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CS5150H датащи(PDF) 11 Page - Cherry Semiconductor Corporation |
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CS5150H датащи(HTML) 11 Page - Cherry Semiconductor Corporation |
11 / 14 page Applications Information: continued 11 Figure 17: CS5150H gate drive waveforms depicting rail to rail swing. The most important aspect of MOSFET performance is RDSON, which effects regulator efficiency and MOSFET thermal management requirements. The power dissipated by the MOSFETs may be estimated as follows; Switching MOSFET: Power = ILOAD2 × RDSON × duty cycle Synchronous MOSFET: Power = ILOAD2 × RDSON × (1 - duty cycle) Duty Cycle = Off Time Capacitor (COFF) The COFF timing capacitor sets the regulator off time: TOFF = COFF × 4848.5 When the VFFB pin is less than 1V, the current charging the COFF capacitor is reduced. The extended off time can be cal- culated as follows: TOFF = COFF × 24,242.5. Off time will be determined by either the TOFF time, or the time out timer, whichever is longer. The preceding equations for duty cycle can also be used to calculate the regulator switching frequency and select the COFF timing capacitor: COFF = , where: Period = Schottky Diode for Synchronous MOSFET A Schottky diode may be placed in parallel with the syn- chronous MOSFET to conduct the inductor current upon turn off of the switching MOSFET to improve efficiency. The CS5150H reference circuit does not use this device due to its excellent design. Instead, the body diode of the syn- chronous MOSFET is utilized to reduce cost and conducts the inductor current. For a design operating at 200kHz or so, the low non-overlap time combined with Schottky forward recovery time may make the benefits of this device not worth the additional expense (see Figure 6, channel 2). The power dissipation in the synchronous MOSFET due to body diode conduction can be estimated by the following equation: Power = Vbd × ILOAD × conduction time × switching frequency Where Vbd = the forward drop of the MOSFET body diode. For the CS5150H demonstration board as shown in Figure 6; Power = 1.6V × 13A × 100ns × 233kHz = 0.48W This is only 1.3% of the 36.4W being delivered to the load. “Droop” Resistor for Adaptive Voltage Positioning Adaptive voltage positioning is used to reduce output volt- age excursions during abrupt changes in load current. Regulator output voltage is offset +40mV when the regula- tor is unloaded, and -40mV at full load. This results in increased margin before encountering minimum and maxi- mum transient voltage limits, allowing use of less capaci- tance on the regulator output (see Figure 7). To implement adaptive voltage positioning, a “droop” resistor must be connected between the output inductor and output capacitors and load. This is normally imple- mented by a PC board trace of the following value: RDROOP = Adaptive voltage positioning can be disabled for improved DC regulation by connecting the VFB pin directly to the load using a separate, non-load current carrying circuit trace. 80mV IMAX 1 switching frequency Period × (1 - duty cycle) 4848.5 VOUT + (ILOAD × RDSON OF SYNCH FET) VIN + (ILOAD × RDSON OF SYNCH FET) - (ILOAD × RDSON OF SWITCH FET) Trace 3 = VGATE(H) (10V/div.) Math 1= VGATE(H) - 5VIN Trace 4 = VGATE(L) (10V/div.) Trace 2 = Inductor Switching Node (5V/div.) |
Аналогичный номер детали - CS5150H |
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Аналогичное описание - CS5150H |
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