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CS5151GN16 Datasheet(PDF) 10 Page - Cherry Semiconductor Corporation |
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CS5151GN16 Datasheet(HTML) 10 Page - Cherry Semiconductor Corporation |
10 / 14 page Applications Information: continued 10 Figure 14: CS5151 demonstration board during power up. Power Good signal is activated when output voltage reaches 1.70V. Selecting External Components The CS5151 can be used with a wide range of external power components to optimize the cost and performance of a particular design. The following information can be used as general guidelines to assist in their selection. NFET Power Transistors Both logic level and standard MOSFETs can be used. The reference designs derive gate drive from the 12V supply which is generally available in most computer systems and use logic level MOSFETs. A charge pump may be easily implemented to support 5V only systems. Multiple MOSFETs may be paralleled to reduce losses and improve efficiency and thermal management. Voltage applied to the MOSFET gate depends on the appli- cation circuit used. The gate driver output is specified to drive to within 1.5V of ground when in the low state and to within 2V of its bias supply when in the high state. In prac- tice, the MOSFET gate will be driven rail to rail due to overshoot caused by the capacitive load it presents to the controller IC. For the typical application where VCC1 = VCC2 = 12V and 5V is used as the source for the regulator output current, the following gate drive is provided; VGATE = 12V - 5V = 7V (see Figure 15). Figure 15: CS5151 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 MOSFET and the Schottky diode may be estimated as follows; Switching MOSFET: Power = ILOAD2 × RDSON × duty cycle Schottky diode: Power = VFORWARD × ILOAD × (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 VOUT + VFORWARD VIN + VFORWARD - (ILOAD × RDSON OF SWITCH FET) Channel 3 = VGATE M1= VGATE - 5VIN Channel 2 = Inductor Switching Node Trace 3 = 12V Input (VCC1) and VCC2) (10V/div.) Trace 4 = 5V Input (2V/div.) Trace 1 = Regulator Output Voltage (1V/div.) Trace 2 = Power Good Signal (2V/div.) |
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