LTC1174
LTC1174-3.3/LTC1174-5
APPLICATIO S I FOR ATIO
Catch Diode Selection
The catch diode carries load current during the off-time. The
average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As V
IN
approaches V
OUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output is
short-circuited. Under this condition the diode must safely
handle I
PEAK
at close to 100% duty cycle. A fast switching diode
must also be used to optimize efficiency. Schottky diodes are
a good choice for low forward drop and fast switching times.
Most LTC1174 circuits will be well served by either a 1N5818,
a MBRS140T3 or a MBR0520L Schottky diode.
Short-Circuit Protection
The LTC1174 is protected from output short by its internal
current limit. Depending on the condition of I
PGM
pin, the
limit is either set to 340mA or 600mA. In addition, the off-
time of the switch is increased to allow the inductor’s
current to decay far enough to prevent any current build-up
(see Figure 2).
I
PGM
= V
IN
I
PGM
= 0
GND
1174 F02
L = 100µH
V
IN
= 13.5V
20µs/DIV
Figure 2. Inductor's Current with Output Shorted
Low-Battery Detector
The low-battery indicator senses the input voltage through
an external resistive divider. This divided voltage connects
to the “–” input of a voltage comparator (Pin 3) which is
U
compared with a 1.25V reference voltage. With the current
going into Pin 3 being negligible, the following expression
is used for setting the trip limit:
⎛
R4
⎞
V
LBTRIP
=
1.25
⎜
1
+
⎟
⎝
R3
⎠
When the LTC1174 is shut down, the low-battery detector
is inactive.
V
IN
R4
3
R3
LTC1174
W
U U
–
+
1.25V
REFERENCE
1174 F03
Figure 3. Low-Battery Comparator
LTC1174 Adjustable/Low Noise Applications
The LTC1174 develops a 1.25V reference voltage between
the feedback (Pin 1) terminal and ground (see Figure 4). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
⎛
R2
⎞
V
OUT
=
1.25
⎜
1
+
⎟
⎝
R1
⎠
For most applications, a 30k resistor is suggested for R1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to the LTC1174. Alternatively, a
capacitor across R2 can be used to increase the switching
frequency for low noise operation.
Inverting Applications
The LTC1174 can easily be set up for a negative output
voltage. If – 5V is desired, the LTC1174-5 is ideal for this
application as it requires the least components. Figure 5
shows the schematic for this application. Note that the
1174fe
9
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
