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參數(shù)資料
| 型號： | LTC3879IUD#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, PQCC16 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-220WEED-4, QFN-16 |
| 文件頁數(shù)： | 5/28頁 |
| 文件大小： | 302K |
| 代理商： | LTC3879IUD#PBF |
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LTC3879
13
3879f
APPLICATIONS INFORMATION
Figure 3 shows how RON relates to switching frequency
for several common output voltages.
When designing for pseudo xed frequency, there is sys-
tematic error because the ION pin voltage is approximately
0.7V, not zero. This causes the ION current to be inversely
proportional to (VIN – 0.7V) and not VIN. The ION current
error increases as VIN decreases. To correct this error, an
additional resistor RON2 can be connected from the ION
pin to the 5.3V INTVCC supply.
R
VV
V
R
ON
2
53
07
=
.– .
.
Likewise, the maximum frequency of operation is deter-
mined by the xed on-time, tON, and the minimum off-time,
tOFF(MIN). The xed on-time is determined by dividing the
duty factor by the nominal frequency of operation:
f
V
Vf
t
Hz
MAX
OUT
IN
OP
OFF MIN
=
+
1
[
]
()
The LTC3879 is a PFM (pulse frequency mode) regula-
tor where pulse density is modulated, not pulse width.
Consequently, frequency increases with a load step and
decreases with a load release. The steady-state operating
frequency, fOP, should be set sufciently below fMAX to
allow for device tolerances and transient response.
Inductor Value Calculation
Given the desired input and output voltages, the induc-
tor value and operation frequency determine the ripple
current:
ΔI
V
fL
V
L
OUT
OP
OUT
IN
=
–
1
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efciency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efciency and operating frequency.
Figure 4. Maximum Switching Frequency vs Duty Cycle
RON (kΩ)
100
SWITCHING
FREQUENCY
(kHz)
1000
10000
3879 F03
VOUT = 1.5V
VOUT = 12V
VOUT = 3.3V
VOUT = 5V
Figure 3. Switching Frequency vs RON
Minimum Off-Time and Dropout Operation
The minimum off-time, tOFF(MIN), is the shortest time
required for the LTC3879 to turn on the bottom MOSFET,
trip the current comparator and then turn off the bottom
MOSFET. This time is typically about 220ns. The minimum
off-time limit imposes a maximum duty cycle of tON/
(tON + tOFF(MIN)). If the maximum duty cycle is reached,
due to a drooping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
VV
tt
t
IN MIN
OUT
ON
OFF MIN
ON
()
=
+
A plot of maximum duty cycle vs. frequency is shown in
Figure 4.
DUTY CYCLE (VOUT/VIN)
DROPOUT
REGION
SWITCHING
FREQUENCY
(MHz)
2
3
3879 F04
1
0
0.25
0.50
0.75
1
4
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| LTC3880EUJ#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 40-QFN RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:PolyPhase® 特色產(chǎn)品:LM3753/54 Scalable 2-Phase Synchronous Buck Controllers 標準包裝:1 系列:PowerWise® PWM 型:電壓模式 輸出數(shù):1 頻率 - 最大:1MHz 占空比:81% 電源電壓:4.5 V ~ 18 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-5°C ~ 125°C 封裝/外殼:32-WFQFN 裸露焊盤 包裝:Digi-Reel® 產(chǎn)品目錄頁面:1303 (CN2011-ZH PDF) 其它名稱:LM3754SQDKR |
| LTC3880EUJ#TRMPBF | 功能描述:IC REG CTRLR BUCK PWM CM 40-QFN RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:PolyPhase® 標準包裝:4,000 系列:- PWM 型:電壓模式 輸出數(shù):1 頻率 - 最大:1.5MHz 占空比:66.7% 電源電壓:4.75 V ~ 5.25 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 85°C 封裝/外殼:40-VFQFN 裸露焊盤 包裝:帶卷 (TR) |
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