LM2745: Max sink/source current

Ambroise Suter 

The LM2745 uses resistive MOSFET based output stages for the drivers.  The resistances are provided in the datasheet: 

The active drive current during the critical MOSFET transition will depend on the MOSFET plateau voltage subtracted from the gate drive voltage, while the critical transition timing will depend on the MOSFET Gate-Drain charge, not the MOSFET total gate charge, since a significant portion of the total gate charge occurs pre and post plateau, before the switching node transition and little current is flowing through the MOSFET or after the switch node has transitioned, when the drop across the FET is low.

For a typical "logic level" MOSFET designed for 5V gate drive, the plateau voltage will be 1.5-2V, and the LM2745 will have about 1.1-1.5A of pull-up drive strength and 2-2.5A of pull-down strength.

If you need assistance identifying the MOSFET's gate transition charge, if you can provide the part-number, or link the datasheet, I can help you.

Ambroise,

During the critical transition time of the MOSFET while it is carrying a high current and simultaneously bridging a large voltage, the gate drive current is approximately:

During MOSFET turn-on: (Vgatedrive - Vplateau) / (Rdrive_pullup + Rext + Rgate)

During High-side MOSFET turn-off: (VIn/2 + Vplateau) / [(Rdrive_pullddown + Rext + Rgate)] (see below)

During Low-side MOSFET turn-off: (Vplateau) / (Rdrive_pullddown + Rext + Rgate)

Rext is the external resistance in series with the gate drive, including resistance in series with the BOOT pin.

Since the high-side driver sinks directly to GND during the high-side Turn-off, the initial turn-off current while the MOSFET transitions can be extremely high as the source of the FET is still very close to VIN, but as the gate discharges, the Gate to Ground voltage drops.  We can approximate the average as VIN/2 + Vplateau

You will also note that the drivers are not symmetric, with about 2.7Ω pull-up and 0.8Ω pull downs.  There are a number of reasons for this:

1) N-channel MOSFETs used in the pull-downs have higher mobility, and thus lower Rdson per unit area than the P-Channel Pull-ups.

2) Vplateau tends to be lower than Vgatedrive/2, so there is generally less drive voltage available during turn-off than turn-on.

3) During the turn-on of the complementary FET, when the switching node swings rapidly, the Gate-Drain capacitance of the MOSFET will drive current into the gate of the MOSFET which must be discharged by the pull-down to prevent unintended dV/dt turn-on of the MOSFET and possible cross-conduction.  To help reduce the risk, it is generally advisable to select MOSFETs with Gate-Drain capacitance lower than their Gate-Source capacitance.

Hi Peter,

Thank you for the detailed response! 

Best regards,

Ambroise