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| 型号： | LTC1649CS#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/16页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 16-SOIC |
| 标准包装： | 50 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 260kHz |
| 占空比： | 93% |
| 电源电压： | 2.7 V ~ 5 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： | 0°C ~ 70°C |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 包装： | 管件 |
LTC1649
APPLICATIO N S I N FOR M ATIO N
EXTERNAL COMPONENT SELECTION
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC1649 circuits. These should be selected primarily by
efficiency loss at full load for each MOSFET. Assuming
roughly 90% efficiency at this current level, this gives a
P MAX value of (2.5V)(10A/0.9)(0.03) = 833mW per FET
and a required R DSON of:
on-resistance considerations; thermal dissipation is often
a secondary concern in high efficiency designs. The
R DS(ON) (Q1) =
(3.3V)(833mW)
(2.5V)(10A 2 )
= 0.011 ?
R DS(ON) (Q2) = = 0.034 ?
LTC1649isdesignedtobeusedwith5Vlogic-levelMOS-
FETs; “standard” threshold MOSFETs with R DS(ON) speci-
fied at 10V only will not provide satisfactory performance.
MOSFET R DS(ON) should be chosen based on input and
output voltage, allowable power dissipation and maxi-
mum required output current. In a typical LTC1649 buck
converter circuit operating in continuous mode, the aver-
age inductor current is equal to the output load current.
This current is always flowing through either Q1 or Q2 with
the power dissipation split up according to the duty cycle:
(3.3V)(833mW)
(3.3V – 2.5V)(10A 2 )
Note that while the required R DS(ON) values suggest large
MOSFETs, the dissipation numbers are less than a watt per
device— large TO-220 packages and heat sinks are not
necessarily required in high efficiency applications. Siliconix
Si4410DY and International Rectifier IRF7801 are two
small, surface mount devices with R ON values of 0.03 ? or
below with 5V of gate drive; both work well in LTC1649
circuits. A higher P MAX value will generally decrease
DC (Q1) =
V OUT
V IN
MOSFET cost and circuit efficiency and increase MOSFET
heat sink requirements.
DC (Q2) = 1 – OUT
V
V IN
Inductor
The inductor is often the largest component in an LTC1649
=
( V IN – V OUT )
V IN
design and should be chosen carefully. Inductor value and
type should be chosen based on output slew rate require-
ments and expected peak current. Inductor value is prima-
The R ON required for a given conduction loss can now be
calculated by rearranging the relation P = I 2 R:
rily controlled by the required current slew rate. The
maximum rate of rise of the current in the inductor is set
by its value, the input-to-output voltage differential and the
R DS(ON) (Q1) =
P MAX (Q1)
DC(Q1)(I MAX 2 )
maximum duty cycle of the LTC1649. In a typical 3.3V to
2.5V application, the maximum rise time will be:
= IN MAX 2
V (P )(Q1)
V OUT (I MAX )
93%
(V IN – V OUT ) AMPS
L SECOND
=
0.744A I
μ s L
R DS(ON) (Q2) =
V IN (P MAX )(Q2)
P MAX (Q2)
DC(Q2)(I MAX 2 )
=
(V IN – V OUT )(I MAX 2 )
P MAX should be calculated based primarily on required
efficiency. A typical high efficiency circuit designed for
3.3V in, 2.5V at 10A out might require no more than 3%
8
where L is the inductor value in μ H. A 2 μ H inductor would
have a 0.37A/ μ s rise time in this application, resulting in a
14 μ s delay in responding to a 5A load current step. During
this 14 μ s, the difference between the inductor current and
the output current must be made up by the output capaci-
tor, causing a temporary droop at the output. To minimize
this effect, the inductor value should usually be in the 1 μ H
to 5 μ H range for most typical 3.3V to 2.xV LTC1649
circuits. Different combinations of input and output volt-
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