Datasheet

LTC3862
30
3862fc
For more information www.linear.com/LTC3862
For the bulk capacitance, which we assume contributes
1% to the total output ripple, the minimum required ca-
pacitance is approximately:
C
I
nV f
OUT
OMAX
OUT
≥
()
.•
••
001
For many designs it will be necessary to use one type of
capacitor to obtain the required ESR, and another type
to satisfy the bulk capacitance. For example, using a
low ESR ceramic capacitor can minimize the ESR step,
while an electrolytic capacitor can be used to supply the
required bulk C.
The voltage rating of the output capacitor must be greater
than the maximum output voltage, with sufficient derating
to account for the maximum capacitor temperature.
Because the ripple current in the output capacitor is a
square wave, the ripple current requirements for this ca
-
pacitor depend on the duty cycle, the number of phases
and the maximum output current. Figure 24 illustrates the
normalized output capacitor ripple current as a function of
duty cycle. In order to choose a ripple current rating for
the output capacitor
, first establish the duty cycle range,
based on the output voltage and range of input voltage.
Referring to Figure 24, choose the worst-case high nor
-
malized ripple current, as a percentage of the maximum
load current.
The output ripple current is divided between the various
capacitors connected in parallel at the output voltage.
Although ceramic capacitors are generally known for low
ESR (especially X5R and X7R), these capacitors suffer
from a relatively high voltage coefficient. Therefore, it is
not safe to assume that the entire ripple current flows in
the ceramic capacitor. Aluminum electrolytic capacitors are
generally chosen because of their high bulk capacitance,
but they have a relatively high ESR. As a result, some
amount of ripple current will flow in this capacitor. If the
ripple current flowing into a capacitor exceeds its RMS
rating, the capacitor will heat up, reducing its effective
capacitance and adversely affecting its reliability. After
the output capacitor configuration has been determined
using the equations provided, measure the individual ca
-
pacitor case temperatures in order to verify good thermal
performance.
Input Capacitor Selection
The input capacitor voltage rating in a boost converter
should comfortably exceed the maximum input voltage.
Although ceramic capacitors can be relatively tolerant of
overvoltage conditions, aluminum electrolytic capacitors
are not. Be sure to characterize the input voltage for any
possible overvoltage transients that could apply excess
stress to the input capacitors.
The value of the input capacitor is a function of the
source impedance, and in general, the higher the source
impedance, the higher the required input capacitance.
The required amount of input capacitance is also greatly
affected by the duty cycle. High output current applica
-
tions that also experience high duty cycles can place great
demands on the input
supply, both in terms of DC current
and ripple current.
The input ripple current in a multi-phase boost converter
is relatively low (compared with the output ripple current),
because this current is continuous and is being divided
between two or more inductors. Nonetheless, significant
stress can be placed on the input capacitor, especially
applicaTions inForMaTion
Figure 24. Normalized Output Capacitor
Ripple Current (RMS) for a Boost Converter
0.1
I
ORIPPLE
/I
OUT
0.9
3862 F24
0.3
0.5
0.7
0.8
0.2
0.4
0.6
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
DUTY CYCLE OR (1-V
IN
/V
OUT
)
1-PHASE
2-PHASE