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ISL6251AHAZ-T Datasheet(PDF) 18 Page - Intersil Corporation |
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ISL6251AHAZ-T Datasheet(HTML) 18 Page - Intersil Corporation |
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18 / 20 page  18 FN9202.1 June 17, 2005 stay on the signal layer, which is isolated by the signal ground to the power signal traces. Component Placement The power MOSFET should be close to the IC so that the gate drive signal, the LGATE, UGATE, PHASE, and BOOT, traces can be short. Place the components in such a way that the area under the IC has less noise traces with high dv/dt and di/dt, such as gate signals and phase node signals. Signal Ground and Power Ground Connection. At minimum, a reasonably large area of copper, which will shield other noise couplings through the IC, should be used as signal ground beneath the IC. The best tie-point between the signal ground and the power ground is at the negative side of the output capacitor on each side, where there is little noise; a noisy trace beneath the IC is not recommended. GND and VDD Pin At least one high quality ceramic decoupling cap should be used to cross these two pins. The decoupling cap can be put close to the IC. LGATE Pin This is the gate drive signal for the bottom MOSFET of the buck converter. The signal going through this trace has both high dv/dt and high di/dt, and the peak charging and discharging current is very high. These two traces should be short, wide, and away from other traces. There should be no other traces in parallel with these traces on any layer. PGND Pin PGND pin should be laid out to the negative side of the relevant output cap with separate traces. The negative side of the output capacitor must be close to the source node of the bottom MOSFET. This trace is the return path of LGATE. PHASE Pin This trace should be short, and positioned away from other weak signal traces. This node has a very high dv/dt with a voltage swing from the input voltage to ground. No trace should be in parallel with it. This trace is also the return path for UGATE. Connect this pin to the high-side MOSFET source. UGATE Pin This pin has a square shape waveform with high dv/dt. It provides the gate drive current to charge and discharge the top MOSFET with high di/dt. This trace should be wide, short, and away from other traces similar to the LGATE. BOOT Pin This pin’s di/dt is as high as the UGATE; therefore, this trace should be as short as possible. CSOP, CSON Pins The current sense resistor connects to the CSON and the CSOP pins through a low pass filter. The CSON pin is also used as the battery voltage feedback. The traces should be away from the high dv/dt and di/di pins like PHASE, BOOT pins. In general, the current sense resistor should be close to the IC. Other layout arrangements should be adjusted accordingly. EN Pin This pin stays high at enable mode and low at idle mode and is relatively robust. Enable signals should refer to the signal ground. DCIN Pin This pin connects to AC adapter output voltage, and should be less noise sensitive. Copper Size for the Phase Node The capacitance of PHASE should be kept very low to minimize ringing. It would be best to limit the size of the PHASE node copper in strict accordance with the current and thermal management of the application. Identify the Power and Signal Ground The input and output capacitors of the converters, the source terminal of the bottom switching MOSFET PGND should connect to the power ground. The other components should connect to signal ground. Signal and power ground are tied together at one point. Clamping Capacitor for Switching MOSFET It is recommended that ceramic caps be used closely connected to the drain of the high-side MOSFET, and the source of the low-side MOSFET. This capacitor reduces the noise and the power loss of the MOSFET. ISL6251, ISL6251A |
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