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參數(shù)資料
| 型號(hào)： | LTC1938IDD#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 4 A SWITCHING REGULATOR, 3300 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10 |
| 文件頁數(shù)： | 8/24頁 |
| 文件大?。?/td> | 272K |
| 代理商： | LTC1938IDD#TRPBF |
LT1938
16
1938f
Soft-Start
The RUN/SS pin can be used to soft-start the LT1938,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC lter to
create a voltage ramp at this pin. Figure 7 shows the start-
up and shut-down waveforms with the soft-start circuit.
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20μA when the RUN/SS
pin reaches 2.3V.
LT1938 can pull large currents from the output through
the SW pin and the VIN pin. Figure 7 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 8 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents ow in the LT1938’s VIN and SW pins, the catch
diode (D1) and the input capacitor (C1). The loop formed
by these components should be as small as possible. These
components, along with the inductor and output capacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and VC nodes small so that the ground
traces will shield them from the SW and BOOST nodes.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT1938 to additional ground planes within the circuit
board and on the bottom side.
Figure 6. To Soft-Start the LT1938, Add a Resisitor
and Capacitor to the RUN/SS Pin
APPLICATIONS INFORMATION
1938 F06
IL
1A/DIV
VRUN/SS
2V/DIV
VOUT
2V/DIV
RUN/SS
GND
0.22
μF
RUN
15k
2ms/DIV
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT1938 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT1938 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1938’s
output. If the VIN pin is allowed to oat and the RUN/SS
pin is held high (either by a logic signal or because it is
tied to VIN), then the LT1938’s internal circuitry will pull
its quiescent current through its SW pin. This is ne if
your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
essentially zero. However, if the VIN pin is grounded while
the output is held high, then parasitic diodes inside the
Figure 7. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LT1938
Runs Only When the Input is Present
VIN
BOOST
GND FB
RUN/SS
VC
SW
D4
MBRS140
VIN
LT1938
1938 F07
VOUT
BACKUP
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