LM5145 datasheet(32/73 Pages) TI2

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Table 8-1. 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 LM25137.

(2)

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

rise over ambient temperature is dependent upon the device total power dissipation and the 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 turnoff. 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 dissipation is offset by the stored energy Eoss2 on

Coss2. For more detail, see also Comparison of deadtime effects on the performance of DC-DC converters with GaN FETs and silicon

MOSFETs, ECCE 2016.

(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, so make sure 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 is switched at zero voltage – current just

communicates from the channel to the body diode or vice-versa during the transition dead times. The LM25137,

with the 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, optimizing the low-side MOSFET for low RDS(on) is critical. 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 LM25137 is an excellent choice to drive TI's portfolio of power MOSFETs.

8.1.1.2 Buck Inductor

For most applications, choose a buck inductance such that the inductor ripple current, ΔIL, is between 30% and

50% of the maximum DC output current at nominal input voltage. Choose the inductance using Equation 11

based on a peak inductor current given by Equation 12.

LM25137

SNVSCU4 – OCTOBER 2025

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