LTC3867
23
3867f
APPLICATIONS INFORMATION
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3867, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
Ceramic capacitors are becoming very popular for small
designs but several cautions should be observed. X7R, X5R
and Y5V are examples of a few of the ceramic materials
used as the dielectric layer, and these different dielectrics
have very different effect on the capacitance value due to
the voltage and temperature conditions applied. Physically,
if the capacitance value changes due to applied voltage
change, there is a concommitant piezo effect which results
in radiating sound! A load that draws varying current at
an audible rate may cause an attendant varying input volt-
age on a ceramic capacitor, resulting in an audible signal.
A secondary issue relates to the energy flowing back into
a ceramic capacitor whose capacitance value is being
reduced by the increasing charge. The voltage can increase
at a considerably higher rate than the constant current being
supplied because the capacitance value is decreasing as
the voltage is increasing! Nevertheless, ceramic capacitors,
when properly selected and used, can provide the lowest
overall loss due to their extremely low ESR.
A small (0.1µF to 1µF) bypass capacitor between the chip
V
IN
pin and ground, placed close to the LTC3867, is also
suggested. A 2.2Ω to 10Ω resistor placed between C
IN
(C1) and V
IN
pin provides further isolation.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The steady-state output ripple (∆V
OUT
) is determined by:
∆V
OUT
≈ ∆I
RIPPLE
ESR+
1
8fC
OUT
where f = operating frequency, C
OUT
= output capacitance
and ∆I
RIPPLE
= ripple current in the inductor. The output
ripple is highest at maximum input voltage since ∆I
RIPPLE
increases with input voltage. The output ripple will be less
than 50mV at maximum V
IN
with ∆I
RIPPLE
= 0.4I
OUT(MAX)
assuming:
C
OUT
required ESR < N • R
SENSE
and
C
OUT
>
1
8f
( )
R
SENSE
( )
The emergence of very low ESR capacitors in small, surface
mount packages makes very small physical implementa-
tions possible. The ability to externally compensate the
switching regulator loop using the I
TH
pin allows a much
wider selection of output capacitor types. The impedance
characteristic of each capacitor type is significantly differ-
ent than an ideal capacitor and therefore requires accurate
modeling or bench evaluation during design. Manufacturers
such as Nichicon, Nippon Chemi-Con and Sanyo should be
considered for high performance through-hole capacitors.
The OS-CON semiconductor dielectric capacitors available
from Sanyo and the Panasonic SP surface mount types
have a good (ESR)(size) product.
Once the ESR requirement for C
OUT
has been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement. Ceramic capacitors from AVX, Taiyo Yuden,
Murata and Tokin offer high capacitance value and very
low ESR, especially applicable for low output voltage
applications.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum
electrolytic and dry tantalum capacitors are both available
in surface mount configurations. New special polymer
surface mount capacitors offer very low ESR also but
have much lower capacitive density per unit volume. In
the case of tantalum, it is critical that the capacitors are
surge tested for use in switching power supplies. Several
excellent choices are the AVX TPS, AVX TPSV, the KEMET
T510 series of surface mount tantalums or the Panasonic
