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CS5157GD16 Datenblatt(PDF) 11 Page - Cherry Semiconductor Corporation |
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CS5157GD16 Datenblatt(HTML) 11 Page - Cherry Semiconductor Corporation |
11 / 14 page Applications Information: continued 11 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: CS5157 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 CS5157 reference circuit does not use this device due to it’s 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 CS5157 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. Input and Output Capacitors These components must be selected and placed carefully to yield optimal results. Capacitors should be chosen to pro- vide acceptable ripple on the input supply lines and regula- tor output voltage. Key specifications for input capacitors are their ripple rating, while ESR is important for output capacitors. For best transient response, a combination of low value/high frequency and bulk capacitors placed close to the load will be required. Output Inductor The inductor should be selected based on its inductance, current capability, and DC resistance. Increasing the induc- tor value will decrease output voltage ripple, but degrade transient response. Thermal Considerations for Power MOSFETs and Diodes In order to maintain good reliability, the junction tempera- ture of the semiconductor components should be kept to a maximum of 150°C or lower. The thermal impedance (junc- tion to ambient) required to meet this requirement can be calculated as follows: Thermal Impedance = TJUNCTION(MAX) - TAMBIENT Power Thermal Management 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.) |
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Ähnliche Beschreibung - CS5157GD16 |
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