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参数资料
| 型号： | LTC1871IMS#TRPBF-1 |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | LEAD FREE, PLASTIC, MSOP-10 |
| 文件页数： | 8/36页 |
| 文件大小： | 354K |
| 代理商： | LTC1871IMS#TRPBF-1 |
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16
LTC1871-1
18711fa
and the RDS(ON) of the MOSFET listed in the manufacturer’s
data sheet.
The power dissipated by the MOSFET in a boost converter is:
P
I
D
RD
k
FET
OMAX
MAX
DS ON
MAX
T
=
+
()
–
1
2
ρ
–
.
()
V
I
D
Cf
O
OMAX
MAX
RSS
185
1
()
The first term in the equation above represents the I2R
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:
TJ = TA + PFET RTH(JA)
The RTH(JA) to be used in this equation normally includes
the RTH(JC) for the device plus the thermal resistance from
the case to the ambient temperature (RTH(CA)). This value
of TJ can then be compared to the original, assumed value
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.
II
I
D
D PEAK
L PEAK
O MAX
MAX
()
–
== +
1
21
χ
The power dissipated by the diode is:
PD = IO(MAX) VD
and the diode junction temperature is:
TJ = TA + PD RTH(JA)
The RTH(JA) to be used in this equation normally includes
the RTH(JC) for the device plus the thermal resistance from
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
ΔV.
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
ΔV. 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:
ESR
V
I
COUT
O
IN PEAK
≤
001
.
()
where:
I
D
IN PEAK
OMAX
MAX
()
–
=+
1
21
χ
For the bulk C component, which also contributes 1% to
the total ripple:
C
I
Vf
OUT
O MAX
O
≥
()
.
001
APPLICATIO S I FOR ATIO
WU
UU
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