Datasheet

LM5109B

SNVS477B –FEBRUARY 2007–REVISED MARCH 2013

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HS Transient Voltages Below Ground

The HS node will always be clamped by the body diode of the lower external FET. In some situations, board

resistances and inductances can cause the HS node to transiently swing several volts below ground. The HS

node can swing below ground provided:

1. HS must always be at a lower potential than HO. Pulling HO more than -0.3V below HS can activate

parasitic transistors resulting in excessive current flow from the HB supply, possibly resulting in damage to

the IC. The same relationship is true with LO and VSS. If necessary, a Schottky diode can be placed

externally between HO and HS or LO and GND to protect the IC from this type of transient. The diode must

be placed as close to the IC pins as possible in order to be effective.

2. HB to HS operating voltage should be 15V or less. Hence, if the HS pin transient voltage is -5V, VDD should

be ideally limited to 10V to keep HB to HS below 15V.

3. Low ESR bypass capacitors from HB to HS and from VDD to VSS are essential for proper operation. The

capacitor should be located at the leads of the IC to minimize series inductance. The peak currents from LO

and HO can be quite large. Any series inductances with the bypass capacitor will cause voltage ringing at the

leads of the IC which must be avoided for reliable operation.

Product Folder Links: LM5109B