SFTB NSC [National Semiconductor], SFTB Datasheet - Page 8

• Manufacturer Part Number: SFTB
• Description: Thin SOT23 1A Load Step-Down DC-DC Regulator
• Manufacturer: NSC [National Semiconductor]

www.national.com
Design Guide
The LM2734Z operates at frequencies allowing the use of
ceramic output capacitors without compromising transient
response. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
capacitor section for more details on calculating output volt-
age ripple.
Now that the ripple current or ripple ratio is determined, the
inductance is calculated by:
where f
current. When selecting an inductor, make sure that it is
capable of supporting the peak output current without satu-
rating. Inductor saturation will result in a sudden reduction in
inductance and prevent the regulator from operating cor-
rectly. Because of the speed of the internal current limit, the
peak current of the inductor need only be specified for the
required maximum output current. For example, if the de-
signed maximum output current is 0.5A and the peak current
is 0.7A, then the inductor should be specified with a satura-
tion current limit of
saturation or peak current of the inductor at the 1.7A typical
switch current limit. The difference in inductor size is a factor
of 5. Because of the operating frequency of the LM2734Z,
ferrite based inductors are preferred to minimize core losses.
This presents little restriction since the variety of ferrite
based inductors is huge. Lastly, inductors with lower series
resistance (DCR) will provide better operating efficiency. For
recommended inductors see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, volt-
age, RMS current rating, and ESL (Equivalent Series Induc-
tance). The recommended input capacitance is 10µF, al-
though 4.7µF works well for input voltages below 6V. The
input voltage rating is specifically stated by the capacitor
manufacturer. Make sure to check any recommended derat-
ings and also verify if there is any significant change in
capacitance at the operating input voltage and the operating
temperature. The input capacitor maximum RMS input cur-
rent rating (I
It can be shown from the above equation that maximum
RMS capacitor current occurs when D = 0.5. Always calcu-
late the RMS at the point where the duty cycle, D, is closest
to 0.5. The ESL of an input capacitor is usually determined
by the effective cross sectional area of the current path. A
large leaded capacitor will have high ESL and a 0805 ce-
ramic chip capacitor will have very low ESL. At the operating
frequencies of the LM2734Z, certain capacitors may have an
ESL so large that the resulting impedance (2πfL) will be
higher than that required to provide stable operation. As a
result, surface mount capacitors are strongly recommended.
Sanyo POSCAP, Tantalum or Niobium, Panasonic SP or
Cornell Dubilier ESR, and multilayer ceramic capacitors
(MLCC) are all good choices for both input and output ca-
s
is the switching frequency and I
RMS-IN
) must be greater than:
>
0.7A. There is no need to specify the
(Continued)
O
is the output
IN
does not
8
pacitors and have very low ESL. For MLCCs it is recom-
mended to use X7R or X5R dielectrics. Consult capacitor
manufacturer datasheet to see how rated capacitance varies
over operating conditions.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired
output ripple and transient response. The initial current of a
load transient is provided mainly by the output capacitor. The
output ripple of the converter is:
When using MLCCs, the ESR is typically so low that the
capacitive ripple may dominate. When this occurs, the out-
put ripple will be approximately sinusoidal and 90˚ phase
shifted from the switching action. Given the availability and
quality of MLCCs and the expected output voltage of designs
using the LM2734Z, there is really no need to review any
other capacitor technologies. Another benefit of ceramic ca-
pacitors is their ability to bypass high frequency noise. A
certain amount of switching edge noise will couple through
parasitic capacitances in the inductor to the output. A ce-
ramic capacitor will bypass this noise while a tantalum will
not. Since the output capacitor is one of the two external
components that control the stability of the regulator control
loop, most applications will require a minimum at 10 µF of
output capacitance. Capacitance can be increased signifi-
cantly with little detriment to the regulator stability. Like the
input capacitor, recommended multilayer ceramic capacitors
are X7R or X5R. Again, verify actual capacitance at the
desired operating voltage and temperature.
Check the RMS current rating of the capacitor. The RMS
current rating of the capacitor chosen must also meet the
following condition:
CATCH DIODE
The catch diode (D1) conducts during the switch off-time. A
Schottky diode is recommended for its fast switching times
and low forward voltage drop. The catch diode should be
chosen so that its current rating is greater than:
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To im-
prove efficiency choose a Schottky diode with a low forward
voltage drop.
BOOST DIODE
A standard diode such as the 1N4148 type is recommended.
For V
small-signal Schottky diode is recommended for greater ef-
ficiency. A good choice is the BAT54 small signal diode.
BOOST CAPACITOR
A ceramic 0.01µF capacitor with a voltage rating of at least
6.3V is sufficient. The X7R and X5R MLCCs provide the best
performance.
BOOST
circuits derived from voltages less than 3.3V, a
I
D1
= I
O
x (1-D)