case of desktop PC applications, those ratings seem some-

what conservative. A rule-of-thumb is increase the 105˚C rat-

ing by 70% for desktop PC applications. The input RMS

ripple current value can be determined by the following

equation:

(14)

and the power loss in each input capacitor is:

(15)

In the case of 333 MHz Pentium II power supply, the maxi-

mum output current is around 14A. Under the worst case

when duty cycle is 50%, the maximum input capacitor RMS

ripple current is half of output current, i.e., 7A. Therefore

three Sanyo 16MV820GX capacitors are necessary under

room temperature (they are rated 1.45A at 105˚C). The

maximum ESR of those capacitors is 44 m

Ω. So the maxi-

mum power loss in each of them is less than (7A)

m

is inversely proportional to the square of the total number of

capacitors, which means the power loss in each capacitor

quickly drops when the number of capacitors increases.

Linear Section — For applications where there is a load tran-

sient requirement such as that the GTL+ supply, low ESR ca-

pacitors should be considered. Make sure that the total ESR

multiplied by the maximum load current is smaller than half

the output voltage regulation window. The output voltage

regulation window should exclude the tolerance of LM2637.

For example, for a 3.3V to 1.5V, 2A design, the initial regula-

2A = 52 m

Ω. A Sanyo 6MV1200DX is sufficient. For applica-

tions where the load is static and for control bandwidth and

stability issue, refer to the guidelines in the

control loop com-

pensation section.

Inductor Selection

Output Inductor. The size of the output inductor is deter-

mined by a number of parameters. Basically the larger the

inductor, the smaller the output ripple voltage, but the slower

the converter’s response speed during a load transient. On

the other hand, a smaller inductor requires higher switching

frequency to maintain the same level of output ripple, and

probably results in a lossier converter, but has less inertia re-

sponding to load transient. In the case of MPU core power

supply, fast recovery of the load voltage from transient win-

dow back to the steady state window is important. That limits

the highest inductance value that can be used. The lowest

inductance value is limited by the highest switching fre-

quency that can be practically employed. As the switching

frequency increases, the switching loss in the FETs tends to

increase, resulting in lower overall efficiency and larger heat

sinks. A good switching frequency is probably a frequency

under which the FET conduction loss is much higher than

the switching loss because the cost of the FET is directly re-

lated to its r

the following equation:

(16)

where V

switching frequency. For commonly used low r

a reasonable switching frequency is 300 kHz. Assume a

peak-peak output ripple voltage is 18 mV, the total output ca-

pacitor ESR is 9 m

Ω, the input voltage is 5V, and output volt-

age is 2.8V, then the inductance value according to the

above equation will be 2 µH. The highest slew rate of the in-

ductor current when the load changes from no load to full

load can be determined as follows:

(17)

where D

around 0.95 for LM2637. For a load transient from 0A to 14A,

the highest current slew rate of the inductor, according to the

above equation, is 0.97 A/µs, and therefore the shortest pos-

sible total recovery time is 14A/(0.97 A/µs) = 14.5 µs. Notice

that output voltage starts to recover whenever the inductor

starts to supply current.

The highest slew rate of the inductor current when the load

changes from full load to no load can be determined from the

same equation but use D

Since the D

−1.4 A/µs. So the approximate total recovery time will be

14A/(1.4 A/µs) = 10 µs.

Often times the power supply designer may have to use a

custom-made inductor for best performance/price ratio. Mi-

crometals offers cost effective iron powder cores that are

widely adopted by motherboard supplies and OEMs. One

important rule when designing an iron power inductor is

never saturate the core or else it will exhibit extremely poor

dynamic performance. Useful inductor design tools can also

be found on their web page, www.micrometals.com. The

user

of

LM2637

can

also

contact

National

for

a

custom-made inductor.

Alternatively the designer may use an open core inductor,

which is lower cost due to its ease of mass production. How-

ever, the open magnetic field may cause some noise prob-

lems to nearby circuitry and may cause EMI issues. How-

ever, no negative reports have been heard so far. Coilcraft

(www.coilcraft.com) offers a wide range of open core in-

ductors. Custom-made parts are also possible. Other than

low cost, the advantages of open core inductors are less

board space and superior dynamic performance.

Input Inductor. The input inductor is for limiting the input

current slew rate during a load transient and normal opera-

tion. In the case that low ESR aluminum electrolytic capaci-

tors are used for the input capacitor bank, input capacitor

voltage change due to capacitor charging/discharging is usu-

ally negligible for the first 20 µs. ESR is by far the dominant

factor in determining the amount of capacitor voltage

undershoot/overshoot during a fast load transient. So the

worst case is when the load changes between no load and

full load. Under that condition the input inductor sees the

highest voltage change across the input capacitors. Assume

the input capacitor bank consists of three 16MV820GX, i.e.,

a total ESR of 15 m

Ω. Whenever there is a sudden load

change, the change in input current has to be initially sup-

ported by the input capacitor bank instead of the input induc-

tor. So for a fast load-swing between 0A and 14A, the voltage

change seen by the input inductor is a ramp from 0V to a

∆V

or vice versa, whereas

∆V = 14A x15mΩ = 210 mV. So this

situation is just as bad as operating under heaviest load. Use

the following equation to determine the minimum inductance

value:

www.national.com

14