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FAN5037 数据表(PDF) 7 Page - Fairchild Semiconductor |
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FAN5037 数据表(HTML) 7 Page - Fairchild Semiconductor |
7 / 11 page PRODUCT SPECIFICATION FAN5037 REV. 1.0.3 9/26/01 7 charged and discharged through the enabling and disabling of the fixed current source. The variable current source is controlled from the error inputs that are received from the current and voltage feedback signals. The oscillator off-time is controlled by the amount of current that is available from the variable current source to charge the external capacitor up to the high threshold level of the comparator. The on-time is set by the constant current source that discharges the external capacitor voltage down to the lower comparator threshold. Figure 2. FAN5037 Detailed Block Diagram IO ION Constant On-Time Oscillator gm gm VH VL VOUT +5V VCCA IFBL IFBH VCCP FBSW SDRV GNDP CEXT 65-5037-07 +12V REF 6 5 1 2 4 3 8 7 Output Voltage Selection The FAN5037 precision reference is trimmed to be 1.2V nominally. When using the FAN5037, the system designer has complete flexibility in choosing the output voltage for one regulator from 1.2V to 3.6V. This is done by appropri- ately selecting the feedback resistors. These could be 0.1% resistors to realize optimum output accuracy. The following equations determines the output voltage of the regulator: For example, for 3.3V: Input Capacitors The number of input capacitors required for the FAN5037 is dependent on their ripple current rating, which assures their rated life. The number required may be determined by where the duty cycle DC = (Vout + Vf,diode) / Vin. For exam- ple, with a 1.5V output at 10A, 5V input, and using the Sanyo capacitors specified in Table 1 which have a 2A ripple current rating, we have DC = (1.5 + .5)/5 = 0.4, and so that we need 3 input capacitors. Short Circuit Considerations The FAN5037 uses a current sensing scheme to limit the load current if an output fault condition occurs. The current sense resistor carries the peak current of the inductor, which is greater than the maximum load current due to ripple current flowing in the inductor. The FAN5037 will begin to limit the output current to the load by reducing the duty cycle of the top-side MOSFET driver when the voltage across the cur- rent-sense resistor exceeds the short circuit comparator threshold voltage (Vth). When this happens the output volt- age will temporarily go out of regulation. As the voltage across the sense resistor becomes larger, the duty cycle of the top-side MOSFET will continue to be reduced until the cur- rent limit value is reached. At this point, the FAN5037 will continuously deliver the limit current at a reduced output voltage level. The short circuit comparator threshold voltage is typically 90mV, with a tolerance of ±10mV. The ripple current flowing through the inductor in Figure 1 is 0.6Apeak. Refer to Application Note AB-23 for detailed discussions. The sense resistor value can be approximated as follows: V OUT 1.2 R4 R5 + R5 --------------------- × = (1) V OUT 1.2 R4 R5 + R5 --------------------- × 1.2 3.48k 2.0k + 2.0k ------------------------------- × 3.3V == = No. Caps I out DC DC 2 – * I rating ---------------------------------------- = (2) No. Caps 10 0.4 0.4 2 – * 2 ----------------------------------- 2.44 == R SENSE V th,min I PK ---------------- 1TF – () × V th,min 0.6A I LOAD,MAX + --------------------------------------------- 1TF – () × == (3) |
类似零件编号 - FAN5037 |
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类似说明 - FAN5037 |
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