Datasheet

5
LTC1574
LTC1574-3.3/LTC1574-5
going into Pin 12 being negligible, the following expres-
sion is used for setting the trip limit:
V
R
R
LBTRIP
=+






125 1
4
3
.
difference between the absolute maximum voltage rating
and the output voltage. A maximum of 12V is specified in
Figure 4, giving the circuit 1.5V of headroom for V
IN
. Note
that the circuit can operate from a minimum of 4V,
making it ideal for a four NiCd cell application. For a
higher output current circuit, please refer to the Typical
Applications section.
Figure 2. Low-Battery Comparator
LTC1574 Adjustable Applications
The LTC1574 develops a 1.25V reference voltage between
the feedback terminal (Pin 10) and ground (see Figure 3).
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
=+






1251
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 LTC1574.
Figure 3. LTC1574 Adjustable Configuration
Inverting Applications
The LTC1574 can easily be set up for a negative output
voltage. If –5V is desired, the LTC1574-5 is ideal for this
application as it requires the least components. Figure 4
shows the schematic for this application. Note that the
output voltage is now taken off the GND pins. Therefore,
the maximum input voltage is now determined by the
Figure 4. Positive-to-Negative 5V Converter
Figure 5. Low Noise 5V to 3.3V Regulator
Low Noise Regulators
In some applications it is important not to introduce any
switching noise within the audio frequency range. Due to
the nature of the LTC1574 during Burst Mode
TM
operation,
there is a possibility that the regulator will introduce audio
noise at some load currents. To circumvent this problem,
a feed-forward capacitor can be used to shift the noise
spectrum up and out of the audio band. Figure 5 shows the
low noise connection with C2 being the feed-forward
capacitor. The peak-to-peak output ripple is reduced to
30mV over the entire load range. A toroidal surface mount
Burst Mode is a trademark of Linear Technology Corporation
APPLICATIO S I FOR ATIO
WUUU
LTC1574
–
+
1.25V
REFERENCE
R4
R3
12
V
IN
1574 • F02
V
FB
R2
R1
10
V
OUT
1574 • F03
100pF
LTC1574
12
LTC1574-5
LB
IN
LB
OUT
I
PGM
GND
V
IN
SHDN
V
OUT
SW
11
6
5
7
10
3, 14
2, 4, 13, 15
50µH**
V
OUT
–5V
45mA
47µF*
16V
× 2
1574 • F04
*
**
AVX TPSD476K016
COILTRONICS CTX50-4
INPUT VOLTAGE
4V TO 12V
0.1µF
+
47µF*
16V
× 2
+
V
IN
L1**
100µH
100µF*
10V
1574 • F05
5
2, 4, 13, 15
56k
33k
7
3, 14
10
12
11
6
V
IN
5V
V
OUT
3.3V
425mA
GND
LTC1574
LB
IN
LB
OUT
I
PGM
SHDN
SW
V
FB
* AVX TPSD107K010
** COILTRONICS CTX100-4
100µF*
10V
C2
6.8nF
+
+