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FAN5099 데이터시트(PDF) 10 Page - Fairchild Semiconductor |
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FAN5099 데이터시트(HTML) 10 Page - Fairchild Semiconductor |
10 / 24 page © 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5099 Rev. 1.1.3 10 Block Diagram Figure 21. Block Diagram Detailed Operation Description FAN5099 combines a high-efficiency, fixed-frequency PWM controller designed for single-phase synchronous buck Point-Of-Load converters with an integrated LDO controller to support GTL-type loads. This controller is ideally suited to deliver low-voltage, high-current power supplies needed in desktop computers, notebooks, workstations, and servers. The controller comes with an integrated boot diode which helps reduce component cost and increase space savings. With this controller, the input to the power supply can be varied from 3V to 24V and the output voltage can be set to regulate at 0.8V to 15V on the switcher output. The LDO output can be con- figured to regulate between 0.8V to 3V and the input to the LDO can be from 1.5V to 5V, respectively. An internal shunt regulator at the VCC pin facilitates the controller operation from either a 5V or 12V power source. VCC Bias Supply FAN5099 can be configured to operate from 5V or 12V for VCC. When 5V supply is used for VCC, no resistor is required to be connected between the supply and the VCC. When the 12V supply is used, a resistor RVCC is connected between the 12V supply and the VCC, as shown in Figure 1. The internal shunt regulator at the VCC pin is capable of sinking 150mA of current to ensure the controller’s internal VCC is maintained at 5.6V maximum. Choose a resistor such that: ■ It is rated to handle the power dissipation. ■ Current sunk within the controller is minimized to prevent IC temperature rise. RVCC Selection (IC) The selection of RVCC is dependent on: ■ Variation of the 12V supply ■ Sum of gate charges of top and bottom FETs (QFET) ■ Switching frequency (FSW) ■ Shunt regulator minimum current (1mA) ■ Quiescent Current of the IC (IQ) Calculate RVCC based on the minimum input voltage for the VCC: For a typical example, where: VINMIN = 11.5V, IQ = 3mA, QFET = 30nC, FSW = 300kHz, RVCC is calculated to be 398.65Ω. PWM Section The FAN5099’s PWM controller combines the conven- tional voltage mode control and current sensing through lower MOSFET RDS_ON to generate the PWM signals. This method of current sensing is loss-less and cost effective. For more accurate current sense requirements, an optional external resistor can be connected with the bottom MOSFET in series. Current Sense Amplifier FB COMP SS Vref Vcc ILIM R ILIM 10μA 10μA Current Limit Comparator Internal Boot Diode Internal Vcc 5.6V Max. OSC R S Q PW M EN R(RAMP) Ramp Generator Enable Vcc C O L O BOOT Vout R RAMP Error Amplifier PWM Comparator C BOOT V IN V IN Shunt Reg HDRV SW LDRV PGND Summing Amplifier Adaptive Gate Drive Circuit RVCC VINMIN 5.6 – IQ 110 • 3 – QFET FSW 1.2 • • ++ () ------------------------------------------------------------------------------------------ = |
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