lm2637mx National Semiconductor Corporation, lm2637mx Datasheet - Page 14

• 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

www.national.com
Applications Information
may have to have a small output inductor to quickly supply
current to the output capacitors when the load suddenly
kicks in and to quickly stop supplying current when the load
is suddenly removed. Multilayer ceramic (MLC) capacitors
can have very low ESR but also a low capacitance value
compared to other kinds of capacitors. Low ESR aluminum
electrolytic capacitors tend to have large sizes and capaci-
tance. Tantalum electrolytic capacitors can have a fairly low
ESR with a much smaller size and capacitance than the
aluminum capacitors. Certain OSCON capacitors present
ultra low ESR and long life span. By the time the total ESR of
the output capacitor bank reaches around 9 mΩ, the capaci-
tance of the aluminum/tantalum/OSCON capacitors is usu-
ally already in the millifarad range. For those capacitors,
ESR is the only factor to consider. MLCs can have the same
amount of total ESR with much less capacitance, most prob-
ably under 100 µF. A very small inductor, ultra fast control
loop and a high switching frequency become necessary in
such a case to deal with the fast charging/discharging rate of
the output capacitor bank.
From a cost savings standpoint, aluminum electrolytic ca-
pacitors are the most popular choice for output capacitors.
They have reasonably long life span and they tend to have
hugh capacitance to withstand the charging or discharging
process during a load transient for a fairly long period. Sanyo
MV-GX and MV-DX series’ give good performance when
enough of the capacitors are paralleled. The 6MV1500GX
capacitor has a typical ESR of 44 mΩ and a capacitance of
1500 µF at a voltage rating of 6.3V. For a detailed procedure
for determining number of output capacitors, refer to the
application note Using Dynamic Voltage Positioning Tech-
nique to Reduce the Cost of Output Capacitors in Advanced
Microprocessor Power Supplies and the associated spread-
sheet for automated design.
Input Capacitors. The challenge on input capacitors is the
RMS ripple current. The large ripple current drawn by the
high-side switch tends to generate quite some heat due to
the capacitor ESR. The RMS ripple current ratings in the
capacitor catalogs are usually specified under 105˚C. In the
case of desktop PC applications, those ratings seem some-
what conservative. A rule-of-thumb is increase the 105˚C
rating by 70% for desktop PC applications. The input RMS
ripple current value can be determined by the following
equation:
and the power loss in each input capacitor is:
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 mΩ. So the maxi-
mum power loss in each of them is less than (7A)
mΩ/3
2
= 0.24W. Note that the power loss in each capacitor is
(Continued)
2
x 44
(14)
(15)
14
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
capacitors 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 regu-
lation window is
plus margin is
±
2A = 52 mΩ. 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
compensation 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
responding 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
related to its r
the following equation:
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
capacitor ESR is 9 mΩ, the input voltage is 5V, and output
voltage is 2.8V, then the inductance value according to the
above equation will be 2 µH. The highest slew rate of the
inductor current when the load changes from no load to full
load can be determined as follows:
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
possible 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
105 mV. Therefore the ESR should be less than 105 mV ÷
o_rip
MAX
is the peak-peak output ripple voltage, f is the
DS_ON
is the maximum allowed duty cycle, which is
±
2%, then the effective window left is
±
9%. Assume the tolerance of the LM2637
. The inductor size can be determined by
MIN
instead of D
MAX
.
DS_ON
±
FET’s,
7% or
(16)
(17)