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CS5155HGDR16 Datasheet(PDF) 11 Page - Cherry Semiconductor Corporation |
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CS5155HGDR16 Datasheet(HTML) 11 Page - Cherry Semiconductor Corporation |
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11 / 14 page  Applications Information: continued 11 Figure 16: CS5155H demonstration board during power up. Power Good signal is activated when output voltage reaches 1.70V. Selecting External Components The CS5155H 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 utilize logic level MOSFETs. A charge pump may be easily implemented to support 5V or 12V only systems (maximum of 20V). Multiple MOSFETs may be paralleled to reduce losses and improve efficiency and thermal management. Voltage applied to the MOSFET gates depends on the application circuit used. Both upper and lower gate driver outputs are specified to drive to within 1.5V of ground when in the low state and to within 2V of their respective bias supplies when in the high state. In practice, the MOS- FET gates will be driven rail to rail due to overshoot caused by the capacitive load they present 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 fol- lowing gate drive is provided; VGATE(H) = 12V - 5V = 7V, VGATE(L) = 12V (see Figure 17). Figure 17: CS5155H 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 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.) 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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