9
LTC1174
LTC1174-3.3/LTC1174-5
Figure 3. Low-Battery Comparator
Figure 5. Positive-to-Negative 5V Converter
Figure 4. LTC1174 Adjustable Configuration
APPLICATIOU
W
U
U
LTC1174
–
+
1.25V
REFERENCE
R4
R3
3
V
IN
1174 F03
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:
V
R
R
OUT
=
+
1 25 1
.
2
1
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.
R2
R1
1
V
OUT
1174 F04
100pF*
6.8nF**
*
**
ADJUSTABLE APPLICATIONS
LOW NOISE APPLICATIONS
LTC1174
V
FB
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
output voltage is now taken off the GND pin. Therefore,
the maximum input voltage is now determined by the
3
SHUTDOWN
2
7
6
8
1
5
4
50
μ
H**
V
–5V
45mA
MBRS140T3
1174 F05
*
**
AVX TPSD476K016
COILTRONICS CTX50-4
INPUT VOLTAGE
4V TO 12V
0.1
μ
F
47
μ
F*
16V
×
2
47
μ
F*
16V
×
2
LTC1174HV-5
GND
LB
IN
LB
OUT
I
PGM
V
IN
V
OUT
SW
+
+
difference between the absolute maximum voltage rating
and the output voltage. A maximum of 12V is specified in
Figure 5, giving the circuit a 1.5V of headroom for V
IN
. Note
that the circuit can operate from a minimum of 4V, making
it ideal for a 4 NiCad cell application. For a higher output
current circuit, please refer to the Typical Applications
section.
Absolute Maximum Ratings and Latchup Prevention
The absolute maximum ratings specify that SW (Pin 5) can
never exceed V
IN
(Pin 6) by more than 0.3V. Normally this
situation should never occur. It could, however, if the
output is held up while the supply is pulled down. A con-
dition where this could potentially occur is when a battery
is supplying power to an LTC1174/LTC1174-3.3/
LTC1174-5 regulator and also to one or more loads in
parallel with the the regulator’s V
IN
. If the battery is dis-
connected while the LTC1174/LTC1174-3.3/LTC1174-5
regulator is supplying a light load and one of the parallel
circuits is a heavy load, the input capacitor of the LTC1174/
LTC1174-3.3/LTC1174-5 regulator could be pulled down
faster than the output capacitor, causing the absolute
maximum ratings to be exceeded. The result is often a
latchup which can be destructive if V
IN
is reapplied. Bat-
tery disconnect is possible as a result of mechanical stress,
bad battery contacts or use of a lithium-ion battery with
a built-in internal disconnect. The user needs to assess
his/her application to determine whether this situation
could occur. If so, additional protection is necessary.