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| 型号： | LTC3812IFE-5#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/34页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16TSSOP |
| 标准包装： | 95 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 93% |
| 电源电压： | 6.2 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： | -40°C ~ 125°C |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 管件 |
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LTC3812-5
APPLICATIONS INFORMATION
MINIMUM OFF-TIME AND DROPOUT OPERATION
The minimum off-time t OFF(MIN) is the smallest amount of
time that the LTC3812-5 is capable of turning on the bot-
tom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 250ns.
The minimum off-time limit imposes a maximum duty
cycle of t ON /(t ON + t OFF(MIN) ). If the maximum duty cycle
this requires a large inductor. There is a tradeoff between
component size, ef?ciency 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 speci?ed maximum, the inductance
should be chosen according to:
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:
L =
V OUT
f I L(MAX)
1
V OUT
V IN(MAX)
V IN(MIN) = V OUT
t ON + t OFF(MIN)
t ON
Once the value for L is known, the type of inductor must
be selected. High ef?ciency converters generally cannot
afford the core loss found in low cost powdered iron cores,
A plot of maximum duty cycle vs frequency is shown in
Figure 8.
INDUCTOR SELECTION
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
forcing the use of more expensive ferrite, molypermalloy
or Kool Mμ ? cores. A variety of inductors designed for
high current, low voltage applications are available from
manufacturers such as Sumida, Panasonic, Coiltronics,
Coilcraft and Toko.
SCHOTTKY DIODE D1 SELECTION
I L =
V OUT
fL
1
V OUT
V IN
The Schottky diode D1 shown in the front page schematic
conducts during the dead time between the conduction of
the power MOSFET switches. It is intended to prevent the
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest ef?ciency operation is obtained at low
frequency with small ripple current. However, achieving
body diode of the bottom MOSFET from turning on and
storing charge during the dead time, which can cause a
modest (about 1%) ef?ciency loss. The diode can be rated
for about one half to one ?fth of the full load current since
2.0
1.5
DROPOUT
REGION
1.0
0.5
0
0
0.25 0.50 0.75
1.0
DUTY CYCLE (V OUT /V IN )
38125 F08
Figure 8. Maximum Switching Frequency vs Duty Cycle
38125fc
17
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