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PT4666C Datasheet(PDF) 11 Page - Texas Instruments |
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PT4666C Datasheet(HTML) 11 Page - Texas Instruments |
11 / 16 page ![]() Application Notes For technical support and further information, visit http://power.ti.com PT4660 & PT4680 Series Operating Features & System Considerations for the PT4660 & PT4680 Dual-Output Converters Over-Current Protection The dual-outputs of the PT4660 and PT4680 series of DC/DC converters have independent output voltage regulation and current limit control. Applying a load current in excess of the current limit threshold at either output will cause the respective output voltage to drop. However, the voltage at Vo2 is derived from Vo1. There- fore a current limit fault on Vo1 will also cause Vo2 to drop. Conversely, a current limit fault applied to Vo2 will only cause Vo2 voltage to drop, and Vo1 will remain in regulation. The current limit is continuous with some current fold- back. This means that at short circuit, the value of the output current can be less than the rated output of the converter. This is to reduce power dissipation when a fault is present. As with any foldback-limited source, if a con- stant current load is applied to the converter with a value greater than the short-circuit current, the output voltage will not come up. Resistive and non-linear load circuits are not affected by this characteristic as long as the cur- rent at startup does not exceed the short-circuit current of the converter. The majority of low-voltage analog and digital applications are not affected by this restric- tion. However, when testing with an electronic load the constant resistance setting should be used. Output Over-Voltage Protection Each output is monitored for over voltage (OV). For fail safe operation and redundancy, the OV fault detection circuitry uses a separate reference to the voltage regula- tion circuits. The OV threshold is fixed, and set nominally 25 % higher than the set-point output voltage. If either output exceeds the threshold, the converter is shutdown and must be actively reset. The OV protection circuit can be reset by momentarily turning the converter off. This is accomplished by either cycling one of the output enable control pins (EN1 or EN2), or by removing the input power to the converter. Note: If Vo1 or Vo2 is adjusted to a higher voltage, the margin between the respective steady- state output voltage and its OV threshold is reduced. This can make the module sensitive to OV fault detection, that may result from random noise and load transients. Over-Temperature Protection The converter has an internal temperature sensor. At a case temperature of approximately 115 °C the converter will shut down, and will automatically restart when the temperature returns to about 100 °C. The analog voltage generated by the sensor is also made available at the TEMP output (pin 5), and can be monitored by the host system for diagnostic purposes. Consult the ‘Pin Descriptions’ section of the data sheet for further infor- mation on this feature. Under-Voltage Lock-Out The Under-Voltage Lock-Out (UVLO) circuit prevents operation of the converter whenever the input voltage to the module is insufficient to maintain output regulation. The UVLO has approximately 2 V of hysterisis. This is to prevent oscillation with a slowly changing input volt- age. Below the UVLO threshold the module is off and the enable control inputs, EN1 and EN2 are inoperative. Primary-Secondary Isolation The PT4460 and PT4680 series of DC/DC converters incorporate electrical isolation between the input termi- nals (primary) and the output terminals (secondary). All converters are production tested to a withstand voltage of 1500 VDC. The isolation complies with UL60950 and EN60950, and the requirements for operational isolation. This allows the converter to be configured for either a positive or negative input voltage source. The regulation control circuitry for these modules is located on the secondary (output) side of the isolation barrier. Control signals are passed between the primary and secondary sides of the converter via a proprietory magnetic coupling scheme. This eliminates the use of opto-couplers. The data sheet ‘Pin Descriptions’ and ‘Pin-Out Information’ provides guidance as to which reference (primary or secondary) that must be used for each of the external control signals. Fuse Requirements To comply with safety agency requirements, these con- verters must be operated with an external input fuse. A fast-acting 250-V fuse is required. Table 1-1 gives the recommended current rating for the product series being used. Table 1-1; Recommended Fuse Rating Product Input Total Fuse Series Bus Iout Rating PT4660 48 V 30 A 7 A PT4680 24 V 20 A 10 A |
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