LM26 datasheet(18/20 Pages) NSC

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MOSFET Selection (Continued)

where Tj_max is the maximum allowed junction temperature

in the FET, Ta_max is the maximum ambient temperature,

Rθja is the junction-to-ambient thermal resistance of the FET,

and TC is the temperature coefficient of the on resistance

which is typically in the range of 10,000ppm/˚C.

If the calculated Rdson_max is smaller than the lowest value

available, multiple FETs can be used in parallel. This effec-

tively reduces the Imax term in the above equation, thus

reducing Rdson. When using two FETs in parallel, multiply

the calculated Rdson_max by 4 to obtain the Rdson_max for

each FET. In the case of three FETs, multiply by 9.

If the selected FET has an Rds value higher than 35.3

Ω,

then two FETs with an Rdson less than 141m

Ω (4 x 35.3mΩ)

can be used in parallel. In this case, the temperature rise on

each FET will not go to Tj_max because each FET is now

dissipating only half of the total power.

TOP FET SELECTION

The top FET has two types of losses: switching loss and

conduction loss. The switching losses mainly consist of

crossover loss and bottom diode reverse recovery loss.

Since it is rather difficult to estimate the switching loss, a

general starting point is to allot 60% of the top FET thermal

capacity to switching losses. The best way to precisely de-

termine switching losses is through bench testing. The equa-

tion for calculating the on resistance of the top FET is thus:

Example: Tj_max = 100˚C, Ta_max = 60˚C, Rqja = 60˚C/W,

Vin_min = 5.5V, Vnom = 5V, and Iload_max = 3.6A.

When using FETs in parallel, the same guidelines apply to

the top FET as apply to the bottom FET.

Loop Compensation

The general purpose of loop compensation is to meet static

and dynamic performance requirements while maintaining

stability. Loop gain is what is usually checked to determine

small-signal performance. Loop gain is equal to the product

of control-output transfer function and the output-control

transfer function (the compensation network transfer func-

tion). Generally speaking it is a good idea to have a loop gain

slope that is -20dB /decade from a very low frequency to well

beyond the crossover frequency. The crossover frequency

should not exceed one-fifth of the switching frequency, i.e.

60kHz in the case of LM2642. The higher the bandwidth is,

the faster the load transient response speed will potentially

be. However, if the duty cycle saturates during a load tran-

sient, further increasing the small signal bandwidth will not

help. Since the control-output transfer function usually has

very limited low frequency gain, it is a good idea to place a

pole in the compensation at zero frequency, so that the low

frequency gain will be relatively large. A large DC gain

means high DC regulation accuracy (i.e. DC voltage

changes little with load or line variations). The rest of the

compensation scheme depends highly on the shape of the

control-output plot.

As shown in Figure 10, the control-output transfer function

consists of one pole (fp), one zero (fz), and a double pole at

fn (half the switching frequency). The following can be done

to create a -20dB /decade roll-off of the loop gain: Place the

first pole at 0Hz, the first zero at fp, the second pole at fz,

and the second zero at fn. The resulting output-control trans-

fer function is shown in Figure 11.

20046214

FIGURE 10. Control-Output Transfer Function

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