LT3957
14
3957f
APPLICATIONS INFORMATION
APPLICATION CIRCUITS
The LT3957 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 LT3957 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
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
effi ciencies and higher switching currents.
Boost Converter: Maximum Output Current Capability
and Inductor Selection
For the boost topology, the maximum average inductor
current is:
I
L(MAX)
= I
O(MAX)
•
1
1−D
MAX
Due to the current limit of its internal power switch, the
LT3957 should be used in a boost converter whose maxi-
mum output current (I
O(MAX)
) is less than the maximum
output current capability by a suffi cient margin (10% or
higher is recommended):
I
O(MAX)
<
V
IN(MIN)
V
OUT
•5A− 0.5 • ΔI
SW
(
)
The inductor ripple current ΔI
SW
has a direct effect on the
choice of the inductor value and the converter’s maximum
output current capability. Choosing smaller values of
ΔI
SW
increases output current capability, but
requires
large inductances and reduces the current loop gain (the
converter will approach voltage mode). Accepting larger
values of ΔI
SW
provides fast transient response and
allows the use of low inductances, but results in higher
input current ripple and greater core losses, and reduces
output current capability.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value of the boost converter can be determined
using the following equation:
L =
V
IN(MIN)
ΔI
SW
•ƒ
•D
MAX
The peak inductor current is the switch current limit (5.9A
typical), and the RMS inductor current is approximately
equal to I
L(MAX)
. The user should choose the inductors
having suffi cient saturation and RMS current ratings.
Boost Converter: Output Diode Selection
To maximize effi ciency, a fast switching diode with low
forward drop and low reverse leakage is desirable. The
peak reverse voltage that the diode must withstand is
equal to the regulator output voltage plus any additional
ringing across its anode-to-cathode during the on-time.
The average forward current in normal operation is equal
to the output current.
It is recommended that the peak repetitive reverse voltage
rating V
RRM
is higher than V
OUT
by a safety margin (a 10V
safety margin is usually suffi cient).
