LM2621MM National Semiconductor, LM2621MM Datasheet - Page 8

• Manufacturer Part Number: LM2621MM
• Description: IC, STEP-UP DC/DC CONVERTER, 8-SOIC
• Manufacturer: National Semiconductor

Specifications of LM2621MM

• Primary Input Voltage: 14V
• No. Of Outputs: 1
• Output Voltage: 14V
• Output Current: 1A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Power Dissipation Pd: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Design Procedure
SETTING THE OUTPUT VOLTAGE
The output voltage of the step-up regulator can be set be-
tween 1.24V and 14V by connecting a feedback resistive
divider made of R
lected as follows:
A value of 150kΩ is suggested for R
selected using the above equation. A 39pF capacitor (C
connected across R
ripple at V
peak output voltage ripple as well as improve the efficiency
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
bootstrapped operation is suggested. This is achieved by
connecting the V
is outside this range, the V
voltage source whose range is between 2.5V and 5V. This
can be the input voltage (V
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
FREQ and V
quency vs R
section of the datasheet for choosing the R
achieve the desired switching frequency. A high switching
frequency allows the use of very small surface mount induc-
tors and capacitors and results in a very small solution size.
A switching frequency between 300kHz and 2MHz is recom-
mended.
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
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OUT
DD
FQ
to the FB pin. This helps reduce the peak-to-
R
“ in the Typical Operating Characteristics
DD
pins. See the graph titled „Switching Fre-
F2
F1
pin (Pin 6) to V
= R
F1
Inductors
and R
helps in feeding back most of the AC
F1
/[(V
OUT
DD
IN
F2
), V
OUT
. The resistor values are se-
pin should be connected to a
) is between 2.5V and 5.0V a
OUT
/ 1.24) −1]
OUT
FQ
) connected between
stepped down using a
F1
. However if the V
. Then, R
TABLE 1. Suggested Manufacturers List
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FQ
F2
value to
can be
Capacitors
OUT
F1
)
8
INDUCTOR SELECTION
The LM2621’s high switching frequency enables the use of a
small surface mount inductor. A 6.8µH 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 100mΩ 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
inductor 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
possible. 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 22µF capacitor is suggested for the
input filter capacitor. It should have a DC working voltage
rating higher than the maximum input voltage. A 68µF tan-
talum capacitor is suggested for the output capacitor. The
DC working voltage rating should be greater than the output
voltage. Very high ESR values (
Table 1 shows a list of the capacitor manufacturers.
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
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
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.
DD
pins. The traces that run between V
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Diodes
>
3Ω) should be avoided.
out
F1
and the FB pin of
, R
F2
, and C
F1
)