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參數(shù)資料
| 型號： | LT3845EFE#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 600 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 5/24頁 |
| 文件大?。?/td> | 263K |
| 代理商： | LT3845EFE#TR |
LT3845
13
3845fb
has a maximum voltage threshold of, typically, 100mV.
Therefore, the peak inductor current is 100mV/RSENSE.
The maximum output load current, IOUT(MAX), is the peak
inductor current minus half the peak-to-peak ripple cur-
rent,
ΔIL.
Allowing adequate margin for ripple current and exter-
nal component tolerances, RSENSE can be calculated as
follows:
R
mV
I
SENSE
OUT MAX
=
70
()
Typical values for RSENSE are in the range of 0.005Ω
to 0.05
Ω.
Operating Frequency
The choice of operating frequency is a trade off between
efciency and component size. Low frequency operation
improves efciency by reducing MOSFET switching losses
and gate charge losses. However, lower frequency opera-
tion requires more inductance for a given amount of ripple
current, resulting in a larger inductor size and higher cost.
If the ripple current is allowed to increase, larger output
capacitors may be required to maintain the same output
ripple. For converters with high step-down VIN to VOUT
ratios, another consideration is the minimum on-time of
the LT3845 (see the Minimum On-time Considerations
section). A nal consideration for operating frequency is
that in noise-sensitive communications systems, it is often
desirable to keep the switching noise out of a sensitive
frequency band. The LT3845 uses a constant frequency
APPLICATIONS INFORMATION
architecture that can be programmed over a 100kHz to
500kHz range with a single resistor from the fSET pin to
ground, as shown in Figure 2. The nominal voltage on the
fSET pin is 1V and the current that ows from this pin is
used to charge an internal oscillator capacitor. The value
of RSET for a given operating frequency can be chosen
from Figure 2 or from the following equation:
RSET(kΩ) = 8.4 10
4 fSW(–1.31)
Table 1 lists typical resistor values for common operating
frequencies.
Table 1. Recommended 1% Standard Values
RSET
fSW
191k
Ω
100kHz
118k
Ω
150kHz
80.6k
Ω
200kHz
63.4k
Ω
250kHz
49.9k
Ω
300kHz
40.2k
Ω
350kHz
33.2k
Ω
400kHz
27.4k
Ω
450kHz
23.2k
Ω
500kHz
Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value, volt-second product, satura-
tion current and/or RMS current.
For a given
ΔIL, The minimum inductance value is calcu-
lated as follows:
LV
VV
fV
I
OUT
IN MAX
OUT
SW
IN MAX
L
≥
Δ
–
()
fSW is the switch frequency.
The typical range of values for
ΔIL is (0.2 IOUT(MAX)) to
(0.5 IOUT(MAX)), where IOUT(MAX) is the maximum load
current of the supply. Using
ΔIL = 0.3 IOUT(MAX) yields a
good design compromise between inductor performance
versus inductor size and cost. A value of
ΔIL=0.3IOUT(MAX)
produces a ±15% of IOUT(MAX) ripple current around the DC
output current of the supply. Lower values of
ΔIL require
larger and more costly magnetics. Higher values of
ΔIL
FREQUENCY (kHz)
0
20
R
SET
(k
Ω
)
40
80
100
120
400
200
!&"# .
60
200
100
500
300
600
140
160
180
Figure 2. Timing Resistor (RSET) Value
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