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LM2688M-ADJ データシート(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM2688M-ADJ
部品情報  Non-synchronous Step-down Controller for Pentium Microprocessors
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2688M-ADJ データシート(HTML) 5 Page - National Semiconductor (TI)

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Application Hints
GENERAL
This is a cost effective non-synchronous buck solution for
powering Pentium and similar microprocessors (AMD’s K5,
Motorola’s PowerPC, etc.) that draws large current at a low
voltage. Fairly high efficiency of 86% or better at 10A load
can be achieved by properly selecting the components.
PIN FUNCTIONS
+V
IN— This is the positive input supply for the controller to
work. A suitable input bypass capacitor must be present at
this pin to minimize voltage transients and to supply the cur-
rent needed to drive the external MOSFET.
Ground—Circuit ground.
Output—Sources pulsed current up to 1.4A maximum to
drive external MOSFET. To minimize turn-on delay of the ex-
ternal MOSFET, copper trace between this pin and the gate
of the MOSFET should be kept as short as possible.
Feedback—Senses the regulator output voltage so that a
feedback control loop is achieved. A resistor divider can be
used to choose the desired output voltage.
ON/OFF—Allows the controller to be shut down by a logic
level signal. If the shutdown feature is not needed, the pin
can be either grounded or left open, both of which enable the
controller.
COMPONENTS SELECTION
Input Filter
Depending on the system requirement, an inductor may or
may not be necessary. When there is no explicit input di/dt
limitation, the filter can be composed of capacitors only. The
inductor plays an important role in limiting the input di/dt so
that the input power rail appears a quiet supply for other
loads. It also limits the inrush current during power on. The
best inductance value is too complicated to calculate but can
be easily determined by experiment. For a typical Pentium
motherboard application, 2µH is recommended. The bulk ca-
pacitors in the input filter should be of low ESR type or oth-
erwise the input di/dt requirement may not be met. A low cost
low ESR electrolytic capacitor manufactured by United
Chemicon, LXV16VB102M, is recommended here.
Output Filter
The output filter plays an extremely important role in meeting
the load transient requirement. From the viewpoint of load
transient response, it is desirable to have a low inductance
and a high output capacitance. However, too low an induc-
tance causes huge ripple current through the MOSFETs
which translates into lower efficiency. Also too big a bank of
output capacitors may not be economical in terms of both
cost of the regulator and motherboard real estate. It may
also require a larger input capacitor bank to stay within input
di/dt specification. The output voltage ripple is determined by
the amount of ripple current in the inductor and the ESR of
the output capacitor bank. For the typical situation where
Pentium processor is the load, a 2.5µH inductor and a bank
of three 1500µF electrolytic capacitors are recommended.
Sanyo’s
ultra
low-ESR
electrolytic
capacitor,
the
6MV1500GX, is recommended.
MOSFET Selection
It is desirable to have the on-resistance of the MOSFET as
low as possible so that its conduction loss is minimized and
high efficiency can be maintained. A good idea is to use two
low-R
dson MOSFETs in parallel so that the total conduction
loss is halved compared with using one MOSFET. In the typi-
cal application circuit, two Fairchild low R
dson (10 mΩ) MOS-
FETs, the NDS8410A, are used in parallel.
Diode Selection
Since current flows through the freewheeling diode when the
MOSFETs are off, efficiency is affected greatly by the for-
ward voltage drop of the diode. Generally a Schottky diode is
used here. Motorola MBR2515L Schottky diode has a for-
ward voltage drop of less than 0.4V at 20A which fits very
well in this application.
Output Voltage Adjustment
Different output voltages can be obtained by using different
combinations of feedback resistors. The formula to calculate
output voltage is VCORE=(R2/R3+1)X1.235V. In the “typical
application” circuit, two jumpers are used to adjust the value
of R2 so that four output voltages can be obtained. When
necessary, C9 and C10 can be used to further compensate
the converter to result in a more stable circuit, the penalty is
a slower transient recovery speed.
Bill of Material
Label
Value
Type
Part Number
Manufacturer
C1
1000µ
Capacitor
LXV16VB102M
United Chemi-Con
C2
1000µ
Capacitor
LXV16VB102M
United Chemi-Con
C3
1000µ
Capacitor
LXV16VB102M
United Chemi-Con
C4
10µ
Capacitor, Tantalum
C5
0.1µ
Capacitor, Ceramic
C6
1500µ
Capacitor
6MV1500GX
Sanyo
C7
1500µ
Capacitor
6MV1500GX
Sanyo
C8
1500µ
Capacitor
6MV1500GX
Sanyo
C9
1n
Capacitor, Ceramic
C10
15n
Capacitor, Ceramic
L1
2.5µ
Inductor
R1
300, 0.5W
Resistor
R2
3.65k 1%
Resistor
R3
2k 1%
Resistor
5
www.national.com


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