lm2738xsdx National Semiconductor Corporation, lm2738xsdx Datasheet - Page 11

• Manufacturer Part Number: lm2738xsdx
• Description: Lm2738 550khz/1.6mhz 1.5a Step-down Dc-dc Switching Regulator
• Manufacturer: National Semiconductor Corporation

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 induc-
tor should be specified with a saturation current limit of >
1.25A. There is no need to specify the saturation or peak cur-
rent 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 in-
ductors is huge. Lastly, inductors with lower series resistance
(R
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.The input volt-
age 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
must be greater than:
Neglecting inductor ripple simplifies the above equation to:
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 effec-
tive cross sectional area of the current path. A large leaded
capacitor will have high ESL and a 0805 ceramic chip capac-
itor will have very low ESL. At the operating frequencies of the
LM2738, leaded capacitors may have an ESL so large that
the resulting impedance (2
quired 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. Con-
sult capacitor manufacturer datasheets to see how rated
capacitance varies over operating conditions.
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
DCR
) will provide better operating efficiency. For recom-
π
fL) will be higher than that re-
IN
does not
RMS-IN
)
11
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 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 of 22 µF of output capacitance. Capaci-
tance, 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 ap-
plications. Like the input capacitor, recommended multilayer
ceramic capacitors are X7R or X5R types.
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.
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 10k. When designing a unity gain converter (Vo = 0.8V), R1
should be between 0Ω and 100Ω, and R2 should not be load-
ed.
PCB LAYOUT CONSIDERATIONS
When planning layout there are a few things to consider when
trying to achieve a clean, regulated output. The most impor-
tant consideration is the close coupling of the GND connec-
tions of the input capacitor and the catch diode D1. These
ground ends should be close to one another and be connect-
ed to the GND plane with at least two through-holes. Place
these components as close to the IC as possible. Next in im-
portance is the location of the GND connection of the output
capacitor, which should be near the GND connections of
C
the bottom layer of a two-layer board except under the switch-
ing node island. The FB pin is a high impedance node and
care should be taken to make the FB trace short to avoid noise
pickup and inaccurate regulation. The feedback resistors
should be placed as close as possible to the IC, with the GND
of R1 placed as close as possible to the GND of the IC. The
V
any other traces that are switching. High AC currents flow
through the V
short and wide as possible. However, making the traces wide
increases radiated noise, so the designer must make this
trade-off. Radiated noise can be decreased by choosing a
OUT
IN
and D1. There should be a continuous ground plane on
trace to R2 should be routed away from the inductor and
IN
, SW and V
I
O
D1
V
and the FB pin. A good value for R2
= I
REF
OUT
OUT
= 0.80V
traces, so they should be as
x (1-D)
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