LM2640MTC-ADJ National Semiconductor, LM2640MTC-ADJ Datasheet - Page 17

• Manufacturer Part Number: LM2640MTC-ADJ
• Description: Dual Adjustable Step-Down Switching Power Supply Controller
• Manufacturer: National Semiconductor

Design Procedure
cifically designed for switching applications. Capacitors such
as these with good high frequency ( 100 kHz) specifications
are not cheap.
Aluminum electrolytic capacitors should generally not be
used in switching regulator applications where the ambient
temperature goes below 0˚C. A typical low-voltage aluminum
electrolytic has an ESR vs. Temperature curve that is fairly
flat from 25˚C to 125˚C. However, a temperature change
from 25˚C to 0˚C will approximately double the ESR, and it
will double again going from 0˚C down to −20˚C.
Tantalum
Solid Tantalum capacitors are best in applications which
must operate over a wide temperature range. A good quality
Tantalum will typically exhibit less than 2:1 change in ESR
over the temperature range of +125˚C to −40˚C. Recom-
mended types are Sprague 593D, Sprague 594D, and AVX
TPS series.
Selecting An Output Capacitor
The required value of output capacitance is directly related
to the specification for the maximum amount of output volt-
age ripple allowed in the application. The value of C
essary to meet the voltage ripple specification can be found
using the approximation:
Where:
I
V
ESR is the equivalent series resistance of the output capaci-
tor.
F is the switching frequency, F
The ESR term predominates in determining output ripple
voltage. Good quality Tantalum capacitors have guaranteed
maximum specifications for ESR, but the typical values for
ESR are usually considerably lower than the maximum limit.
RIPPLE
RIPPLE
is the inductor ripple current.
is the output ripple voltage.
OSC
(Continued)
.
OUT
nec-
17
POWER MOSFETs
Two N-channel logic-level MOSFETs are required for each
output. The voltage rating should be at least 1.2 times the
maximum input voltage.
Maximizing efficiency for a design requires selecting the right
FET. The ON-resistance of the FET determines the ON-state
(conduction) losses, while gate charge defines the losses
during switch transitions. These two parameters require a
trade-off, since reducing ON-resistance typically requires in-
creasing gate capacitance (which increases the charge re-
quired to switch the FET). Improved FETs are currently being
released which are designed specifically for optimized
ON-resistance and gate charge characteristics.
The V
ON time of each switch. In some cases where one FET is on
most of the time, efficiency may be improved slightly by se-
lecting a low ON-resistance FET for one of the FET switches
and a different type with lower gate charge requirement for
the other FET switch. However, for most applications this
would give no measurable improvement.
CURRENT SENSE RESISTOR
A sense resistor is placed between the inductor and the out-
put capacitor to measure the inductor current. The value of
this resistor is set by the current limit voltage of the LM2640
(see Electrical Characteristics) and the maximum (peak) in-
ductor current. The value of the sense resistor can be calcu-
lated from:
Where:
V
Electrical Characteristics).
I
I
TOL is the tolerance (in %) of the sense resistor.
The physical placement of the sense resistors should be as
close as possible to the LM2640 to minimize the lead length
of the connections to the CSH and CSL pins. Keeping short
leads on these connections reduces the amount of switching
noise conducted into the current sense circuitry of the
LM2640.
EXTERNAL DIODES
FET Diodes
Both of the low-side MOSFET switches have an external
Schottky diode connected from drain to source. These di-
odes are electrically in parallel with the intrinsic body diode
present inside the FET. These diodes conduct during the
dead time when both FETs are off and the inductor current
must be supplied by the catch diode (which is either the body
diode or the Schottky diode).
Converter efficiency is improved by using external Schottky
diodes. Since they have much faster turn-off recovery than
the FET body diodes, switching losses are reduced.
The voltage rating of the Schottky must be at least 25%
higher than the maximum input voltage. The average current
rating of the diode needs to be only about 30% of the output
current, because the duty cycle is low.
MAX
RIPPLE
CL
(MIN) is the minimum specified current limit voltage (see
is the maximum output current for the application.
IN
is the inductor ripple current for the application.
and V
OUT
for a specific application determines the
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