Datasheet

LT1912
15
1912fa
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
IN
pin is allowed to float and the RUN/SS
pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT1912’s internal circuitry will pull
its quiescent current through its SW pin. This is fine 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
IN
pin is grounded while
the output is held high, then parasitic diodes inside the
LT1912 can pull large currents from the output through
the SW pin and the V
IN
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 flow in the LT1912’s V
IN
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.
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
1912 F05
LOAD CURRENT (A)
1
INPUT VOLTAGE (V)
4.0
4.5
5.0
10000
3.5
3.0
2.0
10 100 1000
1 1000010 100 1000
2.5
6.0
5.5
TO START
(WORST CASE)
TO RUN
LOAD CURRENT (A)
INPUT VOLTAGE (V)
5.0
6.0
7.0
4.0
2.0
3.0
8.0
TO RUN
V
OUT
= 3.3V
T
A
= 25°C
L = 8.2µH
f = 500kHz
V
OUT
= 5V
T
A
= 25°C
L = 8.2µH
f = 500kHz
TO START
(WORST CASE)
Figure 6. To Soft-Start the LT1912, Add a Resisitor
and Capacitor to the RUN/SS Pin
1912 F06
I
L
1A/DIV
V
RUN/SS
2V/DIV
V
OUT
2V/DIV
RUN/SS
GND
RUN
15k
0.22µF
2ms/DIV
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
V
IN
BOOST
GND FB
RUN/SS
V
C
SW
D4
MBRS140
V
IN
LT1912
1912 F07
V
OUT
BACKUP
APPLICATIONS INFORMATION