pin 2), the module will automatically power up when

the input voltage rises above the UVLO threshold. The

UVLO allows the module to produce a clean transition

during both power-up and power-down, even when the

input voltage is rising or falling slowly. It also reduces

the 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

UVLO threshold includes about 2V of hysteresis. Once

operational, the converter will conform to its operating

specifications when the minimum specified input voltage

is reached.

Over-Current Protection

To protect against load faults, the PT4470/80 series of

DC/DC converters incorporate an output current limit.

Once the load current drawn from the module reaches

the current limit threshold, any attempt by the load to

draw additional current will result in a significant drop

in the module’s regulated output voltage. The current

limit circuitry incorporates a limited amount of foldback.

This has the effect of slightly reducing the output current

from the module when supplying an absolute short circuit.

Upon removal of the load fault, the output voltage from

the converter will automatically recover to its programmed

regulation voltage.

Output Over-Voltage Protection

The PT4470/80 series of DC/DC converters incorporate

circuitry that continually senses the output for an over-

voltage (OV) condition. The OV threshold automatically

tracks the VID output voltage program setting to a level

25% higher than that programmed at the control pins,

VID0 through VID4. If the converter output voltage

exceeds the OV threshold, the converter is immediately

shut down and remains in a latched-off state. To resume

normal operation the converter must be actively reset.

This is accomplished by either cycling the status of the

Inhibit control (pin 3) from “On” to “Off” and then back

“On” again, or by momentarily removing the input power

to the converter. For failsafe operation and redundancy,

the OV protection uses circuitry that is independent of

the converter’s internal feedback loop.

Over-Temperature Protection

Over-temperature protection is provided by an internal

temperature sensor, which closely monitors the tempera-

ture of the converter’s metal case. If the case temperature

exceeds the specified limit (see applicable data sheet),

the converter will shut down. The converter will then

automatically restart when the sensed temperature drops

by about 10°C. When operated outside its recommended

thermal derating envelope (see data sheet SOA curves),

the converter will typcially cycle on and off at intervals

from a few seconds to one or two minutes. This is to ensure

that the internal components are not permanently damaged

from excessive thermal stress.

Primary-Secondary Isolation

Electrical isolation is provided between the input terminals

(primary) and the output terminals (secondary). All con-

verters are production tested to a primary-secondary

withstand voltage of 1500VDC. This specification com-

plies with UL60950 and EN60950 and the requirements

for operational isolation. Operational isolation allows

these converters to be configured for either a positive or

negative input voltage source. The data sheet ‘Pin-Out

Information’ uses shading to indicate which pins are

associated with the primary. They include pins 1 through

5, inclusive.

Fuse Recommendations

If desired, an input fuse may be added to protect against

the application of a reverse input voltage.

Thermal Considerations

Airflow may be necessary to ensure that the module can

supply the desired load current in environments with

elevated ambient temperatures. The required airflow

rate may be determined from the Safe Operating Area

(SOA) thermal derating chart (see converter specifica-

tions). The recommended direction for airflow is into

the longest side of the module’s metal case. See Figure 1.

Recommended direction for airflow is

into (perpendicular to) the longest side.

Figure 1

PT4470, PT4480 Series