Datasheet

LM2738
www.ti.com SNVS556B –APRIL 2008–REVISED APRIL 2013
When selecting an inductor, make sure that it is capable of supporting the peak output 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 inductor need only be
specified for the required maximum output current. For example, if the designed maximum output current is 1.0A
and the peak current is 1.25A, then the inductor should be specified with a saturation current limit of > 1.25A.
There is no need to specify the saturation or peak current of the inductor at the 2.9A typical switch current limit.
Because of the operating frequency of the LM2738, 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 (R
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. The input voltage rating 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
capacitor maximum RMS input current rating (I
RMS-IN
) must be greater than:
(20)
Neglecting inductor ripple simplifies the above equation to:
(21)
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 LM2738, leaded
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, 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 type capacitors due to their tolerance and temperature characteristics. Consult capacitor manufacturer
datasheets 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:
(22)
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 LM2738, 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 of 22 µF of output capacitance. Capacitance, in general, is often increased
when operating at lower duty cycles. Refer to the Circuit Examples at the end of the datasheet for suggested
output capacitances of common applications. Like the input capacitor, recommended multilayer ceramic
capacitors are X7R or X5R types.
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