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參數(shù)資料
| 型號： | LT1622 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Low Input Voltage Current Mode Step-Down DC/DC Controller(低輸入電壓，電流模式步降DC/DC控制器) |
| 中文描述： | 低輸入電壓電流模式降壓DC / DC控制器（低輸入電壓，電流模式步降直流/直流控制器） |
| 文件頁數(shù)： | 11/16頁 |
| 文件大?。?/td> | 223K |
| 代理商： | LT1622 |
11
LTC1622
APPLICATIO
S I
N
FOR
ATIO
U
Transition Loss = 3(V
IN
)
2
I
O(MAX)
C
RSS
(f)
Other losses including C
IN
and C
OUT
ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
W
U
Run/Soft-Start Function
The RUN/SS pin is a dual purpose pin that provides the
soft-start function and a means to shut down the LTC1622.
Soft-start reduces input surge current from V
IN
by gradu-
ally increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 2.5
μ
A current source charges up an external
capacitor C
SS
. When the voltage on the RUN/SS reaches
0.7V the LTC1622 begins operating. As the voltage on
RUN/SS continues to ramp from 0.7V to 1.8V, the internal
current limit is also ramped at a proportional linear rate.
The current limit begins near 0A (at V
RUN/SS
= 0.7V) and
ends at 0.1/R
SENSE
(V
RUN/SS
≥
1.8V). The output current
thus ramps up slowly, reducing the starting surge current
required from the input power supply. If the RUN/SS has
been pulled all the way to ground, there will be a delay
before the current limit starts increasing and is given by:
t
DELAY
= 2.8 10
5
C
SS
in seconds
Pulling the RUN/SS pin below 0.4V puts the LTC1622 into
a low quiescent current shutdown (I
Q
< 15
μ
A).
Foldback Current Limiting
As described in the Output Diode Selection, the worst-
case dissipation occurs with a short-circuited output
when the diode conducts the current limit value almost
continuously. To prevent excessive heating in the diode,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding diode
D
FB
(1N4148 or equivalent) between the output and the I
TH
pin as shown in Figure 5. In a hard short (V
OUT
= 0V), the
current will be reduced to approximately 50% of the
maximum output current.
V
FB
I
TH
V
OUT
LTC1622
R1
1622 F05
R2
D
FB
+
Figure 5. Foldback Current Limiting
Design Example
Assume the LTC1622 is used in a single lithium-ion
battery-powered cellular phone application. The V
IN
will be
operating from a maximum of 4.2V down to a minimum of
2.7V. Load current requirement is a maximum of 1.5A but
most of the time it will be on standby mode, requiring only
2mA. Efficiency at both low and high load current is
important. Output voltage is 2.5V.
In the above application, Burst Mode operation is enabled
by connecting Pin 5 to V
IN
.
Maximum
V
V
+
V
V
OUT
D
IN MIN
D
Duty Cycle
=
+
=
)
%
93
From Figure 2, SF = 57%.
Use the curve of Figure 2 since the operating frequency is
the free running frequency of the LTC1622.
R
SF
I
A
SENSE
OUT
=
( )(
)(
)
=
( )(
)
=
15
100
0 57
.
15 1 5
0 0253
.
.
In the application, a 0.025
resistor is used. For the
inductor, the required value is:
L
kHz
H
MIN
=
=
4 2 2 5
550
0 036
0 025
2 5 0 3
4 2 0 3
1 33
μ
In the application, a 3.9
μ
H inductor is used to reduce
inductor ripple current and thus, output voltage ripple.
For the selection of the external MOSFET, the R
DS(ON)
must be guaranteed at 2.5V since the LTC1622 has to work
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