Datasheet

LT3759
14
3759fc
For more information www.linear.com/3759
The selection of switching frequency is the starting point.
The maximum frequency that can be used is based on the
maximum duty cycle. The conversion ratio as a function
of duty cycle is:
V
OUT
V
IN
=
1
1−D
in continuous conduction mode (CCM). The equations
that follow assume CCM operation.
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
The alternative to CCM, discontinuous conduction mode
(DCM) is not limited by duty cycle to provide high con-
version ratios at a given frequency. The price one pays
is reduced efficiency and substantially higher switching
current.
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:
DI
L
= c • I
L(MAX)
= c • I
O(MAX)
•
1
1- D
MAX
The constant
χ
in the preceding equation represents the
percentage peak-to-peak ripple current in the inductor,
relative to I
L(MAX)
.
The inductor ripple current has a direct effect on the choice
of inductor value. Choosing smaller values of ΔI
L
requires
large inductances and reduces the current loop gain (the
converter will approach voltage mode). Accepting larger
values of ΔI
L
provides fast transient response and allows
the use of low inductances, but results in higher input
current ripple and greater core losses. It is recommended
that
χ
falls within the range of 0.2 to 0.6.
The peak and RMS inductor current are:
I
L(PEAK)
= I
L(MAX)
• 1+
χ
2






I
L(RMS)
= I
L(MAX)
• 1+
χ
2
12
The inductor used with the LT3759 should have a saturation
current rating appropriate to the maximum switch current
selected with the R
SENSE
resistor. Choose an inductor value
based on operating frequency, input and output voltage
to provide a current mode ramp on SENSE during the
switch on-time of approximately 10mV magnitude. The
following equation is useful to estimate the inductor value
for continuous conduction mode operation:
L =
R
SENSE
• V
IN(MIN)
0.01V • f
OSC
• D
MAX
Set the sense voltage at I
L(PEAK)
to be the minimum of the
SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
R
SENSE
=
40mV
I
L(PEAK)
Boost Converter: Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-source voltage rating (V
DS
), the threshold voltage
(V
GS(TH)
), the on-resistance (R
DS(ON)
), the gate to source
and gate to drain charges (Q
GS
and Q
GD
), the maximum
drain current (I
D(MAX)
) and the MOSFET’s thermal resis-
tances (R
θJC
and R
θJA
).
The power MOSFET will see full output voltage, plus a
diode forward voltage, and any additional ringing across
its drain-to-source during its off-time. It is recommended
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