10
LTC1174
LTC1174-3.3/LTC1174-5
1174fe
APPLICATIO S I FOR ATIO
WUUU
output voltage is now taken off the GND pin. Therefore,
the maximum input voltage is now determined by the
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.
Prevention against latchup can be accomplished by sim-
ply connecting a Schottky diode across the SW and V
IN
pins as shown in Figure 6. The diode will normally be
reverse biased unless V
IN
is pulled below V
OUT
at which
time the diode will clamp the (V
OUT
– V
IN
) potential to less
than the 0.6V required for latchup. Note that a low leakage
Schottky should be used to minimize the effect on no-load
supply current. Schottky diodes such as MBR0530, BAS85
and BAT84 work well. Another more serious effect of the
protection diode leakage is that at no load with nothing to
provide a sink for this leakage current, the output voltage
can potentially float above the maximum allowable toler-
ance. To prevent this from occuring, a resistor must be
connected between V
OUT
and ground with a value low
enough to sink the maximum possible leakage current.
Figure 4. LTC1174 Adjustable Configuration
R2
R1
1
V
OUT
1174 F04
100pF*
6.8nF**
*
**
ADJUSTABLE APPLICATIONS
LOW NOISE APPLICATIONS
LTC1174 V
FB
Figure 5. Positive-to-Negative 5V Converter
3
SHUTDOWN
2
7
6
8
1
5
4
50µH**
V
OUT
–5V
45mA
MBRS140T3
1174 F05
*
**
AVX TPSD476K016
COILTRONICS CTX50-4
INPUT VOLTAGE
4V TO 12V
0.1µF
47µF*
16V
×2
47µF*
16V
×2
LTC1174HV-5
LB
IN
LB
OUT
I
PGM
GND
V
IN
V
OUT
SW
+
+
Figure 6. Preventing Absolute Maximum
Ratings from Being Exceeded
1174 F06
V
IN
V
OUT
LATCHUP
PROTECTION
SCHOTTKY
SW
LTC1174
LTC1174-3.3
LTC1174-5
+
