Datasheet

LT3957A
13
3957afa
Figure 4. The RC Filter on SENSE1 Pin and SENSE2 Pin
3957A F04
LT3957A
R
FLT
C
FLT
SENSE2
SGND
SENSE1
APPLICATIONS INFORMATION
On-Chip Power Dissipation and Thermal Lockout (TLO)
The on-chip power dissipation of LT3957A can be estimated
using the following equation:
P
IC
≈ I
2
SW
• D • R
DS(ON)
+ V
2
PEAK
• I
SW
• ƒ • 200pF/A +
V
IN
• (1.6mA + ƒ • 10nC)
where R
DS(ON)
is the internal switch on-resistance which
can be obtained from the Typical Performance Characteris-
tics section. V
SW(PEAK)
is the peak switch off-state voltage.
The maximum power dissipation P
IC(MAX)
can be obtained
by comparing P
IC
across all the V
IN
range at the maximum
output current . The highest junction temperature can be
estimated using the following equation:
T
J(MAX)
≈ T
A
+ P
IC(MAX)
• 42°C/W
It is recommended to measure the IC temperature in steady
state to verify that the junction temperature limit is not
exceeded. A low switching frequency may be required to
ensure T
J(MAX)
does not exceed 125°C.
If LT3957A die temperature reaches thermal lockout
threshold at 165°C (typical), the IC will initiate several
protective actions. The power switch will be turned off.
A soft-start operation will be triggered. The IC will be en-
abled again when the junction temperature has dropped
by 5°C (nominal).
is higher than the input voltage. Remember that boost
converters 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: 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
LT3957A 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 sufficient 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
APPLICATION CIRCUITS
The LT3957A 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 LT3957A can be configured as a boost converter
for the applications where the converter output voltage