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| 型号： | LT1767EMS8#TR-3.3 |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 1500 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, MSOP-8 |
| 文件页数： | 15/16页 |
| 文件大小： | 232K |
| 代理商： | LT1767EMS8#TR-3.3 |
8
LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
APPLICATIONS INFORMATION
WU
U
VOUT USING 47F, 0.1
TANTALUM CAPACITOR
(10mV/DIV)
Figure 3. Output Ripple Voltage Waveform
0.2
s/DIV
1767 F03
VOUT USING 2.2F
CERAMIC CAPACITOR
(10mV/DIV)
VSW
(5V/DIV)
output currents due to reduced peak to peak ripple current.
The following formula gives maximum output current for
continuous mode operation, implying that the peak to
peak ripple (2x the term on the right) is less than the
maximum switch current.
IOUT MAX
() =
Continuous Mode
IP
()
()
()( )( )
VV
V
Lf V
OUT
IN
OUT
IN
2
For VIN = 8V, VOUT = 5V and L = 3.3H,
I
A
OUT MAX
()
=
()
()
()(
)()
=
15
58 5
2 3 3 10
1 25 10
8
15
0 23
1 27
66
.
..
.
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.
When choosing an inductor, consider maximum load
current, core and copper losses, allowable component
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
saturation of the inductor. In these applications, the
soft-start circuit shown in Figure 10 should be used.
2. Calculate peak inductor current at full load current to
ensure that the inductor will not saturate. Peak current
can be significantly higher than output current, espe-
cially with smaller inductors and lighter loads, so don’t
INDUCTOR CHOICE AND MAXIMUM OUTPUT
CURRENT
Maximum output current for a buck converter is equal to
the maximum switch rating (IP) 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
H to 10H range. Lower values are chosen to reduce the
physical size of the inductor, higher values allow higher
Table 2. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size
ESR (Max,
)
Ripple Current (A)
AVX TPS, Sprague 593D
0.1 to 0.3
0.7 to 1.1
AVX TAJ
0.7 to 0.9
0.4
D Case Size
AVX TPS, Sprague 593D
0.1 to 0.3
0.7 to 1.1
C Case Size
AVX TPS
0.2 (typ)
0.5 (typ)
Figure 3 shows a comparison of output ripple for a ceramic
and tantalum capacitor at 200mA ripple current.
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