LM2744 datasheet(19/33 Pages) TI

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LM2744

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SNVS292F – SEPTEMBER 2004 – REVISED MARCH 2013

MOSFETs

Selection of the power MOSFETs is governed by a tradeoff between cost, size, and efficiency. One method is to

determine the maximum cost that can be endured, and then select the most efficient device that fits that price.

Breaking down the losses in the high-side and low-side MOSFETs and then creating spreadsheets is one way to

determine relative efficiencies between different MOSFETs. Good correlation between the prediction and the

bench result is not ensured, however. Single-channel buck regulators that use a controller IC and discrete

MOSFETs tend to be most efficient for output currents of 2-10A.

Losses in the high-side MOSFET can be broken down into conduction loss, gate charging loss, and switching

loss. Conduction, or I2R loss, is approximately:

PC = D (IO

2 x R

DSON-HI x 1.3) (High-Side MOSFET)

(18)

PC = (1 - D) x (IO

2 x R

DSON-LO x 1.3) (Low-Side MOSFET)

(19)

In the above equations the factor 1.3 accounts for the increase in MOSFET RDSON due to heating. Alternatively,

the 1.3 can be ignored and the RDSON of the MOSFET estimated using the RDSON Vs. Temperature curves in the

MOSFET datasheets.

Gate charging loss results from the current driving the gate capacitance of the power MOSFETs, and is

approximated as:

PGC = n x (VDD) x QG x FSW

where

•

‘n’ is the number of MOSFETs (if multiple devices have been placed in parallel)

•

VDD is the driving voltage (see MOSFET Gate Drivers section)

•

QGS is the gate charge of the MOSFET

(20)

If different types of MOSFETs are used, the ‘n’ term can be ignored and their gate charges simply summed to

form a cumulative QG. Gate charge loss differs from conduction and switching losses in that the actual

dissipation occurs in the LM2744, and not in the MOSFET itself.

Switching loss occurs during the brief transition period as the high-side MOSFET turns on and off, during which

both current and voltage are present in the channel of the MOSFET. It can be approximated as:

PSW = 0.5 x VIN x IO x (tr + tf) x FSW

where

•

tR and tF are the rise and fall times of the MOSFET

(21)

Switching loss occurs in the high-side MOSFET only.

For this example, the maximum drain-to-source voltage applied to either MOSFET is 3.6V. The maximum drive

voltage at the gate of the high-side MOSFET is 3.1V, and the maximum drive voltage for the low-side MOSFET

is 3.3V. Due to the low drive voltages in this example, a MOSFET that turns on fully with 3.1V of gate drive is

needed. For designs of 5A and under, dual MOSFETs in SO-8 provide a good tradeoff between size, cost, and

efficiency.

Support Components

CIN2 - A small (0.1 to 1 µF) ceramic capacitor should be placed as close as possible to the drain of the high-side

MOSFET and source of the low-side MOSFET (dual MOSFETs make this easy). This capacitor should be X5R

type dielectric or better.

RCC, CCC- These are standard filter components designed to ensure smooth DC voltage for the chip supply. RCC

should be 1-10

Ω. CCC should be 1 µF, X5R type or better.

CBOOT- Bootstrap capacitor, typically 100nF.

RPULL-UP – This is a standard pull-up resistor for the open-drain power good signal (PWGD). The recommended

value is 10 k

Ω connected to VCC. If this feature is not necessary, the resistor can be omitted.

D1 - A small Schottky diode should be used for the bootstrap. It allows for a minimum drop for both high and low-

side drivers. The MBR0520 or BAT54 work well in most designs.

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