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LT3575 Datasheet(PDF) 9 Page - Linear Technology |
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LT3575 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT8300 9 8300f operaTion effective quiescent current to improve light load efficiency. In this condition, the LT8300 operates in low ripple Burst Mode. The typical 7.5kHz minimum switching frequency Output Voltage The RFB resistor as depicted in the Block Diagram is the only external resistor used to program the output voltage. The LT8300 operates similar to traditional current mode switchers, except in the use of a unique flyback pulse sense circuit and a sample-and-hold error amplifier, which sample and therefore regulate the isolated output voltage from the flyback pulse. Operation is as follows: when the power switch M1 turns off, the SW pin voltage rises above the VIN supply. The amplitude of the flyback pulse, i.e., the difference between the SW pin voltage and VIN supply, is given as: VFLBK = (VOUT + VF + ISEC • ESR) • NPS VF = Output diode forward voltage ISEC = Transformer secondary current ESR = Total impedance of secondary circuit NPS = Transformer effective primary-to-secondary turns ratio The flyback voltage is then converted to a current IRFB by the flyback pulse sense circuit (M2 and M3). This cur- rent IRFB also flows through the internal trimmed 12.23k RREF resistor to generate a ground-referred voltage. The resultingvoltagefeedstotheinvertinginputofthesample- and-hold error amplifier. Since the sample-and-hold error amplifier samples the voltage when the secondary current is zero, the (ISEC • ESR) term in the VFLBK equation can be assumed to be zero. The bandgap reference voltage VBG, 1.223V, feeds to the non-inverting input of the sample-and-hold error ampli- fier. The relatively high gain in the overall loop causes the voltage across RREF resistor to be nearly equal to the applicaTions inForMaTion determines how often the output voltage is sampled and also the minimum load requirement. bandgap reference voltage VBG. The resulting relationship between VFLBK and VBG can be expressed as: VFLBK RFB • RREF = VBG or VFLBK = VBG RREF • RFB = IRFB • RFB VBG = Bandgap reference voltage IRFB = RFB regulation current = 100µA Combination with the previous VFLBK equation yields an equation for VOUT,intermsoftheRFBresistor,transformer turns ratio, and diode forward voltage: VOUT = 100µA • RFB NPS − VF Output Temperature Coefficient The first term in the VOUTequationdoesnothavetempera- ture dependence, but the output diode forward voltage VF hasasignificantnegativetemperaturecoefficient(–1mV/°C to –2mV/°C). Such a negative temperature coefficient pro- duces approximately 200mV to 300mV voltage variation on the output voltage across temperature. Forhighervoltageoutputs,suchas12Vand24V,theoutput diode temperature coefficient has a negligible effect on the output voltage regulation. For lower voltage outputs, such as 3.3V and 5V, however, the output diode temperature coefficientdoescountforanextra2%to5%outputvoltage regulation. For customers requiring tight output voltage regulation across temperature, please refer to other LTC parts with integrated temperature compensation features. |
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