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參數(shù)資料
| 型號： | LTC3862IUH#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | DUAL SWITCHING CONTROLLER, 500 kHz SWITCHING FREQ-MAX, QCC24 |
| 封裝： | 5 X 5 MM, 0.65 MM PITCH, LEAD FREE, PLASTIC, QFN-24 |
| 文件頁數(shù)： | 20/40頁 |
| 文件大?。?/td> | 613K |
| 代理商： | LTC3862IUH#TRPBF |
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LTC3862
27
3862fb
result, some iterative calculation is normally required to
determine a reasonably accurate value.
The power dissipated by the MOSFET in a multi-phase
boost converter with n phases is:
P
I
nD
RD
FET
OMAX
MAX
DS ON
MAX
=
()
()
–
1
2
ρρ
T
OUT
OMAX
MAX
RSS
kV
I
nD
Cf
+
()
–
()
2
1
The rst 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 inversely
related to the gate drive current and has the dimension
of 1/current.
The
ρT term accounts for the temperature coefcient of
the RDS(ON) of the MOSFET, which is typically 0.4%/C.
Figure 19 illustrates the variation of normalized RDS(ON)
over temperature for a typical power MOSFET.
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.
It is tempting to choose a power MOSFET with a very low
RDS(ON) in order to reduce conduction losses. In doing
so, however, the gate charge QG is usually signicantly
higher, which increases switching and gate drive losses.
Since the switching losses increase with the square of
the output voltage, applications with a low output voltage
generally have higher MOSFET conduction losses, and
high output voltage applications generally have higher
MOSFET switching losses. At high output voltages, the
highest efciency is usually obtained by using a MOSFET
with a higher RDS(ON) and lower QG. The equation above
can easily be split into two components (conduction and
switching) and entered into a spreadsheet, in order to
compare the performance of different MOSFETs.
Programming the Current Limit
The peak sense voltage threshold for the LTC3862 is 75mV
at low duty cycle and with a normalized slope gain of
1.00, and is measured from SENSE+ to SENSE–. Figure 20
illustrates the change in the sense threshold with varying
duty cycle and slope gain.
APPLICATIONS INFORMATION
Figure 19. Normalized Power MOSFET RDS(ON) vs Temperature
JUNCTION TEMPERATURE (°C)
–50
TNORMALIZED
ON
RESISTANCE
1.0
1.5
150
3862 F19
0.5
0
50
100
2.0
DUTY CYCLE (%)
50
MAXIMUM
CURRENT
SENSE
THRESHOLD
(mV)
60
70
80
55
65
75
20
40
60
80
3862 F20
100
10
030
50
70
90
SLOPE = 0.625
SLOPE = 1.66
SLOPE = 1
Figure 20. Maximum Sense Voltage Variation
with Duty Cycle and Slope Gain
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