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UC2914 датащи(PDF) 11 Page - Texas Instruments |
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UC2914 датащи(HTML) 11 Page - Texas Instruments |
11 / 14 page 11 UC1914 UC2914 Resistive Load: () () CT RV I R T R V R PL CC MAX LOAD START PL CC PL min = •+ − • • • +• − 10 2 2 4 RC LOAD LOAD • Output Current Softstart The external MOSFET output current can be increased at a user-defined rate to ensure that the output voltage comes up in a controlled fashion by adding capacitor CSS, as shown in Fig. 5. The chip does place one con- straint on the soft start time and that is that the charge pump time constant has to be much less than the softstart time constant to ensure proper soft start opera- tion. The time constant determining the startup time of the charge pump is given by: τ CP OUT PUMP RC =• ROUT is the output impedance of the charge pump given by: R fCP OUT PUMP = • 1 where fPUMP is the charge pump frequency (125kHz) and CP = CP1 = CP2 are the charge pump flying capacitors. For typical values of CP1, CP2 and CPUMP (0.01µF) and a switching frequency of 125kHz, the output impedance is 800 Ω and the charge pump time constant is 8µs. The charge pump should be close to being fully charged in 3 time constants or 24 µs. By placing a capacitor from VCC to IMAX, the voltage at IMAX, which sets the maximum output current of the FET, will exponentially decay from VCC to the desired value set by R1 and R2. The output current of the MOSFET will be controlled via soft start as long as the soft start time constant ( τSS) is much greater than the charge pump time constant τCP, given by () τ SS SS RR C =• 12 Minimizing Total Dropout Under Low Voltage Operation In a typical application, the UC3914 will be used to con- trol the output current of an external NMOS during hot swapping situations. Once the load has been fully charged, the desired output voltage on the load, VOUT, will be required to be as close to VCC as possible to minimize total dropout. For a resistive load, RLOAD, the output voltage is given by: V R RR R VCC OUT LOAD LOAD SENSE DS on = ++ • () RSENSE was picked to set the fault current, IFAULT. RDS(on), the on-resistance of the NMOS, should be made as small as possible to ensure VOUT is as close to VCC as possible. For a given NMOS, the manufacturer will specify the RDS(on) for a certain VGS (maybe 7V to 10V). The source potential of the NMOS is VOUT. In order to ensure sufficient VGS, this requires the gate of the NMOS, which is the output of the linear amplifier, to be many volts higher than VCC. The UC3914 provides the capability to generate this voltage through the addition of 3 capacitors, CP1, CP2 and CPUMP as shown in Fig. 6. These capacitors should be used in conjunction with the complementary output drivers and internal diodes in- cluded on-chip to create a charge pump or voltage tripler. The circuit boosts VCC by utilizing capacitors CP1, CP2 and CPUMP in such a way that the voltage at VPUMP approximately equals (3 • VCC )–(5•VDIODE), al- most tripling the input supply voltage to the chip. APPLICATION INFORMATION (cont.) 6 11 12 9 8 7 C PUMP L O A D C LOAD V OUT M1 2 OUT VOUTS VPUMP VCC R2 18 16 C SS R1 C1 IMAX REF OSC PMP 5 OSCB PMPB CP2 CP1 Figure 5. MOSFET soft start diagram. UDG-98160 Figure 6. Charge pump block diagram. UDG-97058 |
Аналогичный номер детали - UC2914 |
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Аналогичное описание - UC2914 |
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