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ILC6377SOADJ 데이터시트(PDF) 7 Page - Fairchild Semiconductor |
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ILC6377SOADJ 데이터시트(HTML) 7 Page - Fairchild Semiconductor |
7 / 12 page ILC6376/77 7 ©2001 Fairchild Semiconductor Corporation 1kHz < < 20kHz The EXT1 and EXT2 pins are provided so as to drive exter- nal transistors; thus allowing design flexibility. The EXT1 output drive signal has the same timing as the gate drive to the internal P-channel MOSFET i.e. EXT1 output is low as long as the internal MOSFET is on. Both EXT1 and EXT2 pins are capable of driving 1000pF gate capacitance. For example, a high output current application circuit using an external P-channel MOSFET is shown in figure 2. P-Channel Boost Circuit The ILC6376/77 includes a unique P-Channel MOSFET architecture with built-in charge pump to maintain low on- resistance even at low input voltages. As shown in figure 3, a 2200pF ceramic capacitor and a schottky diode (MBR0520L or equivalent) allows the gate voltage of the internal P-Chan- nel MOSFET to be driven negative; thus reducing the switch on-resistance. This technique can be employed to increase efficiency at low input voltages and high output currents. Note that the voltage between VIN and P_BST should not exceed 10V, otherwise damage to the device may occur. For high input voltage applications the schottky diode should be replaced by a low voltage zener diode so that the P_BST pin is clamped to a safe negative voltage. Adjustable Output (ILC6376/77SOADJ) For adjustable output voltage ILC6376/77SOADJ should be used. All connections to the ILC6376/77SOADJ are the same as ILC6376/77SOXX, except for the feedback voltage divider network shown in figure 4. The output voltage, VOUT, can be calculated from the following equation: VOUT = VFB (1 + RFB1/RFB2), where VFB is approximately 1V and RFB1 + RFB2 < 2M Ω The feedback compensation capacitor should be chosen such that the pole frequency f is between 1kHz and 20kHz: The pole frequency should generally be set at 5kHz. The value of CFB calculated from the above equation may require some adjustment depending on the output inductor (L) and output capacitor (CL) values chosen. Example for 3V output: RFB1 = 400k Ω RFB2 = 200k Ω CFB = 100pF PC Board Layout As with all switching DC-DC converter designs, good PC board layout is critical for optimum performance. The heavy lines indicated in figure 1 schematic should be wide printed circuit board traces and should be kept as short as is practical. A large ground plane with as much copper area as is allowable should be used. All external components should be mounted as close to the IC as possible. For ILC6376/77SOADJ, the feedback resistors and their associ- ated wiring should be kept away from the inductor location and the vicinity of inductive flux. VOUT ILC6376/77 (TOP VIEW) 3 4 5 1 2 6 7 8 + CL + VIN S/D CSS SD1 L *CIN EXT1 Fig.2 1Amp output current application using external MOSFET VOUT 5 6 CFB RFB1 RFB2 RFB1 + RFB2 < 2MW CFB chosen so that To pin 8 (Note: rest of circuit is same as Fig.1) 2 x π x CFB x RFB1 1kHz < 1 < 20kHz Fig.4 Adjustable output using ILC6376/77SOADJ Figure 3. P-Channel Negative CBST Voltage between Vin and P_BST must be less than 10V. ILC6376/77 (TOP VIEW) 3 4 5 1 2 6 7 8 SD2 Boost Circuit 2200pF Schottky MBR0520L 2 x π x CFB x RFB1 1 |
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