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參數(shù)資料
| 型號： | LTC1174-5 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | High Efficiency Step-Down and Inverting DC/DC Converter(高效率,步降DC/DC變換器) |
| 中文描述： | 高效率降壓和倒轉DC / DC轉換器（高效率，步降直流/直流變換器） |
| 文件頁數(shù)： | 7/20頁 |
| 文件大小： | 246K |
| 代理商： | LTC1174-5 |
7
LTC1174
LTC1174-3.3/LTC1174-5
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically there are two kinds of losses in an
inductor, core and copper
Core losses are dependent on the peak-to-peak ripple
current and the core material. However it is independent of
the physical size of the core. By increasing the inductance
the inductor’s peak-to-peak ripple current will decrease,
therefore reducing core loss. Utilizing low core loss mate-
rial, such as molypermalloy or Kool M
μ
will allow users to
concentrate on reducing copper loss and preventing satu-
ration. Figure 1 shows the effect of different core material on
the efficiency of the LTC1174. The CTX core is Kool M
μ
and
the CTXP core is powdered iron (material 52).
Although higher inductance reduces core loss, it increases
copper loss as it requires more windings. When space is not
a premium larger gauge wire can be used to reduce the wire
resistance. This also prevents excessive heat dissipation.
C
IN
In continuous mode the source current of the P-channel
MOSFET is a square wave of duty cycle V
OUT
/V
IN
. To prevent
large voltage transients, a low ESR input capacitor sized for
Figure 1. Efficiency Using Different Types of
Inductor Core Material
(Refer to Functional Diagram)
circuitry is shut off, reducing quiescent current from
0.45mA to 0.13mA. When the output capacitor discharges
by the amount of the hysteresis of the comparator A1, the
P-channel switch turns on again and the process repeats
itself.
Operating Frequency and Inductor
Since the LTC1174 utilizes a constant off-time architecture,
its operating frequency is dependent on the value of V
IN
. The
frequency of operation can be expressed as:
f
t
V
V
V
V
Hz
( )
OFF
IN
OUT
IN
D
=
+
1
where t
OFF
= 4
μ
s and V
D
is the voltage drop across the diode.
Note that the operating frequency is a function of the input
and ouput voltage.
Although the size of the inductor does not affect the fre-
quency, it does affect the ripple current. The peak-to-peak
ripple current is given by:
I
V
V
L
A
RIPPLE
OUT
D
P P
=
+
(
)
4 10
6
By choosing a smaller inductor, a low ESR output filter
capacitor has to be used (see C
IN
and C
OUT
). Moreover, core
loss will also increase (see Inductor Core Selection section)
due to higher ripple current.
OPERATIOU
APPLICATIU
W
U
U
LOAD CURRENT (mA)
1
E
10
100
500
100
90
80
70
60
50
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
CTX50-4
CTX50-4P
1174 F01
LOAD CURRENT (mA)
1
E
10
100
500
100
90
80
70
60
50
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
CTX100-4
CTX100-4P
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