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參數(shù)資料
| 型號(hào)： | LTC1626 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Low Voltage, High Efficiency Step-Down DC/DC Converter(低壓，高效率，步降DC/DC變換器) |
| 中文描述： | 低，高效率步電壓型DC / DC轉(zhuǎn)換器（低壓，高效率，步降直流/直流變換器） |
| 文件頁(yè)數(shù)： | 6/12頁(yè) |
| 文件大?。?/td> | 242K |
| 代理商： | LTC1626 |
6
LTC1626
APPLICATIO
S I
FOR
ATIO
U
inductor current should be continuous during burst peri-
ods. Hence, the peak-to-peak inductor ripple current must
not exceed 25mV/R
SENSE
.
To account for light load conditions, the I
OUT(MAX)
is then
given by:
I
OUT(MAX)
= 150mV/R
SENSE
– 25mV/2R
SENSE
(A)
= 137.5mV/R
SENSE
(A)
Solving for R
SENSE
and allowing a margin of variations in
the LTC1626 and external component values yields:
R
SENSE
= 100mV/I
OUT(MAX)
(
)
The LTC1626 switch is capable of supplying a maximum
of 1.2A of output current. Therefore, the minimum value
of R
SENSE
that can be used is 0.083
. A graph for
selecting R
SENSE
versus maximum output current is given
in Figure 2.
W
U
U
Operating Frequency Considerations
For most applications, the LTC1626 should be operated in
the 100kHz to 300kHz range. This range can be extended,
however, up to 600kHz, to accommodate smaller size/
valued inductors, such as low profile types, with a slight
decrease in efficiency due to gate charge losses. Some
experimentation may be required to determine the opti-
mum operating frequency for a particular set of external
components and operating conditions.
C
T
and L Selection
The value of C
T
is calculated from the desired continuous
mode operating frequency:
(
+
(
C
V
V
)
V
V
V
V
f
F
T
IN
3300
OUT
(
IN
D
IN
BE
O
=
)
)(
)( )
( )
where V
D
is the drop across the Schottky diode D1 and V
BE
is a base-emitter voltage drop (0.6V).
The complete expression for operating frequency is given
by:
f
t
V
V
+
V
V
Hz
O
OFF
IN
OUT
IN
D
≈
( )
1
where:
t
C
V
V
OFF
T
IN
BE
=
(
)( )
(
) (
)
3300
sec
Figure 3 is a graph of operating frequency versus power
supply voltage for the 2.5V regulator circuit shown in
Figure 1 (C
T
= 270pF). Note that the frequency is relatively
constant with supply voltage but drops as the supply
voltage approaches the regulated output voltage.
To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
R
SENSE
of peak-to-peak ripple current. This results in the
following expression for L:
)( )(
Using an inductance smaller than the above value will
result in inductor current being discontinuous. As a con-
L
R
SENSE
≥
( )
(
) ( )
5 2 10
5
.
C
V
H
T
REG
MAXIMUM OUTPUT CURRENT (A)
0.5
0.4
0.3
0.2
0.1
0
R
S
)
1626 F02
0.6
0.8
1.0
0.4
0
0.2
Figure 2. Selecting R
SENSE
During a short circuit of the regulator output to ground, the
peak current is determined by:
I
SC
= 150mV/R
SENSE
(A)
In this condition, the LTC1626 automatically extends the
off-time period of the P-channel MOSFET switch to allow
the inductor current to decay far enough to prevent any
current buildup. The resulting ripple current causes the
average current to be approximately I
OUT(MAX)
.
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