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参数资料
| 型号： | LT3845MPFE#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/26页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16TSSOP |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 500kHz |
| 电源电压： | 4 V ~ 60 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -55°C ~ 125°C |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 带卷 (TR) |
LT3845
APPLICATIONS INFORMATION
has a maximum voltage threshold of, typically, 100mV.
Therefore, the peak inductor current is 100mV/R SENSE .
The maximum output load current, I OUT(MAX) , is the peak
inductor current minus half the peak-to-peak ripple cur-
rent, ΔI L .
Allowing adequate margin for ripple current and exter-
nal component tolerances, R SENSE can be calculated as
follows:
architecture that can be programmed over a 100kHz to
500kHz range with a single resistor from the f SET pin to
ground, as shown in Figure 2. The nominal voltage on the
f SET pin is 1V and the current that ?ows from this pin is
used to charge an internal oscillator capacitor. The value
of R SET for a given operating frequency can be chosen
from Figure 2 or from the following equation:
R SET(kΩ) = 8.4 ? 10 4 ? f SW(–1.31)
R SENSE =
70mV
I OUT(MAX)
Table 1 lists typical resistor values for common operating
frequencies.
Typical values for R SENSE are in the range of 0.005Ω
to 0.05Ω.
Table 1. Recommended 1% Standard Values
R SET (kΩ) f SW (kHz)
Operating Frequency
The choice of operating frequency is a trade off between
ef?ciency and component size. Low frequency operation
improves ef?ciency 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
191
118
80.6
63.4
49.9
40.2
33.2
27.4
23.2
100
150
200
250
300
350
400
450
500
ripple. For converters with high step-down V IN to V OUT
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
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 ΔI L , The minimum inductance value is calcu-
lated as follows:
200
180
160
L ≥ V OUT ?
V IN(MAX) – V OUT
f SW ? V IN(MAX) ? Δ I L
140
f SW is the switch frequency.
120
100
80
60
40
20
0
100
200
300
400
500
600
The typical range of values for ΔI L is (0.2 ? I OUT(MAX) ) to
(0.5 ? I OUT(MAX) ), where I OUT(MAX) is the maximum load
current of the supply. Using ΔI L = 0.3 ? I OUT(MAX) yields a
good design compromise between inductor performance
versus inductor size and cost. A value of ΔI L = 0.3 ? I OUT(MAX)
produces a ±15% of I OUT(MAX) ripple current around the DC
FREQUENCY (kHz)
3845 F2
Figure 2. Timing Resistor (R SET ) Value
output current of the supply. Lower values of ΔI L require
larger and more costly magnetics. Higher values of ΔI L
3845fd
14
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相关代理商/技术参数 |
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