LT8330
16
8330fa
For more information
www.linear.com/LT8330
Be sure to check that D
MAX
and D
MIN
obey :
D
MAX
< 1-Minimum Off-Time
(MAX)
•
f
OSC(MAX)
and
D
MIN
> Minimum On-Time
(MAX)
•
f
OSC(MAX)
where Minimum Off-Time, Minimum On-Time and f
OSC
are specified in the Electrical Characteristics table.
Inverting Converter: Inductor, Output Diode and Input
Capacitor Selections
The selections of the inductor, output diode and input
capacitor of an inverting converter are similar to those of
the SEPIC converter. Please refer to the corresponding
SEPIC converter sections.
Inverting Converter: Output Capacitor Selection
The inverting converter requires much smaller output
capacitors than those of the boost, flyback and SEPIC
converters for similar output ripples. This is due to the fact
that, in the inverting converter, the inductor L2 is in series
with the output, and the ripple current flowing through the
output capacitors are continuous. The output ripple voltage
is produced by the ripple current of L2 flowing through
the ESR and bulk capacitance of the output capacitor:
∆
V
OUT(P–P)
= ∆
I
L2
•
ESR
COUT
+
1
8
•
f
•
C
OUT
⎛
⎝
⎜
⎞
⎠
⎟
After specifying the maximum output ripple, the user can
select the output capacitors according to the preceding
equation.
The ESR can be minimized by using high quality X5R or
X7R dielectric ceramic capacitors. In many applications,
ceramic capacitors are sufficient to limit the output volt-
age ripple.
The RMS ripple current rating of the output capacitor
needs to be greater than:
I
RMS(COUT)
> 0.3
•
∆
I
L2
Inverting Converter: Selecting the DC Coupling
Capacitor
The DC voltage rating of the DC coupling capacitor (C
DC
,
as shown in Figure 7) should be larger than the maximum
input voltage minus the output voltage (negative voltage):
V
CDC
> V
IN(MAX)
– V
OUT
C
DC
has nearly a rectangular current waveform. During
the switch off-time, the current through C
DC
is I
IN
, while
approximately –I
O
flows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
I
RMS(CDC)
>
I
O(MAX)
•
D
MAX
1
−
D
MAX
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for C
DC
.
applicaTions inForMaTion
