Datasheet

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LM2735
SNVS485F –JUNE 2007–REVISED APRIL 2013 www.ti.com
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 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
IN
does not 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 depending 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 voltage and the
operating temperature. The ESL of an input capacitor is usually determined by the effective cross sectional 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 ceramic 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 capacitor manufacturer datasheet to see how rated capacitance varies over operating
conditions.
OUTPUT CAPACITOR
The LM2735 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.
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 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 resistance (ESR):
(8)
When using MLCCs, the ESR is typically so low that the capacitive 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,
recommended multilayer ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired
operating voltage 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
OUT
and the FB pin.
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