LM5145 datasheet(31/74 Pages) TI2

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8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

The LM5137 is a dual-channel or dual-phase, synchronous, buck controller used to convert a higher input

voltage to one or two lower output voltages. The following sections consider the power-train and compensation

components and provide specific circuit design examples for single- and dual-output implementations. To

expedite and streamline the process of designing of an LM5137-based regulator, a comprehensive LM5137

Quickstart Calculator is available for download to assist the designer with component selection for a given

application.

8.1.1 Power Train Components

A comprehensive understanding of the buck regulator power train components is critical to successfully

completing an efficient and reliable synchronous buck regulator design. The following sections discuss the

following:

• Power MOSFETs

• Buck inductor

• Input and output capacitors

• EMI input filter

8.1.1.1 Power MOSFETs

The choice of power MOSFETs has an outsized impact on DC/DC regulator performance. A MOSFET with

low on-state resistance, RDS(on), reduces conduction loss, whereas low parasitic capacitances enable faster

transition times and reduced switching loss. Normally, the lower the RDS(on) of a MOSFET, the higher the gate

charge and output charge (QG and QOSS, respectively), and vice-versa. As a result, the product of RDS(on) and

QG is commonly specified as a MOSFET figure-of-merit. Low thermal resistance of a given package makes sure

that the MOSFET power dissipation does not result in excessive MOSFET die temperature.

The main parameters affecting power MOSFET selection in an LM5137 application are as follows:

• RDS(on) at VGS = 5V

• Drain-source voltage rating, BVDSS, is typically 40V, 60V or 80V, depending on the maximum input voltage.

• Gate charge parameters at VGS = 5V

• Output charge, QOSS, at the relevant input voltage

• Body diode reverse recovery charge, QRR

• Gate threshold voltage, VGS(th), derived from the Miller plateau evident in the QG versus VGS plot in the

MOSFET data sheet. With a Miller plateau voltage typically in the range of 2.5V to 3.2V, the 5V gate drive

amplitude of the LM5137 provides an adequately-enhanced MOSFET when on and a margin against Cdv/dt

shoot-through when off.

The MOSFET-related power losses for one channel are summarized by the equations presented in Table 8-1,

where suffixes 1 and 2 represent high-side and low-side MOSFET parameters, respectively. While the influence

of inductor ripple current is considered, second-order loss modes, such as those related to parasitic inductances

and switch-node voltage ringing, are not included. A comprehensive Quickstart Calculator available from the

LM5137 product folder provides power loss calculations based on the entered MOSFET parameters, including

RDS(on) and QG.

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LM5137

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