LT1912 Linear Technology Corporation, LT1912 Datasheet - Page 15

• Manufacturer Part Number: LT1912
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT1912 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
LT1912 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 LT1912’s
output. If the V
pin is held high (either by a logic signal or because it is
tied to V
its quiescent current through its SW pin. This is fi 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 V
the output is held high, then parasitic diodes inside the
LT1912 can pull large currents from the output through
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
IN
), then the LT1912’s internal circuitry will pull
6.0
5.5 TO START
5.0
4.5
4.0
3.5
3.0
2.5
2.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1
1
TO RUN
TO RUN
(WORST CASE)
V
T
L = 8.2μH
f = 500kHz
TO START
(WORST CASE)
V
T
L = 8.2μH
f = 500kHz
IN
A
A
OUT
OUT
= 25°C
= 25°C
pin is allowed to fl oat and the RUN/SS
= 3.3V
= 5V
10
10
LOAD CURRENT (A)
LOAD CURRENT (A)
100
100
IN
1000
1000
pin is grounded while
1912 F05
10000
10000
the SW pin and the V
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 fl ow in the LT1912’s V
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.
0.22μF
RUN
15k
V
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 LT1912
Runs Only When the Input is Present
IN
Figure 6. To Soft-Start the LT1912, Add a Resisitor
and Capacitor to the RUN/SS Pin
MBRS140
D4
RUN/SS
GND
V
RUN/SS
V
IN
C
LT1912
GND FB
IN
BOOST
pin. Figure 7 shows a circuit that
SW
IN
2ms/DIV
and SW pins, the catch
LT1912
1912 F07
1912 F06
V
BACKUP
OUT
15
I
1A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
1912f