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| 型号： | LTC3808EDE#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 15/28页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 14-DFN |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 825kHz |
| 占空比： | 100% |
| 电源电压： | 2.75 V ~ 9.8 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： | -40°C ~ 85°C |
| 封装/外壳： | 14-WFDFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
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LTC3808
APPLICATIO S I FOR ATIO
V IN > 5V) may work fine at lower voltages (e.g., 3.3V).
Selecting the N-channel MOSFET is typically easier, since
for a given R DS(ON) , the gate charge and turn-on and turn-
off delays are much smaller than for a P-channel MOSFET.
Using a Sense Resistor
A sense resistor R SENSE can be connected between SENSE +
and SENSE – to sense the output load current. In this case,
the source of the P-channel MOSFET is connected to
SENSE – pin and the drain is connected to SW pin of
LTC3808. Therefore the current comparator monitors the
voltage developed across R SENSE instead of V DS of the
P-channel MOSFET. The output current that the LTC3808
can provide in this case is given by:
spread spectrum operation. Pulling the PLLLPF to V IN
selects 750kHz operation; pulling the PLLLPF to GND
selects 300kHz operation.
Alternatively, the LTC3808 will phase-lock to a clock signal
applied to the SYNC/MODE pin with a frequency between
250kHz and 750kHz (see Phase-Locked Loop and Fre-
quency Synchronization).
To further reduce EMI, the nominal 550kHz frequency will
be spread over a range with frequencies between 460kHz
and 635kHz when spread spectrum modulation is
enabled (see Spread Spectrum Modulation with
SYNC/MODE and PLLLPF Pins).
Inductor Value Calculation
– RIPPLE
I OUT ( MAX ) =
? V SENSE ( MAX )
R SENSE
I
2
Given the desired input and output voltages, the inductor
value and operating frequency, f OSC , directly determine
the inductor’s peak-to-peak ripple current:
V OUT V IN – V OUT
Setting ripple current as 40% of I OUT(MAX) and using
Figure 1 to choose SF, the value of R SENSE is:
I RIPPLE =
V IN f OSC ? L
?
R SENSE =
5
6
? SF ?
? V SENSE ( MAX )
I OUT ( MAX )
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
See the P-channel R DS(ON) selection in Power MOSFET
Selection.
Variation in the resistance of a sense resistor is much
smaller than the variation in on-resistance of the external
MOSFET. Therefore the load current is well controlled with
a sense resistor. However the sense resistor causes extra
I 2 R losses in addition to the I 2 R losses of the MOSFET.
Therefore, using a sense resistor lowers the efficiency of
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I OUT(MAX). Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
? OUT
LTC3808, especially for large load current.
Operating Frequency and Synchronization
L ≥
V IN – V OUT
f OSC ? I RIPPLE
V
V IN
The choice of operating frequency, f OSC , is a trade-off
between efficiency and component size. Low frequency
operation improves efficiency by reducing MOSFET switch-
ing losses, both gate charge loss and transition loss.
However, lower frequency operation requires more induc-
tance for a given amount of ripple current.
Burst Mode Operation Considerations
The choice of R DS(ON) and inductor value also determines
the load current at which the LTC3808 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
The internal oscillator for the LTC3808’s controller runs at
a nominal 550kHz frequency when the PLLLPF pin is left
floating and the SYNC/MODE pin is not configured for
I BURST ( PEAK ) =
1 ? V SENSE ( MAX )
?
4 R DS ( ON )
3808f
15
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| LTC3808EGN#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 16-SSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:2,500 系列:- PWM 型:电流模式 输出数:1 频率 - 最大:500kHz 占空比:96% 电源电压:4 V ~ 36 V 降压:无 升压:是 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-40°C ~ 125°C 封装/外壳:24-WQFN 裸露焊盘 包装:带卷 (TR) |
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