Datasheet

LT3757/LT3757A
25
3757afe
For more information www.linear.com/LT3757
Inverting Converter: Switch Duty Cycle and Frequency
For an inverting converter operating in CCM, the duty
cycle of the main switch can be calculated based on the
negative output voltage (V
OUT
) and the input voltage (V
IN
).
The maximum duty cycle (D
MAX
) occurs when the con-
verter has the minimum input voltage:
D
MAX
=
V
OUT
−
V
D
V
OUT
− V
D
− V
IN(MIN)
Inverting Converter: Inductor, Sense Resistor, Power
MOSFET, Output Diode and Input Capacitor Selections
The selections of the inductor, sense resistor, power
MOSFET, output diode and input capacitor of an invert-
ing 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 flow-
ing through the ESR and bulk capacitance of the output
capacitor:
ΔV
OUT(P–P)
= ΔI
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 10) should be larger than the maxi-
mum 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
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