LTC1871 Linear Technology, LTC1871 Datasheet - Page 26

• 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
LTC1871
MOSFET to ensure the V
rating for the device.
During the MOSFET’s on-time, the control circuit limits the
maximum voltage drop across the power MOSFET to
about 150mV (at low duty cycle). The peak inductor
current is therefore limited to 150mV/R
tionship between the maximum load current, duty cycle
and the R
The V
and is reduced to about 100mV at a duty cycle of 92% due
to slope compensation, as shown in Figure 8. The constant
‘ ’ in the denominator represents the ripple current in the
inductors relative to their maximum current. For example,
if 30% ripple current is chosen, then = 0.30. The
accounts for the temperature coefficient of the R
the MOSFET, which is typically 0.4%/ C. Figure 9 illus-
trates the variation of normalized R
ture for a typical power MOSFET.
Another method of choosing which power MOSFET to use
is to check what the maximum output current is for a given
R
discrete values.
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be
known. This power dissipation is a function of the duty
cycle, the load current and the junction temperature itself.
As a result, some iterative calculation is normally required
to determine a reasonably accurate value. Since the con-
troller is using the MOSFET as both a switching and a
sensing element, care should be taken to ensure that the
converter is capable of delivering the required load current
I
26
R
O MAX
DS(ON)
DS ON
(
(
SENSE(MAX)
)
)
since MOSFET on-resistances are available in
DS(ON)
V
V
SENSE MAX
SENSE MAX
R
I
O MAX
DS ON
(
of the power MOSFET is:
term is typically 150mV at low duty cycle
(
(
(
U
)
)
)
)
•
DS
•
U
1
1
remains below the maximum
2
2
1
1
•
•
W
DS(ON)
T
T
•
•
DS(ON)
V
V
over tempera-
V
V
IN MIN
O
O
IN MIN
(
(
U
. The rela-
1
V
1
DS(ON)
V
D
D
)
)
T
term
1
1
of
over all operating conditions (load, line and temperature)
and for the worst-case specifications for V
the R
data sheet.
The power dissipated by the MOSFET in a SEPIC converter
is:
The first term in the equation above represents the I
losses in the device and the second term, the switching
losses. The constant k = 1.7 is an empirical factor inversely
related to the gate drive current and has the dimension of
1/current.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
The R
the R
the board to the ambient temperature in the enclosure.
This value of T
assumption for the junction temperature in the iterative
calculation process.
SEPIC Converter: Output Diode Selection
To maximize efficiency, a fast-switching diode with low
forward drop and low reverse leakage is desired. The
output diode in a SEPIC converter conducts current during
the switch off-time. The peak reverse voltage that the
diode must withstand is equal to V
average forward current in normal operation is equal to the
output current, and the peak current is equal to:
The power dissipated by the diode is:
P
T
P
I
FET
D PEAK
k
J
D
(
TH(JC)
DS(ON)
= T
•
TH(JA)
= I
V
O(MAX)
A
IN MIN
I
)
+ P
O MAX
(
for the device plus the thermal resistance from
(
to be used in this equation normally includes
of the MOSFET listed in the manufacturer’s
FET
1
)
J
• V
)
can then be used to check the original
•R
•
V
2
D
1
O
TH(JA)
–
D
•
1 85
MAX
D
.
I
O MAX
MAX
(
•
I
O MAX
(
)
2
•
•
R
V
V
)
DS ON
IN MIN
O
•
(
(
1
IN(MAX)
D
–
V
MAX
)
D
D
)
SENSE(MAX)
•
MAX
D
MAX
1
+ V
•
C
•
RSS
O
. The
T
and
2
•
R
f