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參數(shù)資料
| 型號： | LTC1772 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23 |
| 中文描述： | 恒定頻率電流模式降壓DC / DC控制器，采用SOT - 23 |
| 文件頁數(shù)： | 10/16頁 |
| 文件大?。?/td> | 188K |
| 代理商： | LTC1772 |
10
LTC1772
APPLICATIO
S I
N
FOR
ATIO
U
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 continu-
ously. 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 diodes
D
FB1
and D
FB2
between the output and the I
TH
/RUN 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.
W
U
In the application, a 0.03
resistor is used. For the
inductor, the required value is:
L
kHz
H
MIN
=
+
+
=
4 2 2 5
.
550
0 0288
0 03
.
2 5
4 2
.
0 3
0 3
.
2 00
.
.
.
.
.
μ
In the application, a 5.6
μ
H inductor is used to reduce ripple
current.
For the selection of the external MOSFET, the R
DS(ON)
must be guaranteed at 2.5V since the LTC1772 has to work
down to 2.7V. Let’s assume that the MOSFET dissipation
is to be limited to P
P
= 250mW and its thermal resistance
is 50
°
C/W. Hence the junction temperature at T
A
= 25
°
C
will be 37.5
°
C and
δ
p = 0.005 (37.5 – 25) = 0.0625. The
required R
DS(ON)
is then given by:
P
DC I
OUT
+
The P-channel MOSFET requirement can be met by an
Si6433DQ.
The requirement for the Schottky diode is the most strin-
gent when V
OUT
= 0V, i.e., short circuit. With a 0.03
R
SENSE
resistor, the short-circuit current through the
Schottky is 0.1/0.03 = 3.3A. An MBRS340T3 Schottky
diode is chosen. With 3.3A flowing through, the diode is
rated with a forward voltage of 0.4V. Therefore, the worst-
case power dissipated by the diode is 1.32W. The addition
of D
FB1
and D
FB2
(Figure 5) will reduce the diode dissipa-
tion to approximately 0.66W.
The input capacitor requires an RMS current rating of at
least 0.75A at temperature, and C
OUT
will require an ESR
of 0.1
for optimum efficiency.
R
p
DS ON
(
P
2
1
)
.
(
)
(
)
=
0 11
δ
Figure 5. Foldback Current Limiting
V
FB
I
TH
/RUN
V
OUT
LTC1772
R1
1772 F05
R2
D
FB1
D
FB2
+
Design Example
Assume the LTC1772 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.
Maximum
V
V
+
V
V
OUT
D
IN MIN
(
D
Duty Cycle
=
+
=
)
%
93
From Figure 2, SF = 57%.
R
SF
I
SENSE
OUT
=
( )(
12
)(
)
=
( )(
)
=
100
0 57
.
0 0317
.
.
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