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ISL6609CBZ-T Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL6609CBZ-T Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 12 page 9 FN9221.2 April 27, 2009 Application Information MOSFET and Driver Selection The parasitic inductances of the PCB and of the power devices’ packaging (both upper and lower MOSFETs) can cause serious ringing, exceeding absolute maximum rating of the devices. The negative ringing at the edges of the PHASE node could increase the bootstrap capacitor voltage through the internal bootstrap diode, and in some cases, it may overstress the upper MOSFET driver. Careful layout, proper selection of MOSFETs and packaging, as well as the proper driver can go a long way toward minimizing such unwanted stress. The selection of D2-PAK, or D-PAK packaged MOSFETs, is a much better match (for the reasons discussed) for the ISL6609A. Low-profile MOSFETs, such as Direct FETs and multi-SOURCE leads devices (SO-8, LFPAK, PowerPAK), have low parasitic lead inductances and can be driven by either ISL6609 or ISL6609A (assuming proper layout design). The ISL6609, missing the 3 Ω integrated BOOT resistor, typically yields slightly higher efficiency than the ISL6609A. Layout Considerations A good layout helps reduce the ringing on the switching node (PHASE) and significantly lower the stress applied to the output drives. The following advice is meant to lead to an optimized layout: • Keep decoupling loops (VCC-GND and BOOT-PHASE) as short as possible. • Minimize trace inductance, especially on low-impedance lines. All power traces (UGATE, PHASE, LGATE, GND, VCC) should be short and wide, as much as possible. • Minimize the inductance of the PHASE node. Ideally, the source of the upper and the drain of the lower MOSFET should be as close as thermally allowable. • Minimize the current loop of the output and input power trains. Short the source connection of the lower MOSFET to ground as close to the transistor pin as feasible. Input capacitors (especially ceramic decoupling) should be placed as close to the drain of upper and source of lower MOSFETs as possible. In addition, connecting the thermal pad of the QFN package to the power ground through a via, or placing a low noise copper plane underneath the SOIC part is recommended for high switching frequency, high current applications. This is to improve heat dissipation and allow the part to achieve its full thermal potential. Upper MOSFET Self Turn-On Effects at Startup Should the driver have insufficient bias voltage applied, its outputs are floating. If the input bus is energized at a high dV/dt rate while the driver outputs are floating, because of self-coupling via the internal CGD of the MOSFET, the UGATE could momentarily rise up to a level greater than the threshold voltage of the MOSFET. This could potentially turn on the upper switch and result in damaging inrush energy. Therefore, if such a situation (when input bus powered up before the bias of the controller and driver is ready) could conceivably be encountered, it is a common practice to place a resistor (RUGPH) across the gate and source of the upper MOSFET to suppress the Miller coupling effect. The value of the resistor depends mainly on the input voltage’s rate of rise, the CGD/CGS ratio, as well as the gate-source threshold of the upper MOSFET. A higher dV/dt, a lower CDS/CGS ratio, and a lower gate-source threshold upper FET will require a smaller resistor to diminish the effect of the internal capacitive coupling. For most applications, a 5k to 10k Ω resistor is typically sufficient, not affecting normal performance and efficiency. The coupling effect can be roughly estimated with the following equations, which assume a fixed linear input ramp and neglect the clamping effect of the body diode of the upper drive and the bootstrap capacitor. Other parasitic components such as lead inductances and PCB capacitances are also not taken into account. These equations are provided for guidance purpose only. FIGURE 3. TYPICAL UPPER-GATE DRIVE TURN-ON PATH FIGURE 4. TYPICAL LOWER-GATE DRIVE TURN-ON PATH Q1 D S G RGI1 RG1 BOOT RHI1 CDS CGS CGD RLO1 PHASE VCC UGATE VCC Q2 D S G RGI2 RG2 RHI2 CDS CGS CGD RLO2 GND LGATE ISL6609, ISL6609A |
Similar Part No. - ISL6609CBZ-T |
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Similar Description - ISL6609CBZ-T |
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