LM2635MX National Semiconductor, LM2635MX Datasheet - Page 10

• Manufacturer Part Number: LM2635MX
• Description: IC REG SYNCH BUCK 5-BIT 20-SOIC
• Manufacturer: National Semiconductor

Specifications of LM2635MX

• Applications: Power Supplies
• Current - Supply: 2.5mA
• Voltage - Supply: 4.5 V ~ 5.5 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM2635MX; LM2635MXTR

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Applications Information
driving capability of the error amplifier is limited to around
3 mA, it is a good idea to have a high impedance path from
the output of the error amplifier to the output of the converter.
From the above equations it can be told that a larger R
result in a smaller C
an R
the inverting input pin of the error amplifier. Calculations
show that the following combination is a good one: R
C
For a different application or different type of output capaci-
tors, a different compensation scheme may be necessary.
The user can either follow the steps above to figure the
appropriate component values or contact the factory for
help.
MOSFET SELECTION
The selection of MOSFET switches affects both the effi-
ciency of the whole converter and the current limit setting.
From an efficiency point of view it is suggested that for the
high-side switch, only logic level MOSFETs be used. Stan-
dard MOSFETs can be used for the low side switch when
12V is used to power the BOOTV pin. The lower loss asso-
ciated with the MOSFETs is two-fold — Ohmic loss and
switching loss. The Ohmic loss is easy to calculate whereas
the switching loss is much more difficult to estimate. In
general the switching loss is directly proportional to the
switching frequency. As the power MOSFET technology ad-
vances, lower and lower gate charge devices will be avail-
able. That should allow the user to go to higher switching
frequencies without the penalty of losing too much efficiency.
As an example, let us select the MOSFETs for a converter
with a target efficiency of 80% at a load of 2.8V, 14A.
Assume the inductors lose 1W, the capacitors lose 0.75W
and the total switching loss at 300 kHz is 3.2W. The total
allowed power loss is 9.8W, so the MOSFET Ohmic loss
should not exceed 4.9W. Assume the two switches have the
same conduction loss, i.e., 2.5W each, then the ON resis-
tance for the two switches is:
The low side switch ON resistance is much higher than the
high side because at 2.8V the duty cycle is higher than 50%
and becomes even larger at full load. For the high side
switch, an IRL3202 (TO-220 package) or IRL3202S (D
should be sufficient. For the low side switch, an IRL3303
(TO-220 package) or IRL3303S (D
cient. Since each FET is dissipating 3.2W/2 + 2.5W = 4.1W,
it is suggested that appropriate heat sinks be used in the
case of TO-220 package or large enough copper area be
connected to the drain in the case of surface mount pack-
age.
CAPACITOR SELECTION
The selection of capacitors is an extremely important step
when designing a converter for a load such as the Pentium
II. Since the typical slew rate of the load current during a
large load transient is around 20A/µs to 30A/µs, the switch-
1
= 0.022 µF, R
1
can also bring error due to the bias current required by
1
= 5.6 kΩ, C
1
, C
2
and a larger R
2
= 820 pF.
2
PAK) should be suffi-
1
. However, too large
(Continued)
2
= 51Ω,
2
PAK)
2
will
10
ing converter has to rely on the output capacitors to take
care of the first few microseconds. Under such a current
slew rate, ESR of the output capacitors is more of a concern
than the ESL. Depending on the kind of capacitors being
used, capacitance of the output capacitors may or may not
be an important factor. When the output capacitance is too
low, the converter 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 capacitances. 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 capacitance of the aluminum/
tantalum/OSCON capacitors is usually already in the milli-
farad 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 probably 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 point of view, aluminum electrolytic
capacitors are the most popular choice for output capacitors.
They have reasonably long life span and they tend to have
huge capacitance to withstand the charging or discharging
process during a load transient for a fairly long period. Sanyo
MV-GX series gives good performance when enough of the
capacitors are paralleled. The 6MV1500GX capacitor has a
typical ESR of 44 mΩ. Five of these capacitors should be
sufficient in the case of on-board power supply for a Pentium
II motherboard.
The challenge for input capacitors is the ripple current. The
large ripple current drawn by the high side switch tends to
generate quite some heat due to the capacitor ESR. The
ripple current ratings in the capacitor catalogs are usually
specified under the highest allowable temperature. In the
case of desktop applications, those ratings seem too con-
servative. A good way to ensure enough number of capaci-
tors is through lab evaluation. The input current RMS ripple
value can be determined by the following equation:
and the power loss in each input capacitor is:
In the case of Pentium II power supply, the maximum 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. It is found that three Sanyo
16MV820GX capacitors are enough under room tempera-
ture. The typical ESR of those capacitors is 44 mΩ. So the
power loss in each of them is around (7A)
0.24W. Note that the power loss in each capacitor is in-
versely 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.
2
x 44 mΩ/3
2
=