LM2637M datasheet(15/17 Pages) NSC

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Applications Information (Continued)

(18)

where (di/dt)

max is the maximum allowable input current slew

rate, which is 0.1 A/µs in the case of Pentium II power supply

and

∆V is equal to maximum load current times input capaci-

tor ESR. So the input inductor size, according to the above

equation, should be 2.1 µH.

Dynamic Positioning of Load Voltage

The following is just a quick overview of a technique called

dynamic voltage positioning. For a detailed explanation and

examples please refer to our application note

Using Dy-

namic Voltage Positioning Technique to Reduce the Cost of

Output Capacitors in Advanced Microprocessor Power Sup-

plies. An associated spreadsheet is also available for auto-

mated design.

Since the typical MPU core voltage’s steady state regulation

window is fairly large, it is a good idea to dynamically posi-

tion the steady state output voltage in the steady state regu-

lation window with respect to load current level so that the

output voltage has more headroom for load transient re-

sponse. This needs load current information. There are at

least two simple ways to implement this idea with LM2637.

One is to utilize the output inductor DC resistance, see

Fig-

ure 9. The average voltage across the output inductor is ac-

tually that across its DC resistance, which is proportional to

load current.

Since the switching node voltage V

A toggles between the in-

put voltage and ground at the switching frequency, it is im-

possible to choose node A as the feedback point, otherwise

the dynamic performance will suffer and the system may

have noise problems. Using a low pass filter network around

the inductor, such as the one shown in the figure, seems to

be a good idea. The feedback point is node C.

Since at switching frequency the impedance of the 0.1 µF is

much less than 5 k

Ω, so the toggling voltage at node A will

mainly drop across the 5 k

Ω resistor and node C will be

much quieter than A. However, V

CB average is still the ma-

jority of V

AB average, because of the ratio of the resistor di-

vider. So in steady state V

C = IO xrL +VCORE, where rL is the

inductor DC resistance. So at no load, output voltage is

equal to V

C, and at full load, output voltage is IO xrL lower

than V

C. To further utilize the steady state regulation window,

a resistor can be connected between the FB pin and ground

to increase the no-load output voltage to close to the upper

limit of the window.

A possible drawback of the scheme in

Figure 9 is slow tran-

sient recovery speed. Since the 5 k

Ω resistor and the 0.1 µF

capacitor have a large time constant, the settling of node C

to its steady state value during a load transient may take a

few milliseconds. Depends on the interaction between the

compensation network and the 0.1 µF capacitor, V

CORE may

take different routes to reach its steady state value. This is

undesired when the load transient happens more than 1000

times per second. Reducing the time constant will result in a

more fluctuating V

C, due to a less effective low pass filter.

Fine tuning the parameters may generate an acceptable de-

sign.

Another way to implement the dynamic voltage ppsitioning is

through the use of a separate resistor, such as the 4 m

Ω re-

sistor in

Figure 10 above. The advantage of this implementa-

tion over the previous one is a much faster recovery speed of

CORE from transient level to steady state level. A fine-tuned

compensation network will give good response as shown in

Figure 11. The disadvantage is additional power loss. The to-

tal power loss can be 0.78W at 14A of load current. The cost

of the resistor can be minimized by using a PCB etch

resistor.

PCB Layout Considerations

There are several points to consider.

DS100848-27

FIGURE 9. Dynamic Voltage Positioning by Utilizing

Output Inductor DC Resistance

DS100848-28

FIGURE 10. Dynamic Voltage Positioning by Using a

Stand-Alone Resistor

DS100848-29

FIGURE 11. Load Transient Response with DVP: 0A to

14A, ESR = 9.4 m

Ω, Droop Resistor = 4mΩ

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