lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 14

• Manufacturer Part Number: lm27342sdx
• Description: 2 Mhz 1.5a/2a Wide Input Range Step-down Dc-dc Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

www.national.com
therefore are safer when there is a severe overload or even
shorted output. Their physical sizes are usually smaller than
the Ferrite core inductors. The downside is their fringing flux
and higher power dissipation due to relatively high AC loss,
especially at high frequencies.
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 Equivalent Series Inductance (ESL).
The recommended input capacitance is 10 µF, although 4.7
µF works well for input voltages below 6V. 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
must be greater than:
where r is the ripple ratio defined earlier, I
current, and D is the duty cycle. 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 ca-
pacitor 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 LM27341/
LM27342, certain 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, Tan-
talum or Niobium, Panasonic SP or Cornell Dubilier Low ESR
are all good choices for input capacitors and have acceptable
ESL. Multilayer ceramic capacitors (MLCC) have very low
ESL. For MLCCs it is recommended to use X7R or X5R di-
electrics. Consult the capacitor manufacturer's datasheet 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 LM27341/2's loop
compensation is designed for ceramic capacitors. A minimum
of 22 µF is required at 2 MHz (33 uF at 1 MHz) while 47 - 100
µF is recommended for improved transient response and
higher phase margin. The output voltage ripple of the con-
verter 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. Another benefit of ceramic capac-
itors 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 capac-
itor will bypass this noise while a tantalum will not.
The transient response is determined by the speed of the
control loop and the ability of the output capacitor to provide
π
fL) will be higher than that re-
OUT
is the output
IN
does not
RMS-IN
)
14
the initial current of a load transient. Capacitance can be in-
creased significantly with little detriment to the regulator sta-
bility. However, increasing the capacitance provides diminin-
shing improvement over 100 uF in most applications,
because the bandwidth of the control loop decreases as out-
put capacitance increases. If improved transient performance
is required, add a feed forward capacitor. This becomes es-
pecially important for higher output voltages where the band-
width of the LM27341/LM27342 is lower. See Feed Forward
Capacitor and Frequency Synchronization sections.
Check the RMS current rating of the capacitor. The RMS cur-
rent rating of the capacitor chosen must also meet the follow-
ing condition:
where I
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 (OPTIONAL)
For circuits with input voltages V
>0.75V. a small-signal Schottky diode is recommended. A
good choice is the BAT54 small signal diode. The cathode of
the diode is connected to the BOOST pin and the anode to a
5V voltage rail.
BOOST CAPACITOR
A ceramic 0.1 µF 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
ue for R2 is 1 kΩ.
FEED FORWARD CAPACITOR (OPTIONAL)
A feed forward capacitor C
sponse of the converter. Place C
value of C
above, the pole of the output capacitor and R
capacitor will increase the crossover frequency of the design,
thus a larger minimum output capacitance is required for de-
signs using C
capacitance greater than or equal to 44 uF.
OUT
FF
is the output current, and r is the ripple ratio.
should place a zero in the loop response at, or
FF
. C
FF
OUT
I
D1
should only be used with an output
= I
and the FB pin. A good starting val-
OUT
FF
can improve the transient re-
x (1-D)
IN
FF
< 5V and duty cycles (D)
in parallel with R1. The
LOAD
. The C
FF