LM5145 datasheet(37/69 Pages) TI

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MOSFET body diode reverse recovery charge, QRR, depends on many parameters, particularly forward current, current transition

speed, and temperature.

The high-side (control) MOSFET carries the inductor current during the PWM on time (or D interval) and

typically incurs most of the switching losses, so it is imperative to choose a high-side MOSFET that balances

conduction and switching loss contributions. The total power dissipation in the high-side MOSFET is the sum of

the following:

• Losses due to conduction

• Switching (voltage-current overlap)

• Output charge

• Typically two-thirds of the net loss attributed to body diode reverse recovery

The low-side (synchronous) MOSFET carries the inductor current when the high-side MOSFET is off (or 1–D

interval). The low-side MOSFET switching loss is negligible as it is switched at zero voltage – current just

commutates from the channel to the body diode or vice versa during the transition dead times. The LM25143,

with its adaptive gate drive timing, minimizes body diode conduction losses when both MOSFETs are off. Such

losses scale directly with switching frequency.

In high step-down ratio applications, the low-side MOSFET carries the current for a large portion of the switching

period. Therefore, to attain high efficiency, it is critical to optimize the low-side MOSFET for low RDS(on). In cases

where the conduction loss is too high or the target RDS(on) is lower than available in a single MOSFET, connect

two low-side MOSFETs in parallel. The total power dissipation of the low-side MOSFET is the sum of the losses

due to channel conduction, body diode conduction, and typically one-third of the net loss attributed to body diode

reverse recovery. The LM25143 is well suited to drive TI's portfolio of NexFET™ power MOSFETs.

10.1.1.5 EMI Filter

As expressed by Equation 22, switching regulators exhibit negative input impedance, which is lowest at the

minimum input voltage.

IN(min)

(22)

An underdamped LC filter exhibits a high output impedance at the resonant frequency of the filter. For stability,

the filter output impedance must be less than the absolute value of the converter input impedance.

GND

LIN

CIN

OUT

GND

Figure 10-2. Buck Regulator With π-Stage EMI Filter

Referencing the filter schematic in Figure 10-2, the EMI filter design steps are as follows:

• Calculate the required attenuation of the EMI filter at the switching frequency, where CIN represents the

existing capacitance at the input of the switching converter.

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LM5145 Datasheet(PDF) 37 Page - Texas Instruments

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