Datasheet

LTC3838
25
3838fa
+
+
–
L
IN
1µH
ESR
(BULK)
V
IN
ESL
(BULK)
C
IN(BULK)
ESR
(CERAMIC)
ESL
(CERAMIC)
I
PULSE(PHASE1)
C
IN(CERAMIC)
I
PULSE(PHASE2)
3838 F06
Figure 6. Circuit Model for Input Capacitor
Ripple Current Simulation
APPLICATIONS INFORMATION
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs temperature curve in the
power MOSFET data sheet. For low voltage MOSFETs,
0.5% per degree (°C) can be used to estimate δ as an
approximation of percentage change of R
DS(ON)
:
δ = 0.005/°C • (T
J
– T
A
)
where T
J
is estimated junction temperature of the MOSFET
and T
A
is ambient temperature.
C
IN
Selection
In continuous mode, the source current of the top N-
channel MOSFET is a square wave of duty cycle V
OUT
/
V
IN
. To prevent large voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The worst-case RMS current occurs by assuming
a single-phase application. The maximum RMS capacitor
current is given by:
I
RMS
≅I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
–1
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT(MAX)
/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
also be paralleled to meet size or height requirements in
the design. Due to the high operating frequency of the
LTC3838, additional ceramic capacitors should also be
used in parallel for C
IN
close to the IC and power switches
to bypass the high frequency switching noises. Typically
multiple X5R or X7R ceramic capacitors are put in parallel
with either conductive-polymer or aluminum-electrolytic
types of bulk capacitors. Because of its low ESR, the
ceramic capacitors will take most of the RMS ripple cur-
rent. Vendors do not consistently specify the ripple current
rating for ceramics, but ceramics could also fail due to
excessive ripple current. Always consult the manufacturer
if there is any question.
Figure 6 represents a simplified circuit model for calculat-
ing the ripple currents in each of these capacitors. The
input inductance (L
IN
) between the input source and the
input of the converter will affect the ripple current through
the capacitors. A lower input inductance will result in less
ripple current through the input capacitors since more
ripple current will now be flowing out of the input source.
For simulations with this model, look at the ripple current
during steady-state for the case where one phase is fully
loaded and the other was not loaded. This will in general
be the worst case for ripple current since the ripple cur-
rent from one phase will not be cancelled by ripple current
from the other phase.
Note that the bulk capacitor also has to be chosen for
RMS rating with ample margin beyond its RMS current
per simulation with the circuit model provided. For a lower
V
IN
range, a conductive-polymer type (such as Sanyo
OS-CON) can be used for its higher ripple current rating
and lower ESR. For a wide V
IN
range that also require
higher voltage rating, aluminum-electrolytic capacitors are