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参数资料
| 型号： | LT1956-5IGN |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 570 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, PLASTIC, SSOP-16 |
| 文件页数： | 16/24页 |
| 文件大小： | 437K |
| 代理商： | LT1956-5IGN |
23
LT1956/LT1956-5
APPLICATIO S I FOR ATIO
WU
UU
point where switch current is 1.5A. The first step is to use
the following formula to calculate the load current where
the switcher must use continuous mode. If your load
current is less than this, use the discontinuous mode
formula to calculate minimum inductor needed. If load
current is higher, use the continuous mode formula.
Output current where continuous mode is needed:
I
VI
VV
V
CONT
IN
P
IN
OUT
IN
OUT
F
=
++
+
() ( )
()(
)
22
4
Minimum inductor discontinuous mode:
L
VI
fI
MIN
OUT
P
=
2
()(
)
()( )
Minimum inductor continuous mode:
L
VV
fV
V
I
VV
V
MIN
IN
OUT
IN
OUT
P
OUT
F
IN
=
++
+
()(
)
()(
)
–
()
21
For a 12V to –12V converter using the LT1956 with peak
switch current of 1.5A and a catch diode of 0.63V:
IA
CONT =
++
+
=
() ( . )
()(
.
)
.
12
1 5
4 1212 1212 0 63
0 370
22
For a load current of 0.25A, this says that discontinuous
mode can be used and the minimum inductor needed is
found from:
LH
MIN ==
212 0 25
500 10
1 5
53
32
()( .
)
(
)( . )
.
In practice, the inductor should be increased by about
30% over the calculated minimum to handle losses and
variations in value. This suggests a minimum inductor of
7
H for this application, but looking at the ripple voltage
chart shows that output ripple voltage could be reduced by
a factor of two by using a 18
H inductor. There is no rule
of thumb here to make a final decision. If modest ripple is
needed and the larger inductor does the trick, go for it. If
ripple is noncritical use the smaller inductor. If ripple is
extremely critical, a second filter may have to be added in
any case, and the lower value of inductance can be used.
Keep in mind that the output capacitor is the other critical
factor in determining output ripple voltage. Ripple shown
on the graph (Figure 15) is with a capacitor ESR of 0.1
.
This is reasonable for an AVX type TPS “D” or “E” size
surface mount solid tantalum capacitor, but the final
capacitor chosen must be looked at carefully for ESR
characteristics.
Ripple Current in the Input and Output Capacitors
Positive-to-negative converters have high ripple current in
both the input and output capacitors. For long capacitor
lifetime, the RMS value of this current must be less than
the high frequency ripple current rating of the capacitor.
The following formula will give an
approximate value for
RMS ripple current.
This formula assumes continuous
mode and large inductor value. Small inductors will give
somewhat higher ripple current, especially in discontinu-
ous mode. The exact formulas are very complex and
appear in Application Note 44, pages 29 and 30. For our
purposes here I have simply added a fudge factor (ff). The
value for ff is about 1.2 for higher load currents and
L
≥15H. It increases to about 2.0 for smaller inductors at
lower load currents.
Capacitor I
ff I
V
RMS
OUT
IN
= ()(
)
ff = 1.2 to 2.0
Diode Current
Average diode current is equal to load current. Peak diode
current will be considerably higher.
Peak diode current:
Continuous Mode
I
VV
V
VV
Lf V
V
Discontinuous Mode
IV
Lf
OUT
IN
OUT
IN
OUT
IN
OUT
=
+
=
()
(
)(
)
()( )(
)
()(
)
()( )
2
Keep in mind that during start-up and output overloads,
average diode current may be much higher than with
normal loads. Care should be used if diodes rated less than
1A are used, especially if continuous overload conditions
must be tolerated.
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