LM2746MXAX datasheet(16/24 Pages) NSC

16 / 24 page

Application Information (Continued)

4SP560M electrolytic capacitor will give an equivalent ESR

of 14m

Ω. The capacitance of 560 µF is enough to supply

energy even to meet severe load transient demands.

MOSFETs

Selection of the power MOSFETs is governed by a tradeoff

between cost, size, and efficiency. One method is to deter-

mine 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 guaran-

teed, 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 I

2R loss, is approximately:

C =D(IO

2 xR

DSON-HI x 1.3)

(High-Side MOSFET)

C =(1-D)x(IO

2 xR

DSON-LO x 1.3)

(Low-Side MOSFET)

In the above equations the factor 1.3 accounts for the in-

crease in MOSFET R

DSON due to heating. Alternatively, the

1.3 can be ignored and the R

DSON of the MOSFET estimated

using the R

DSON 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:

GC =nx(VDD)xQG xfSW

where ‘n’ is the number of MOSFETs (if multiple devices

have been placed in parallel), V

DD is the driving voltage (see

MOSFET Gate Drivers section) and Q

GS is the gate charge

of the MOSFET. If different types of MOSFETs are used, the

‘n’ term can be ignored and their gate charges simply

summed to form a cumulative Q

G. Gate charge loss differs

from conduction and switching losses in that the actual

dissipation occurs in the LM2746, 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 cur-

rent and voltage are present in the channel of the MOSFET.

It can be approximated as:

SW =0.5xVIN xIO x(tr +tf)xfSW

where t

R and tF are the rise and fall times of the MOSFET.

Switching loss occurs in the high-side MOSFET only.

For this example, the maximum drain-to-source voltage ap-

plied to either MOSFET is 3.6V. The maximum drive voltage

at the gate of the high-side MOSFET is 3.1V, and the maxi-

mum 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

IN2 - 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 MOS-

FETs make this easy). This capacitor should be X5R type

dielectric or better.

CC,CCC- These are standard filter components designed to

ensure smooth DC voltage for the chip supply. R

CC should

be 1-10

Ω.C

CC should 1 µF, X5R type or better.

BOOT- Bootstrap capacitor, typically 100nF.

PULL-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 V

CC. If this feature is not necessary,

the resistor can be omitted.

1 - 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.

CS - Resistor used to set the current limit. Since the design

calls for a peak current magnitude (I

OUT+0.5*

∆I

OUT) of 4.8A,

a safe setting would be 6A. (This is below the saturation

current of the output inductor, which is 7A.) Following the

equation from the Current Limit section, a 1.3k

Ω resistor

should be used.

FADJ - This resistor is used to set the switching frequency of

the chip. The resistor value is calculated from equation in

Normal Operation section. For 300 kHz operation, a 97.6 k

Ω

resistor should be used.

SS - The soft-start capacitor depends on the user require-

ments and is calculated based on the equation given in the

section titled START UP/SOFT-START. Therefore, for a

700µs delay, a 12nF capacitor is suitable.

Control Loop Compensation

The LM2746 uses voltage-mode (‘VM’) PWM control to cor-

rect changes in output voltage due to line and load tran-

sients. One of the attractive advantages of voltage mode

control is its relative immunity to noise and layout. However

VM requires careful small signal compensation of the control

loop for achieving high bandwidth and good phase margin.

The control loop is comprised of two parts. The first is the

power stage, which consists of the duty cycle modulator,

output inductor, output capacitor, and load. The second part

is the error amplifier, which for the LM2746 is a 9MHz

op-amp used in the classic inverting configuration. Figure 12

shows the regulator and control loop components.

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