LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 16

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

APPLICATIONS INFORMATION
LT3758
switch ripple current percentage can be calculated using
the following equation:
χ is used in subsequent design examples to calculate induc-
tor value. ΔV
The LT3758 switching controller incorporates 100ns timing
interval to blank the ringing on the current sense signal
immediately after M1 is turned on. This ringing is caused
by the parasitic inductance and capacitance of the PCB
trace, the sense resistor, the diode, and the MOSFET. The
100ns timing interval is adequate for most of the LT3758
applications. In the applications that have very large and
long ringing on the current sense signal, a small RC fi lter
can be added to fi lter out the excess ringing. Figure 5
shows the RC fi lter on the SENSE pin. It is usually suf-
fi cient to choose 22Ω for R
Keep R
(typical) fl owing out of the SENSE pin. Adding R
affect the SENSE current limit threshold:
16
V
χ =
SENSE_ILIM
80
FLT
mV
’s resistance low. Remember that there is 65μA
Figure 5. The RC Filter on the SENSE Pin
SENSE
−
Δ
= 110mV – 65μA • R
V
0 5 . •
LT3758
SENSE
GND
is the ripple voltage across R
Δ
SENSE
V
GATE
SENSE
FLT
and 2.2nF to 10nF for C
R
C
FLT
FLT
FLT
R
3758 F05
M
SENSE
1
SENSE
FLT
FLT
will
.
.
APPLICATION CIRCUITS
The LT3758 can be confi gured as different topologies. The
fi rst topology to be analyzed will be the boost converter,
followed by the fl yback, SEPIC and inverting converters.
Boost Converter: Switch Duty Cycle and Frequency
The LT3758 can be confi gured as a boost converter for
the applications where the converter output voltage is
higher than the input voltage. Remember that boost con-
verters are not short-circuit protected. Under a shorted
output condition, the inductor current is limited only by
the input supply capability. For applications requiring a
step-up converter that is short-circuit protected, please
refer to the Applications Information section covering
SEPIC converters.
The conversion ratio as a function of duty cycle is
in continuous conduction mode (CCM).
For a boost converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (V
duty cycle (D
minimum input voltage:
Discontinuous conduction mode (DCM) provides higher
conversion ratios at a given frequency at the cost of reduced
effi ciencies and higher switching currents.
Boost Converter: Inductor and Sense Resistor Selection
For the boost topology, the maximum average inductor
current is:
Then, the ripple current can be calculated by:
I
D
ΔI
V
L MAX
V
MAX
(
OUT
L
IN
=
χ
=
=
)
OUT
=
•
1
V
I
I
−
OUT
L MAX
1
O MAX
) and the input voltage (V
(
MAX
D
(
V
−
) occurs when the converter has the
OUT
)
V
)
=
IN MIN
•
1
χ
(
−
•
D
I
O MAX
1
)
MAX
(
)
•
1
−
D
1
IN
MAX
). The maximum
3758f