AN4402 datasheet(6/39 Pages) STMICROELECTRONICS

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Active clamp of the L99MC6 and L9733

AN4402

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Active clamp of the L99MC6 and L9733

By nature, an inductive load such as a relay coil develops a voltage across its terminals in

order to resist the current variations passing through it. According to Lenz's law, the voltage

across the inductor is proportional to its inductance and to the rate of change of the current:

VL = L diL / dt. This voltage can reach very high values when the current is abruptly turned

off.

Opening mechanically a circuit with an energized inductor without any protection will result

in a high voltage surge across the load of possibly several hundreds of volts, which can

cause an electrical arc.

During the turn-off phase of a MOSFET driving an inductive load the voltage across the

drain and the source of the MOSFET (VDS) will increase until the MOSFET breakdown

voltage is reached. It is the so-called avalanche effect. Operating in avalanche can have a

negative impact on the lifetime of standard MOSFETs.

The output stages of the L99MC6 and of the L9733 consist of protected n-channel

MOSFETs which can be driven either in low-side or high-side configuration. In order to avoid

avalanche conditions during the switch-off of an inductive load, the L99MC6 and the L9733

integrate a so-called active clamp, which limits VDS below the MOSFETs' breakdown

(Figure 2). Indeed, during the turn-off of inductive loads, the output MOSFET is driven in

linear (and high dissipation) mode. This results in a higher control capability of this type of

loads without additional devices.

Figure 2. Low-side and high-side configured output driving inductive loads

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