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参数资料
| 型号： | LM2621MDC |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 2.85 A SWITCHING REGULATOR, 2000 kHz SWITCHING FREQ-MAX, UUC |
| 封装： | DIE |
| 文件页数： | 11/13页 |
| 文件大小： | 306K |
| 代理商： | LM2621MDC |
Design Procedure (Continued)
ficiency of the step-up regulator, because a set hysteresis of
30mV at the FB pin is used for the gated oscillator control
scheme.
BOOTSTRAPPING
When the output voltage (V
OUT) is between 2.5V and 5.0V a
bootstrapped operation is suggested. This is achieved by
connecting the V
DD pin (Pin 6) to VOUT. However if the VOUT
is outside this range, the V
DD pin should be connected to a
voltage source whose range is between 2.5V and 5V. This
can be the input voltage (V
IN), VOUT stepped down using a
linear regulator, or a different voltage source available in the
system. This is referred to as non-bootstrapped operation.
The maximum acceptable voltage at the BOOT pin (Pin 7) is
10V.
SETTING THE SWITCHING FREQUENCY
The switching frequency of the oscillator is selected by
choosing an external resistor (R
FQ)
connected between
FREQ and V
DD pins. See the graph titled Switching Fre-
quency vs R
FQ“ in the Typical Operating Characteristics sec-
tion of the datasheet for choosing the R
FQ value to achieve
the desired switching frequency. A high switching frequency
allows the use of very small surface mount inductors and ca-
pacitors and results in a very small solution size. A switching
frequency between 300kHz and 2MHz is recommended.
INDUCTOR SELECTION
The LM2621’s high switching frequency enables the use of a
small surface mount inductor. A 6.8H shielded inductor is
suggested. The inductor should have a saturation current
rating higher than the peak current it will experience during
circuit operation (see graph titled Peak Inductor Current vs.
Load Current“ in the Typical Performance Characteristics
section). Less than 100m
ESR is suggested for high effi-
ciency.
Open-core inductors cause flux linkage with circuit compo-
nents and interfere with the normal operation of the circuit.
They should be avoided. For high efficiency, choose an in-
ductor with a high frequency core material, such as ferrite, to
reduce the core losses. To minimize radiated noise, use a
toroid, pot core or shielded core inductor. The inductor
should be connected to the SW pin as close to the IC as pos-
sible. See
Table 1 for a list of the inductor manufacturers.
OUTPUT DIODE SELECTION
A Schottky diode should be used for the output diode. The
forward current rating of the diode should be higher than the
load current, and the reverse voltage rating must be higher
than the output voltage. Do not use ordinary rectifier diodes,
since slow switching speeds and long recovery times cause
the efficiency and the load regulation to suffer.
Table 1 shows
a list of the diode manufacturers.
INPUT AND OUTPUT FILTER CAPACITORS SELECTION
Tantalum chip capacitors are recommended for the input and
output filter capacitors. A 22F capacitor is suggested for the
input filter capacitor. It should have a DC working voltage rat-
ing higher than the maximum input voltage. A 68F tantalum
capacitor is suggested for the output capacitor. The DC
working voltage rating should be greater than the output volt-
age. Very high ESR values (>3
) should be avoided. Table
1 shows a list of the capacitor manufacturers.
TABLE 1. Suggested Manufacturers List
Inductors
Capacitors
Diodes
Coilcraft
Tel: (800) 322-2645
Fax: (708) 639-1469
Sprague/ Vishay
Tel: (207) 324-4140
Fax: (207) 324-7223
Motorola
Tel: (800) 521-6274
Fax: (602) 244-6609
Coiltronics
Tel: (407) 241-7876
Fax: (407) 241-9339
Kemet
Tel: (864) 963-6300
Fax: (864) 963-6521
International Rectifier (IR)
Tel: (310) 322-3331
Fax: (310) 322-3332
Pulse Engineering
Tel: (619) 674-8100
Fax: (619) 674-8262
Nichicon
Tel: (847) 843-7500
Fax: (847) 843-2798
General Semiconductor
Tel: (516) 847-3222
Fax: (516) 847-3150
PC BOARD LAYOUT
High switching frequencies and high peak currents make a
proper layout of the PC board an important part of design.
Poor design can cause excessive EMI and ground-bounce,
both of which can cause malfunction and loss of regulation
by corrupting voltage feedback signal and injecting noise
into the control section.
Power components - such as the inductor, input and output
filter capacitors, and output diode - should be placed as
close to the regulator IC as possible, and their traces should
be kept short, direct and wide. The ground pins of the input
and output filter capacitors and the PGND and SGND pins of
LM2621 should be connected using short, direct and wide
traces. The voltage feedback network (R
F1,RF2, and CF1)
should be kept very close to the FB pin. Noisy traces, such
as from the SW pin, should be kept away from the FB and
V
DD pins. The traces that run between Vout and the FB pin of
the IC should be kept away from the inductor flux. Always
provide sufficient copper area to dissipate the heat due to
power loss in the circuitry and prevent the thermal protection
circuitry in the IC from shutting the IC down.
LM2621
www.national.com
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