LTC1871 Linear Technology, LTC1871 Datasheet - Page 16

• 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
and the R
data sheet.
The power dissipated by the MOSFET in a boost 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 in-
versely related to the gate drive current and has the dimen-
sion 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 case to the ambient temperature (R
of T
used in the iterative calculation process.
Boost 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 boost converter conducts current during
the switch off-time. The peak reverse voltage that the
diode must withstand is equal to the regulator output
voltage. The average forward current in normal operation
is equal to the output current, and the peak current is equal
to the peak inductor current.
The power dissipated by the diode is:
16
T
P
P
I
D PEAK
J
J
D
FET
(
TH(JC)
can then be compared to the original, assumed value
= T
TH(JA)
= I
O(MAX)
A
DS(ON)
k V
)
+ P
for the device plus the thermal resistance from
1
•
to be used in this equation normally includes
I
O MAX
–
I
FET
L PEAK
(
O
D
(
of the MOSFET listed in the manufacturer’s
1 85
• V
MAX
.
• R
D
)
U
•
)
TH(JA)
2
1
I
O MAX
–
•
(
1
U
R
D
DS ON
MAX
2
(
)
•
)
•
1
•
C
I
W
O MAX
D
–
RSS
(
MAX
D
TH(CA)
MAX
•
)
•
f
T
). This value
U
2
R
and the diode junction temperature is:
The R
the R
the board to the ambient temperature in the enclosure.
Remember to keep the diode lead lengths short and to
observe proper switch-node layout (see Board Layout
Checklist) to avoid excessive ringing and increased
dissipation.
Boost Converter: Output Capacitor Selection
Contributions of ESR (equivalent series resistance), ESL
(equivalent series inductance) and the bulk capacitance
must be considered when choosing the correct compo-
nent for a given output ripple voltage. The effects of these
three parameters (ESR, ESL and bulk C) on the output
voltage ripple waveform are illustrated in Figure 12e for a
typical boost converter.
The choice of component(s) begins with the maximum
acceptable ripple voltage (expressed as a percentage of
the output voltage), and how this ripple should be divided
between the ESR step and the charging/discharging V.
For the purpose of simplicity we will choose 2% for the
maximum output ripple, to be divided equally between
the ESR step and the charging/discharging V. This
percentage ripple will change, depending on the require-
ments of the application, and the equations provided
below can easily be modified.
For a 1% contribution to the total ripple voltage, the ESR
of the output capacitor can be determined using the
following equation:
where:
T
I
ESR
IN PEAK
J
TH(JC)
(
= T
TH(JA)
COUT
A
+ P
)
for the device plus the thermal resistance from
to be used in this equation normally includes
D
1
0 01
I
• R
IN PEAK
. •
(
2
TH(JA)
V
•
O
)
1
I
O MAX
–
(
D
MAX
)