Datasheet

LT1934/LT1934-1
12
1934fe
APPLICATIONS INFORMATION
maximum duty cycle of the LT1934, requiring a higher
input voltage to maintain regulation.
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT1934 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
LT1934 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 LT1934’s
output. If the V
IN
pin is allowed to fl oat and the SHDN pin
is held high (either by a logic signal or because it is tied
Figure 3. The Minimum Input Voltage Depends
on Output Voltage, Load Current and Boost Circuit
Minimum Input Voltage V
OUT
= 3.3V
Minimum Input Voltage V
OUT
= 5V
LOAD CURRENT (mA)
3.5
INPUT VOLTAGE (V)
4.0
4.5
5.0
5.5
6.0
0.1 10 100
1934 G12
3.0
1
LT1934
V
OUT
= 3.3V
T
A
= 25°C
BOOST DIODE TIED TO OUTPUT
V
IN
TO START
V
IN
TO RUN
LOAD CURRENT (mA)
5
INPUT VOLTAGE (V)
6
7
8
0.1 10 100
1934 G13
4
1
LT1934
V
OUT
= 5V
T
A
= 25°C
BOOST DIODE TIED TO OUTPUT
V
IN
TO START
V
IN
TO RUN
to V
IN
), then the LT1934’s internal circuitry will pull 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 SHDN 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 LT1934 can
pull large currents from the output through the SW pin
and the V
IN
pin. Figure 4 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
Figure 4. 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 LT1934 Runs Only When the Input
is Present
V
IN
BOOST
GND FB
SHDN SW
5
D4
V
IN
4
1
6
23
1M
100k
LT1934
1934 F07
V
OUT
BACKUP
D4: MBR0530
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 5 shows
the high current paths in the buck regulator circuit. Note
that large, switched currents fl ow in the power switch,
the catch diode (D1) and the input capacitor (C2). The
loop formed by these components should be as small as
possible. Furthermore, the system ground should be tied
to the regulator ground in only one place; this prevents
the switched current from injecting noise into the system
ground. 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, and tie this ground plane to system
ground at one location, ideally at the ground terminal of
the output capacitor C1. Additionally, the SW and BOOST
nodes should be kept as small as possible. Finally, keep
the FB node as small as possible so that the ground pin