Datasheet

LT3758/LT3758A
15
3758afd
ApplicAtions inForMAtion
Due to the current limit function of the SENSE pin, R
SENSE
should be selected to guarantee that the peak current sense
voltage V
SENSE(PEAK)
during steady state normal operation
is lower than the SENSE current limit threshold (see the
Electrical Characteristics table). Given a 20% margin,
V
SENSE(PEAK)
is set to be 80mV. Then, the maximum
switch ripple current percentage can be calculated using
the following equation:
c =
∆V
SENSE
80mV − 0.5 • ∆V
SENSE
c
is used in subsequent design examples to calculate in-
ductor value. ∆V
SENSE
is the ripple voltage across R
SENSE
.
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 filter
can be added to filter out the excess ringing. Figure 5
shows the RC filter on the SENSE pin. It is usually suf-
ficient to choose 22Ω for R
F LT
and 2.2nF to 10nF for C
FLT
.
Keep R
F LT
’s resistance low. Remember that there is 65µA
(typical) flowing out of the SENSE pin. Adding R
F LT
will
affect the SENSE current limit threshold:
V
SENSE_ILIM
= 110mV – 65µA • R
F LT
APPLICATION CIRCUITS
The LT3758 can be configured as different topologies. The
first topology to be analyzed will be the boost converter,
followed by the flyback, SEPIC and inverting converters.
Boost Converter: Switch Duty Cycle and
Frequency
The
LT3758 can be configured 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:
V
OUT
V
IN
=
1
1− D
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
OUT
) and the input voltage (V
IN
). The maximum
duty cycle (D
MAX
) occurs when the converter has the
minimum input voltage:
D
MAX
=
V
OUT
− V
IN(MIN)
V
OUT
Discontinuous conduction mode (DCM) provides higher
conversion ratios at a given frequency at the cost of reduced
efficiencies and higher switching currents.
Boost Converter: Inductor and Sense Resistor Selection
For the boost topology, the maximum average inductor
current is:
I
L(MAX)
=I
O(MAX)
•
1
1− D
MAX
Then, the ripple current can be calculated by:
∆I
L
= c • I
L(MAX)
= c • I
O(MAX)
•
1
1− D
MAX
Figure 5. The RC Filter on the SENSE Pin
C
FLT
3758 F05
LT3758
R
FLT
R
SENSE
M
1
SENSE
GATE
GND