参数资料
型号: LTC3608IWKG#TRPBF
厂商: Linear Technology
文件页数: 13/26页
文件大小: 0K
描述: IC REG BUCK SYNC ADJ 8A 52QFN
特色产品: 8A, 18V Synchronous Step-Down Regulator
标准包装: 2,000
类型: 降压(降压)
输出类型: 可调式
输出数: 1
输出电压: 0.6 V ~ 17.8 V
输入电压: 4 V ~ 18 V
PWM 型: 电流模式
电流 - 输出: 8A
同步整流器:
工作温度: -40°C ~ 125°C
安装类型: 表面贴装
封装/外壳: 52-VFQFN 裸露焊盘
包装: 带卷 (TR)
供应商设备封装: 52-QFN(7x8)裸露焊盘
LTC3608
APPLICATIONS INFORMATION
V OUT
R VON1
30k
R VON2
100k
R C
C VON
0.01μF
V ON
LTC3608
I TH
2.0
1.5
DROPOUT
REGION
C C
(2a)
1.0
0.5
DUTY CYCLE (V OUT /V IN )
V OUT
INTV CC
R VON1
3k
10k
Q1
2N5087
R VON2
10k
C VON
0.01μF
R C
C C
V ON
LTC3608
I TH
3608 F02
0
0 0.25 0.50 0.75 1.0
3608 F03
Figure 3. Maximum Switching Frequency vs Duty Cycle
V IN(MIN) = V OUT
V OUT = 0.6V ? 1 +
Δ I L = ? OUT ? ? 1 ? OUT ?
V OUT ?
V OUT
? f Δ I L(MAX) ? ?
V IN(MAX) ?
(2b)
Figure 2. Correcting Frequency Shift with Load Current Changes
Minimum Off-time and Dropout Operation
The minimum off-time, t OFF(MIN) , is the smallest amount
of time that the LTC3608 is capable of turning on the bot-
tom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 320ns.
The minimum off-time limit imposes a maximum duty
cycle of t ON /(t ON + t OFF(MIN) ). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
t ON + t OFF(MIN)
t ON
A plot of Maximum Duty Cycle vs Frequency is shown in
Figure 3.
Setting the Output Voltage
The LTC3608 develops a 0.6V reference voltage between
the feedback pin, V FB , and the signal ground as shown in
Figure 6. The output voltage is set by a resistive divider
according to the following formula:
? R2 ?
?
? R1 ?
To improve the frequency response, a feed forward capaci-
tor C1 may also be used. Great care should be taken to
route the V FB line away from noise sources, such as the
inductor or the SW line.
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
? V ? ? V ?
? f L ? ? V IN ?
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I OUT(MAX) . The largest ripple current
occurs at the highest V IN . To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
? ? ?
L = ? ? ? 1 ? ?
3608fc
  
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