lm2637mx National Semiconductor Corporation, lm2637mx Datasheet - Page 15

• Manufacturer Part Number: lm2637mx
• Description: Motherboard Power Supply Solution With A 5-bit Programmable Switching Controller And Two Linear Regulator Controllers
• Manufacturer: National Semiconductor Corporation

Applications Information
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.
However, the open magnetic field may cause some noise
problems to nearby circuitry and may cause EMI issues.
However, no negative reports have been heard so far. Coil-
craft (www.coilcraft.com) offers a wide range of open core
inductors. 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 mΩ. 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 x 15 mΩ = 210 mV. So this
situation is just as bad as operating under heaviest load. Use
the following equation to determine the minimum inductance
value:
where (di/dt)
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-
MIN
max
of LM2637 is at 0%, the slew rate is therefore
is the maximum allowable input current slew
(Continued)
(18)
15
tion the steady state output voltage in the steady state
regulation window with respect to load current level so that
the output voltage has more headroom for load transient
response. 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
Figure 9. The average voltage across the output inductor is
actually that across its DC resistance, which is proportional
to load current.
Since the switching node voltage V
input voltage and ground at the switching frequency, it is
impossible to choose node A as the feedback point, other-
wise 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 kΩ, so the toggling voltage at node A will
mainly drop across the 5 kΩ resistor and node C will be
much quieter than A. However, V
majority of V
divider. So in steady state V
the inductor DC resistance. So at no load, output voltage is
equal to V
than V
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.
FIGURE 9. Dynamic Voltage Positioning by Utilizing
C
. To further utilize the steady state regulation window,
C
, and at full load, output voltage is I
AB
Output Inductor DC Resistance
average, because of the ratio of the resistor
C
= I
O
x r
CB
A
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toggles between the
+ V
average is still the
CORE
, where r
O
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