Datasheet

I
RMS-OUT
= I
OUT
x
r
12
'V
OUT
= 'i
L
x (R
ES
R
+
1
8 x f
SW
x C
OUT
)
I
RMS-IN
= I
OUT
x
D x
r
2
12
1 - D +
LM27341, LM27342, LM27341-Q1, LM27342-Q1
www.ti.com SNVS497E –NOVEMBER 2008–REVISED APRIL 2013
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 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
RMS-IN
) must be
greater than:
where
• r is the ripple ratio defined earlier
• I
OUT
is the output current
• D is the duty cycle (18)
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
LM27341/LM27342, 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. Sanyo POSCAP, Tantalum 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 dielectrics. 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 output 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 converter is:
(19)
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. 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.
The transient response is determined by the speed of the control loop and the ability of the output capacitor to
provide the initial current of a load transient. Capacitance can be increased significantly with little detriment to the
regulator stability. However, increasing the capacitance provides dimininshing improvement over 100 uF in most
applications, because the bandwidth of the control loop decreases as output capacitance increases. If improved
transient performance is required, add a feed forward capacitor. This becomes especially important for higher
output voltages where the bandwidth of the LM27341/LM27342 is lower. See FEED FORWARD CAPACITOR
(OPTIONAL) and FREQUENCY SYNCHRONIZATION sections.
Check the RMS current rating of the capacitor. The RMS current rating of the capacitor chosen must also meet
the following condition:
where
• I
OUT
is the output current
• r is the ripple ratio. (20)
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