lm2735xsdx National Semiconductor Corporation, lm2735xsdx Datasheet - Page 10

• Manufacturer Part Number: lm2735xsdx
• Description: 520khz/1.6mhz - Space-efficient Boost And Sepic Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
One must also ensure that the minimum current limit (2.1A)
is not exceeded, so the peak current in the inductor must be
calculated. The peak current (I
lated by:
or
When selecting an inductor, make sure that it is capable of
supporting the peak input current without saturating. Inductor
saturation will result in a sudden reduction in inductance and
prevent the regulator from operating correctly. Because of the
speed of the internal current limit, the peak current of the in-
ductor need only be specified for the required maximum input
current. For example, if the designed maximum input current
is 1.5A and the peak current is 1.75A, then the inductor should
be specified with a saturation current limit of >1.75A. There is
no need to specify the saturation or peak current of the in-
ductor at the 3A typical switch current limit.
Because of the operating frequency of the LM2735, ferrite
based inductors are preferred to minimize core losses. This
presents little restriction since the variety of ferrite-based in-
ductors is huge. Lastly, inductors with lower series resistance
(DCR) will provide better operating efficiency. For recom-
mended 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 10 µF to 44 µF de-
pending on the application. The capacitor manufacturer
specifically states the input voltage rating. Make sure to check
any recommended deratings and also verify if there is any
significant change in capacitance at the operating input volt-
age and the operating temperature. The ESL of an input
capacitor is usually determined by the effective cross sec-
tional area of the current path. At the operating frequencies
of the LM2735, 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. Multilayer ce-
ramic capacitors (MLCC) are 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 capac-
itor manufacturer datasheet to see how rated capacitance
varies over operating conditions.
OUTPUT CAPACITOR
The LM2735 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. The output ca-
pacitor 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
impedance will therefore determine the maximum voltage
perturbation. The output ripple of the converter is a function
of the capacitor’s reactance and its equivalent series resis-
tance (ESR):
IL
pk
IL
= I
pk
OUT
= I
IN
LPK
/ D' + ΔI
+ ΔI
) in the inductor is calcu-
L
L
IN
does not
10
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 LM2735, 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 capacitances 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 4.7
µF of output capacitance. Like the input capacitor, recom-
mended multilayer ceramic capacitors are X7R or X5R.
Again, verify actual capacitance at the desired operating volt-
age and temperature.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the following equation where
R1 is connected between the FB pin and GND, and R2 is
connected between V
A good value for R1 is 10kΩ.
COMPENSATION
The LM2735 uses constant frequency peak current mode
control. This mode of control allows for a simple external
compensation scheme that can be optimized for each appli-
cation. A complicated mathematical analysis can be complet-
ed to fully explain the LM2735’s internal & external compen-
sation, but for simplicity, a graphical approach with simple
equations will be used. Below is a Gain & Phase plot of a
LM2735 that produces a 12V output from a 5V input voltage.
The Bode plot shows the total loop Gain & Phase without ex-
ternal compensation.
FIGURE 5. Setting Vout
OUT
and the FB pin.
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