AN4402 datasheet(7/39 Pages) STMICROELECTRONICS

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AN4402

Active clamp of the L99MC6 and L9733

2.1

Low-side configured outputs

Right after the turn-off of a low-side switch, the inductor current decreases, and VL goes

negative with the sign convention (Figure 3). Consequently, the MOSFET drain-source

voltage VDS increases until VDS reaches VZENER_LS + VF + VGS_TH = VCL_LS. At this

moment the MOSFET is turned back on in linear mode.

The control loop consisting of a zener between the MOSFET gate and drain (Figure 3) is

called the gate-drain clamp in the rest of the document. This circuits maintains VDS (=VD,

since the source is connected to GND in this case) to VCL_LS until the energy stored in the

inductor is completely dissipated (waveform on Figure 2). VCL_LS has been chosen below

the MOSFET breakdown in order to avoid the avalanche conditions. This protection reduces

the stress applied to the MOSFET.

Figure 3. Low-side of a L99MC6 during the turn-off phase

2.2

High-side configured outputs

For a high-side configured output, either the gate-drain clamp or the gate-source clamp is

activated (Figure 7 and Figure 8), depending on the conditions.

We can distinguish two cases:

•

Case 1: VBAT < VCL_LS - VCL_HS

•

Case 2: VBAT > VCL_LS - VCL_HS

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