LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 25

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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 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, fl yback 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 fl owing through the
output capacitors are continuous. The output ripple voltage
is produced by the ripple current of L2 fl owing through the
ESR and bulk capacitance of the output capacitor:
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 suffi cient to limit the output volt-
age ripple.
The RMS ripple current rating of the output capacitor
needs to be greater than:
Inverting Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor
(C
maximum input voltage minus the output voltage (nega-
tive voltage):
C
the switch off-time, the current through C
approximately –I
DC
DC
I
V
Δ
RMS(COUT)
CDC
has nearly a rectangular current waveform. During
V
, as shown in Figure 10) should be larger than the
OUT P P
> V
( – )
IN(MAX)
> 0.3 • ΔI
=
Δ
O
I
– V
fl ows during the on-time. The RMS
L
2
•
OUT
L2
⎛
⎜
⎝
ESR
COUT
+
8
• •
f C
1
DC
OUT
is I
⎞
⎟
⎠
IN
, while
rating of the coupling capacitor is determined by the fol-
lowing equation:
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for C
Board Layout
The high speed operation of the LT3758 demands careful
attention to board layout and component placement. The
Exposed Pad of the package is the only GND terminal of
the IC, and is important for thermal management of the
IC. Therefore, it is crucial to achieve a good electrical and
thermal contact between the Exposed Pad and the ground
plane of the board. For the LT3758 to deliver its full output
power, it is imperative that a good thermal path be pro-
vided to dissipate the heat generated within the package.
It is recommended that multiple vias in the printed circuit
board be used to conduct heat away from the IC and into
a copper plane with as much area as possible.
To prevent radiation and high frequency resonance
problems, proper layout of the components connected
to the IC is essential, especially the power paths with
higher di/dt. The following high di/dt loops of different
topologies should be kept as tight as possible to reduce
inductive ringing:
• In boost confi guration, the high di/dt loop contains
• In fl yback confi guration, the high di/dt primary loop
• In SEPIC confi guration, the high di/dt loop contains
• In inverting confi guration, the high di/dt loop con-
the output capacitor, the sensing resistor, the power
MOSFET and the Schottky diode.
contains the input capacitor, the primary winding, the
power MOSFET and the sensing resistor. The high
di/dt secondary loop contains the output capacitor,
the secondary winding and the output diode.
the power MOSFET, sense resistor, output capacitor,
Schottky diode and the coupling capacitor.
tains power MOSFET, sense resistor, Schottky diode
and the coupling capacitor.
I
RMS CDC
(
DC
)
.
>
I
O MAX
(
)
•
− 1
D
MAX
D
MAX
LT3758
25
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