LTC1871 Linear Technology, LTC1871 Datasheet - Page 25

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
It is worth noting here that SEPIC converters that operate
at high duty cycles (i.e., that develop a high output voltage
from a low input voltage) can have very high input cur-
rents, relative to the output current. Be sure to check that
the maximum load current will not overload the input
supply.
SEPIC Converter: Inductor Selection
For most SEPIC applications the equal inductor values will
fall in the range of 10 H to 100 H. Higher values will
reduce the input ripple voltage and reduce the core loss.
Lower inductor values are chosen to reduce physical size
and improve transient response.
Like the boost converter, the input current of the SEPIC
converter is calculated at full load current and minimum
input voltage. The peak inductor current can be signifi-
cantly higher than the output current, especially with
smaller inductors and lighter loads. The following formu-
las assume CCM operation and calculate the maximum
peak inductor currents at minimum V
The ripple current in the inductor is typically 20% to 40%
(i.e., a range of ‘ ’ from 0.20 to 0.40) of the maximum
average input current occurring at V
and I
function of the output current results in the following
equations for calculating the inductor value:
By making L1 = L2 and winding them on the same core, the
value of inductance in the equation above is replace by 2L
where
I
I
L
L PEAK
L PEAK
1
2
I
(
L
(
:
L1
V
IN MIN
I
(
L
)
= I
)
•
•
I
O MAX
f
)
(
L2
1
1
•
D
. Expressing this ripple current as a
MAX
2
U
)
2
•
1
•
•
I
–
D
I
O MAX
O MAX
MAX
D
U
(
(
MAX
)
)
•
•
V
V
V
IN MIN
O
W
IN MIN
(
V
(
IN MIN
IN(MIN)
IN
V
(
:
D
)
)
)
V
and I
D
U
O(MAX)
due to mutual inductance. Doing this maintains the same
ripple current and energy storage in the inductors. For
example, a Coiltronix CTX10-4 is a 10 H inductor with two
windings. With the windings in parallel, 10 H inductance
is obtained with a current rating of 4A (the number of turns
hasn’t changed, but the wire diameter has doubled).
Splitting the two windings creates two 10 H inductors
with a current rating of 2A each. Therefore, substituting 2L
yields the following equation for coupled inductors:
Specify the maximum inductor current to safely handle
I
current rating for the inductor should be checked at the
minimum input voltage (which results in the highest
inductor current) and maximum output current.
SEPIC Converter: Power MOSFET Selection
The power MOSFET serves two purposes in the LTC1871:
it represents the main switching element in the power
path, and its R
element for the control loop. Important parameters for the
power MOSFET include the drain-to-source breakdown
voltage (BV
resistance (R
gate-to-source and gate-to-drain charges (Q
respectively), the maximum drain current (I
the MOSFET’s thermal resistances (R
The gate drive voltage is set by the 5.2V INTV
dropout regulator. Consequently, logic-level threshold
MOSFETs should be used in most LTC1871 applications.
If low input voltage operation is expected (e.g., supplying
power from a lithium-ion battery), then sublogic-level
threshold MOSFETs should be used.
The maximum voltage that the MOSFET switch must
sustain during the off-time in a SEPIC converter is equal to
the sum of the input and output voltages (V
result, careful attention must be paid to the BV
fications for the MOSFETs relative to the maximum actual
switch voltage in the application. Many logic-level devices
are limited to 30V or less. Check the switching waveforms
directly across the drain and source terminals of the power
L(PK)
L
1
specified in the equation above. The saturation
L
2
DSS
2
DS(ON)
V
•
), the threshold voltage (V
IN MIN
DS(ON)
(
I
L
) versus gate-to-source voltage, the
•
)
f
•
represents the current sensing
D
MAX
TH(JC)
LTC1871
GS(TH)
O
and R
GS
D(MAX)
+ V
DSS
), the on-
and Q
IN
25
TH(JA)
CC
). As a
speci-
) and
low
GD
).
,