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| 型号： | LT1767EMS8#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 1.5A 8MSOP |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1.2 V ~ 20 V |
| 输入电压： | 3 V ~ 25 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 1.25MHz |
| 电流 - 输出： | 1.5A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-TSSOP，8-MSOP（0.118"，3.00mm 宽） |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 8-MSOP |
LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
APPLICATIO N S I N FOR M ATIO N
Table 2. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size ESR (Max, ? ) Ripple Current (A)
and reduces the current at which discontinuous operation
occurs. The following formula gives maximum output
current for continuous mode operation, implying that the
AVX TPS, Sprague 593D
AVX TAJ
0.1 to 0.3
0.7 to 0.9
0.7 to 1.1
0.4
peak to peak ripple (2x the term on the right) is less than
the maximum switch current.
D Case Size
AVX TPS, Sprague 593D
C Case Size
AVX TPS
0.1 to 0.3
0.2 (typ)
0.7 to 1.1
0.5 (typ)
Continuous Mode
I OUT ( MAX ) =
I P ?
( V OUT )( V IN ? V OUT )
2 ( L )( f )( V IN )
Figure 3 shows a comparison of output ripple for a ceramic
Discontinuous operation occurs when
and tantalum capacitor at 200mA ripple current.
I OUT ( DIS ) =
(V OUT )
2 ( L )( f )
( 5 ) ( 8 ? 5 )
(
)(
) ( )
I OUT ( ) = 1 . 5 ?
? 6 6
V OUT USING 47μF, 0.1?
TANTALUM CAPACITOR
(10mV/DIV)
V OUT USING 2.2 μ F
CERAMIC CAPACITOR
(10mV/DIV)
V SW
(5V/DIV)
For V IN = 8V, V OUT = 5V and L = 3.3 μ H,
MAX
2 3 . 3 ? 10 1 . 25 ? 10 8
= 1 . 5 ? 0 . 23 = 1 . 27 A
Note that the worst case (minimum output current avail-
able) condition is at the maximum input voltage. For the
same circuit at 15V, maximum output current would be
only 1.1A.
0.2 μ s/DIV
Figure 3. Output Ripple Voltage Waveform
1767 F03
When choosing an inductor, consider maximum load
current, core and copper losses, allowable component
INDUCTOR CHOICE AND MAXIMUM OUTPUT
CURRENT
Maximum output current for a buck converter is equal to
the maximum switch rating (I P ) minus one half peak to
peak inductor current. In past designs, the maximum
switch current has been reduced by the introduction of
slope compensation. Slope compensation is required at
duty cycles above 50% to prevent an affect called
subharmonic oscillation (see Application Note 19 for
details). The LT1767 has a new circuit technique that
maintains a constant switch current rating at all duty
cycles. (Patent Pending)
For most applications, the output inductor will be in the
1 μ H to 10 μ H range. Lower values are chosen to reduce the
physical size of the inductor, higher values allow higher
output currents due to reduced peak to peak ripple current,
height, output voltage ripple, EMI, fault current in the
inductor, saturation, and of course, cost. The following
procedure is suggested as a way of handling these some-
what complicated and conflicting requirements.
1. Choose a value in microhenries from the graphs of
maximum load current. Choosing a small inductor with
lighter loads may result in discontinuous mode of
operation, but the LT1767 is designed to work well in
either mode.
Assume that the average inductor current is equal to
load current and decide whether or not the inductor
must withstand continuous fault conditions. If maxi-
mum load current is 0.5A, for instance, a 0.5A inductor
may not survive a continuous 2A overload condition.
Also, the instantaneous application of input or release
from shutdown, at high input voltages, may cause
sn1767 1767fas
8
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