LM5145 datasheet(29/62 Pages) TI

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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 RDS(on) × QG

is commonly specified as a MOSFET figure-of-merit. Low thermal resistance ensures that the MOSFET power

dissipation does not result in excessive MOSFET die temperature.

The main parameters affecting power MOSFET selection in a LM5145-Q1 application are as follows:

• RDS(on) at VGS = 7.5 V

• Drain-source voltage rating, BVDSS, typically 60 V, 80 V, or 100 V, depending on maximum input voltage

• Gate charge parameters at VGS = 7.5 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 5 V, the 7.5-V gate drive

amplitude of the LM5145-Q1 provides an adequately-enhanced MOSFET when on and a margin against

Cdv/dt shoot-through when off.

The MOSFET-related power losses are summarized by the equations presented in Table 9-1, 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 SW node

ringing, are not included. Consult the LM5145-Q1 Quickstart Calculator to assist with power loss calculations.

Table 9-1. Buck Regulator 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 gate driver resistance of the LM5145-Q1.

(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, ensure that the MOSFET RDS(on) is rated at VGS = 4.5 V.

(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 Coss2 of the low-side

MOSFET. Eoss1, the energy of Coss1, is dissipated at turnon, 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

the losses due to conduction, switching (voltage-current overlap), output charge, and 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 deadtimes. The LM5145-Q1,

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