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NCP1000 Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1000 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 14 page NCP1000, NCP1001, NCP1002 http://onsemi.com 10 reliability in overload conditions. Figure 20 shows the timing waveforms in auto restart mode. The VCC pin receives its startup power from the high voltage startup circuit. Once the undervoltage lockout trip point is exceeded, the high voltage startup circuit turns off, and the VCC pin receives its power from the auxiliary winding of the power transformer. Once the converter is enabled, the VCC voltage will be clamped by the 8.6 V limiter. Since the voltage limiter will regulate the VCC voltage at 8.6 V, it must shunt all excess current based on the input impedance to this pin. A resistor is required between the auxiliary winding filter capacitor and the VCC pin to limit the current. Optocoupler Fail−safe Circuit The NCP100x has the ability to sense an open optocoupler and protect the load in the event of a failure. This circuit operates by sensing the current in the VCC limiter, and detecting a high current which is an indication of an open optocoupler. The VCC pin receives the output of a current source which is created by the voltage drop between the auxiliary winding and the VCC limiter across the shunt resistor. The Vcc limiter will clamp the VCC voltage to approximately 8.6 V. Any current that is available at this pin, that is not needed for either the chip bias current, or the opto current is shunted through this limiter. The opto fail−safe circuit operates on the premise that under an open opto condition, the opto current will all be shunted through the VCC limiter, and the output voltage (and therefore the auxiliary winding voltage) will increase. The increase in auxiliary winding voltage will cause an amplified increase in the current into the VCC pin. To detect an open opto condition, the current in the limiter is measured and if it exceeds 10 milliamps, the chip will shut down and go into burst mode operation. After a shutdown signal, the optocoupler fail−safe circuit will enable the divide−by−eight counter and attempt to restart the unit after every eight VCC cycles. For this circuit to operate properly, the shunt resistor must be chosen prudently. There is a range of values for RS that will determine the operation of this circuit. On one extreme, a large value of RS will minimize the bias current, which will have the effect of maximizing efficiency, while reducing the response to an open optocoupler. The other extreme is the minimum value of RS, which will maximize the bias current into the chip and minimize the voltage overshoot in the event of an open optocoupler. For minimum bias current: Rsmax + (V AUX min * 8.8 volts) I CC1max where: VAUXmin is the minimum expected DC voltage from the auxiliary winding. Typically, this voltage will vary between "5% to "10% from it’s nominal value. ICC1max is the maximum rated bias current for the device used. This value can found in the tables on the data sheet. For the best optocoupler fail−safe response: Rs min + (V AUX max * 7.2 volts) 1.0mA ) I trip where: VAUXmax is the maximum expected DC voltage from the auxiliary winding. Itrip is the minimum trip current for the optocoupler fail−safe. This information can be found in the tables under Current Limit and Thermal Protection, as well as in Figure 12. The value of RS that is used in the circuit must be between the two extreme values calculated. Setting it closer to RSmin will optimize the optocoupler fail−safe feature, while setting it closer to the RSmax value will minimize the bias current Thermal Shutdown and Package The internal Thermal Shutdown block protects the device in the event that the maximum junction temperature is exceeded. When activated, typically at 140 °C, one input of the Driver is held low to disable the Power Switch Circuit. Thermal shutdown activation is non−latching and the Power Switch Circuit is allowed resume operation when the junction temperature falls below 110 °C. The thermal shutdown feature is provided to prevent catastrophic device failures from accidental overheating. It is not intended to be used as a substitute for proper heatsinking. The die in the 8−pin dual−in−line package is mounted on a special heat tab copper alloy lead frame. The tab consists of pins 3, 6, 7, 8 is specifically designed to improve the thermal conduction from the die to the printed circuit board. This permits the use of standard layout and mounting practices while having the ability to halve the junction to air thermal resistance. |
Similar Part No. - NCP1000_06 |
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Similar Description - NCP1000_06 |
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