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IR21571 Datasheet(PDF) 8 Page - International Rectifier |
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IR21571 Datasheet(HTML) 8 Page - International Rectifier |
8 / 17 page IR21571(S) 8 www.irf.com Supply Bypassing and PC Board Layout Rules Component selection and placement on the pc board is extremely important when using power control ICs. VCC should be bypassed to COM as close to the IC terminals as possible with a low ESR/ESL capacitor, as shown in Figure 1 below. A rule of thumb for the value of this bypass capacitor is to keep its minimum value at least 2500 times the value of the total input capacitance (Ciss) of the power transistors being driven. This decoupling capacitor can be split between a higher valued electrolytic type and a lower valued ceramic type connected in parallel, although a good quality electrolytic (e.g., 10 µF) placed immediately adjacent to the VCC and COM terminals will work well. In a typical application circuit, the supply voltage to the IC is normally derived by means of a high value startup resistor (1/4W) from the rectified line voltage, in combination with a charge pump from the output of the half-bridge. With this type of supply arrangement, the internal 15.6V zener clamp diode from VCC to COM will determine the steady state IC supply voltage. Connecting the IC Ground (COM) to the Power Ground Both the low power control circuitry and low side gate driver output stage grounds return to this lead within the IC. The COM lead should be connected to the bottom terminal of the current sense resistor in the source of the low side power MOSFET using an individual pc board trace, as shown in Figure 2. In addition, the ground return path of the timing components and VCC decoupling capacitor should be connected directly to the IC COM lead, and not via separate traces or jumpers to other ground traces on the board. This connection technique prevents high current ground loops from interfering with the sensitive timing component operation, and allows the entire control circuit to reject common-mode noise due to output switching. Description of Operation & Component Selection Tips Figure 1: Supply bypassing PCB layout example Figure 2: COM lead connection PCB layout example C VCC (surface mount) D Boot (surface mount) C BOOT (surface mount) C VCC (through hole) pin 1 IR21571 C VCC (surface mount) C VCC (through hole) IR21571 pin 1 timing components V BUS return R CS (through hole) |
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