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FAN5230QSC 데이터시트(PDF) 9 Page - Fairchild Semiconductor |
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FAN5230QSC 데이터시트(HTML) 9 Page - Fairchild Semiconductor |
9 / 16 page FAN5230 REV. 2.8.5 10/17/01 9 Functional Description The FAN5230 is a high efficiency and high precision DC/DC controller for notebook and other portable applications. It provides all of the voltages necessary for system electronics: 5V, 3.3V, 12V, and both 3.3V-ALWAYS and 5V-ALWAYS. Utilization of both input and output voltage feedback in a current-mode control allows for fast loop response over a wide range of input and output variations. Current sense based on MOSFET RDS,on gives maximum efficiency, while also permitting the use of a sense resistor for high accuracy. 3.3V and 5V Architecture The 3.3V and 5V switching regulator outputs of the FAN5230 are generated from the unregulated input voltage using synchronous buck converters. Both high side and low- side MOSFETs are N-channel. The 3.3V and 5V switchers have pins for current sensing and for setting of output over-current threshold using MOSFET RDS,on. Each converter has a pin for voltage-sense feedback, a pin that shuts down the converter, and a pin for generating the boost voltage to drive the high-side MOSFET. If the 5V switcher is not used, connect SDN5 (pin 17) to SGND (pin 14). If the 3.3V switcher is not used, connect SDN3.3 (pin 11) to SGND (pin 14). The following discussion of the FAN5230 design will be done with reference to Figures 1 through 4, showing the internal block diagram of the IC. 3.3V and 5V PWM Current Sensing Peak current sensing is done on the low side driver because of the very low duty-cycle on the high side MOSFET. The current is sampled 50ns after turn on and the value is held for current feedback and over-current limit. 3.3V and 5V PWM Loop Compensation The 3.3V and 5V control loops of the FAN5230 function as voltage mode with current feedback for stability. They each have an independent voltage feedback pin, as shown in Fig- ure 1. They use voltage feed-forward to guarantee loop rejec- tion of input voltage variation: that is to say that the PWM (pulse width modulation) ramp amplitude is varied as a func- tion of the input voltage. Compensation of the control loops is done entirely internally using current-mode feedback com- pensation. This scheme allows the bandwidth and phase mar- gin to be almost independent of output capacitance and ESR. 3.3V and 5V PWM Current Limit The 3.3V and 5V converters each sense the voltage across their own low-side MOSFET to determine whether to enter current limit. If an output current in excess of the current limit threshold is measured then the converter enters a pulse skipping mode where Iout is equal to the over-current (OC) set limit. After 8 clock cycles then the regulator is latched off (HSD and LSD off). This is the likely scenario in the case of a "soft" short. If the short is "hard" it will instantly trigger the under-voltage protection which again will latch the regulator off (HSD and LSD off) after a 2µs delay. Selection of a current-limit set resistor must include the tolerance of the current-limit trip point, the MOSFET on resistance and temperature coefficient, and the ripple current, in addition to the maximum output current. Example: Maximum DC output current on the 5V is 5A, the MOSFET RDS,on is 17mΩ, and the inductor is 5µH at a current of 5A. Because of the low RDS,on, the low-side MOS- FET will have a maximum temperature (ambient + self-heat- ing) of only 75°C, at which its RDS,on increases to 20mΩ. Peak current is DC output current plus peak ripple current: where T is the maximum period, VO is output voltage, and L is the inductance. This current generates a voltage on the low-side MOSFET of 7A • 20m Ω = 140mV. The current limit threshold is typically 150mV (worst-case 135mV) with R2 = 1K Ω, and so this value is suitable. R2 could be increased a further 10% if additional noise margin is deemed necessary. Precision Current Limit Precision current limiting can be achieved by placing a discrete sense resistor between the source of the low-side MOSFET and ground. In this case, current limit accuracy is set by the tolerance of the IC, +10%. Figure 4. Using a Precision Current Sense Resistor Shutdown (SDWN) The SDWN pin turns off all 5 converters (+5V, +3.3V, and +12V, 5V/3.3V-ALWAYS) and puts the FAN5230 into a low- power mode (Shutdown mode). Ipk Idc + TV0 2L = 5A + 4µsec • 5V 2 • 5µH = 7A ≈ HSD SW LSD ISEN GND |
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