DocID027090 Rev 1

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PM8851

Application guidelines

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6

Application guidelines

6.1

Power supply

The PM8851 driver is intended to drive power MOSFETs used in power conversion

topologies at high speed. The accurate supply voltage definition guarantees an effective

driving in every condition. The voltage present at the IN_TH pin is used for the threshold

definition. It could be the same voltage used to supply the device providing the signal

applied to the IN pin, or it can be derived by the VCC pin, eventually using a voltage divider.

It is mainly suggested to provide IN_TH voltage starting from VCC voltage.

For example, an auxiliary, unregulated voltage can be used to be connected to both

PM8851 VCC pin and the input of a linear regulator that provides a well regulated supply

voltage for logic circuitry. The same low voltage is then provided to the IN_TH pin of the

PM8851.

If the IN_TH is derived directly by the VCC pin, the structure illustrated in Figure 12 can be

used.

It is mandatory to properly connect a 100 nF ceramic cap as close as possible to the VCC

pin to bypass the current's spikes absorbed by VCC during the gate charging.

Also IN_TH voltage should be filtered with a ceramic capacitor (10 nF to 100 nF), especially

when long traces are used to supply it; when derived by VCC a lighter filtering is allowed.

6.2

Layout suggestions

The small package of the PM8851 allows to place it very close to the gate of the driven

MOSFET: this reduces the risk of injecting high frequency noise produced by the driving

current running between the OUT pin and the MOSFET's gate pin.

Figure 11. Shared supply configuration

Figure 12. Independent supply configuration