LM2734YMKX National Semiconductor, LM2734YMKX Datasheet - Page 10

• Manufacturer Part Number: LM2734YMKX
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,BICMOS,TSOP,6PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2734YMKX

• Rohs Compliant: NO

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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
rent. When selecting an inductor, make sure that it is capable
of supporting the peak output current without saturating. In-
ductor saturation will result in a sudden reduction in induc-
tance and prevent the regulator from operating correctly.
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 designed maxi-
mum output current is 0.5A and the peak current is 0.7A, then
the inductor should be specified with a saturation current limit
of >0.7A. There is no need to specify the 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 LM2734, ferrite based inductors
are preferred to minimize core losses. This presents little re-
striction since the variety of ferrite based inductors is huge.
Lastly, inductors with lower series resistance (DCR) will pro-
vide 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, voltage,
RMS current rating, and ESL (Equivalent Series Inductance).
The recommended input capacitance is 10µF, although 4.7µF
works well for input voltages below 6V. The input voltage rat-
ing is specifically stated by the capacitor manufacturer. Make
sure to check any recommended deratings and also verify if
there is any significant change in capacitance at the operating
input voltage and the operating temperature. The input ca-
pacitor maximum RMS input current rating (I
greater than:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate 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 ceramic chip
capacitor will have very low ESL. At the operating frequencies
of the LM2734, 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 capacitors and have
very low ESL. For MLCCs it is recommended to use X7R or
X5R dielectrics. Consult capacitor manufacturer datasheet to
see how rated capacitance varies over operating conditions.
s
is the switching frequency and I
O
is the output cur-
RMS-IN
IN
) must be
does not
10
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired out-
put ripple and transient response. The initial current of a load
transient is provided mainly by the output capacitor. The out-
put ripple of the converter is:
When using MLCCs, the ESR is typically so low that the ca-
pacitive ripple may dominate. When this occurs, the output
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
LM2734, there is really no need to review any other capacitor
technologies. Another benefit of ceramic capacitors is their
ability to bypass high frequency noise. A certain amount of
switching edge noise will couple through parasitic capaci-
tances in the inductor to the output. A ceramic 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. Capaci-
tance can be increased significantly 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 tem-
perature.
Check the RMS current rating of the capacitor. The RMS cur-
rent rating of the capacitor chosen must also meet the follow-
ing 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 effi-
ciency. 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.
OUTPUT VOLTAGE
The output voltage is set using the following equation where
R2 is connected between the FB pin and GND, and R1 is
connected between V
is 10kΩ.
BOOST
circuits derived from voltages less than 3.3V, a
O
I
D1
and the FB pin. A good value for R2
= I
O
x (1-D)