LM5145 datasheet(30/70 Pages) TI

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9.1.1.4 Power MOSFETs

The choice of power MOSFETs has significant 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 ensures that

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

The main parameters affecting power MOSFET selection in a LM5148 application are as follows:

• RDS(on) at VGS = 5 V

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

• Gate charge parameters at VGS = 5 V

• 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 V to 3 V, the 5-V gate drive

amplitude of the LM5148 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 9-1,

where suffixes one and two 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 SW node ringing, are not included. Consult the LM5148 Quickstart Calculator to assist with

power loss calculations.

Table 9-1. MOSFET Power Losses

POWER LOSS MODE

HIGH-SIDE MOSFET

LOW-SIDE MOSFET

MOSFET conduction(2)

(3)

cond1

OUT

DS(on)1

cond2

OUT

DS(on)2

c ¨

MOSFET switching

sw1

OUT

Negligible

MOSFET gate drive(1)

Gate1

Gate2

MOSFET output

charge(4)

Coss

oss2

oss1

oss2

Body diode

conduction

N/A

cond

OUT

dt1

OUT

dt2

Body diode

reverse recovery(5)

RR2

(1)

Gate drive loss is apportioned based on the internal gate resistance of the MOSFET, externally added series gate resistance and the

relevant driver resistance of the LM5148.

(2)

MOSFET RDS(on) has a positive temperature coefficient of approximately 4500 ppm/°C. The MOSFET junction temperature, TJ, and its

rise over ambient temperature is dependent upon the device total power dissipation and its thermal impedance. When operating at or

near minimum input voltage, make sure that the MOSFET RDS(on) is rated for the available gate drive voltage.

(3)

D' = 1–D is the duty cycle complement.

(4)

MOSFET output capacitances, Coss1 and Coss2, are highly non-linear with voltage. These capacitances are charged losslessly by

the inductor current at high-side MOSFET turn-off. During turn-on, however, a current flows from the input to charge the output

capacitance of the low-side MOSFET. Eoss1, the energy of Coss1, is dissipated at turn-on, but this is offset by the stored energy Eoss2 on

Coss2.

(5)

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. It is, therefore, 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

LM5148

SNVSC01 – FEBRUARY 2023

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