LT1934 LINER [Linear Technology], LT1934 Datasheet - Page 11

• Manufacturer Part Number: LT1934
• Description: Micropower Step-Down Switching Regulators in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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
applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1934’s
output. If the V
is held high (either by a logic signal or because it is tied to
Figure 3. The Minimum Input Voltage Depends
on Output Voltage, Load Current and Boost Circuit
6.0
5.5
5.0
4.5
4.0
3.5
3.0
8
7
6
5
4
0.1
0.1
Minimum Input Voltage V
Minimum Input Voltage V
LT1934
V
T
BOOST DIODE TIED TO OUTPUT
LT1934
V
T
BOOST DIODE TIED TO OUTPUT
A
A
OUT
OUT
IN
= 25 C
= 25 C
pin is allowed to float and the SHDN pin
= 3.3V
= 5V
V
V
V
V
IN
IN
U
IN
IN
TO START
TO START
1
1
TO RUN
LOAD CURRENT (mA)
TO RUN
LOAD CURRENT (mA)
U
10
10
W
OUT
OUT
100
100
= 3.3V
1934 G12
= 5V
1934 G13
U
V
quiescent current through its SW pin. This is fine 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
is held high, then parasitic diodes inside the LT1934 can
pull large currents from the output through the SW pin and
the V
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 5 shows
the high current paths in the buck regulator circuit. Note
that large, switched currents flow 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 and
V
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
IN
IN
), then the LT1934’s internal circuitry will pull its
IN
D4: MBR0530
pin. Figure 4 shows a circuit that will run only when
1M
100k
D4
5
4
V
SHDN
IN
GND
2
LT1934
BOOST
IN
FB
SW
3
pin is grounded while the output
LT1934/LT1934-1
1
6
1934 F07
11
V
BACKUP
OUT
1934f