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UCC3890N Datasheet(PDF) 7 Page - Texas Instruments |
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UCC3890N Datasheet(HTML) 7 Page - Texas Instruments |
7 / 9 page UCC1890 UCC2890 UCC3890 low the regulation point of 1.25V. The minimum value for TOFF is 8.75 µs. So in this case ROFF = 8.75 µs • (1.24V − 0.4V) 150pF • 3.4V = 15k OTHER APPLICATION CONSIDERATIONS Output Capacitor: For best regulation of the output voltage or current, the output capacitor should be a low ESR type. This is especially true when operating in cur- rent sense mode with a non-linear load such as a bat- tery. If a low ESR capacitor cannot be used, excellent regulation can also be achieved by placing a low pass R/C filter between the current shunt and the CS input. No Load Operation: The UCC3890 is inherently pro- tected for short circuits, but not for open circuits. If the load is removed, the output voltage will quickly rise up to the regulation point. Once the output is above the regulation voltage, the oscillator will drop to the mini- mum frequency set by RS/CT. With no load on the out- put, even at this low frequency the output voltage can quickly rise to a dangerous level. To protect against this, it is recommended that a zener or other voltage clamp always be connected across the output. The clamp should be chosen to be above the normal range of out- put voltage, but low enough to protect the output ca- pacitor. In current sense operation, removal of the load will also break the regulation loop, in which case a sim- ple clamp on the output may not be adequate. In current sense mode it is recommended that a second zener be connected from the output to the FB pin, the breakdown voltage of this clamp chosen to be high enough so that it will not conduct during normal operation, but will con- duct at least 2V lower than the breakdown voltage of the other clamp. Gate Drive for the External FET: The UCC3890 is guaranteed to be able to deliver at least 1mA of steady state current to the gate of the external FET at ITON = 2mA. If ITON is higher than 2mA, 80% of the additional current is available to drive the FET gate. If, as in the design example above, a moderate sized FET such as the IRF820 is used, the operating frequency is 100kHz, and the minimum ITON at low line is 2.8mA, then the available gate drive current may be adequate. The IRF820 needs about 13nC to charge the gate on each cycle. At 100kHz, this is equivalent to 1.3mA steady state; below the minimum 1.64mA available. In some combinations of a larger FET, and/or higher frequency operation, the current available for driving the gate may not be adequate. In that case extra current may be pro- vided by connecting a resistor RDD from the line input to the VDD pin. This resistor should be sized so that under all conditions the current input to VDD is below the 7.5mA absolute maximum limit. RDD will likely need to be a power resistor. UNITRODE CORPORATION 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. (603) 424-2410 • FAX (603) 424-3460 APPLICATION INFORMATION (cont.) 7 |
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