7.3.3.2 Gate Charge and Gate Timing

Power MOSFETs typically specify the total gate charge required to energize and de-energize parameter. The total gate charge informs the gate-to-source rise times and fall times. Make sure the maximum gate-to-source rise times and fall times maximum are approximately between 20 ns and 30 ns. Because the typical high-side driver pull-up resistance is approximately 5 Ω, use a maximum gate capacitance between 4 nF and 6 nF . Design a maximum total gate charge between 20 nC and 30 nC.

Internal non-overlap timing prevents both the high-side and low-side MOSFET of the illumination buck converter from energizing simultaneously. Typical non-overlap timing of approximately 35 ns usually gives sufficient margins. The DLPA4000 device measures the gate-to-source voltage of the external MOSFETs to determine whether a MOSFET energized or not. This measurement is done at the pins of the DLPA4000. For the low-side MOSFET this measurement is done between ILLUM_LSIDE_DRIVE and ILLUM_A_GND. Similarly, for the high-side MOSFET the device measures the gate-to-source voltage between the ILLUM_HSIDE_DRIVE pin and the ILLUM_A_SW pin. Because of the location of these measurement nodes, do not insert any additional drivers or circuitry between the DLPA4000 and the external power MOSFETs of the buck converter. Delays can lead to incorrect on-off detection of the FETs and cause shoot-through currents if the user inserts additional circuitry. Shoot-through currents reduce efficiency, and more seriously damage the power MOSFETs.

LED selection switches require no specific criteria regarding the gate charge or gate timing. Timing of the LED selection signals is in the microsecond range rather than nanosecond range.