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LTC1574_1

High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode

Linear Technology 

LTC1574

LTC1574-3.3/LTC1574-5

APPLICATIO S I FOR ATIO

going into Pin 12 being negligible, the following expres-

sion is used for setting the trip limit:

VLBTRIP

=



1.251 +

R4

R3 

VIN

R4

LTC1574

12

–

+

R3

1.25V

REFERENCE

1574 • F02

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:

VOUT

=



1.251

+

R2

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 LTC1574.

LTC1574

VFB 10

VOUT

R2

100pF R1

1574 • F03

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

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 VIN. 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.

INPUT VOLTAGE

4V TO 12V

5

VIN

12

LTC1574-5

LBIN

SHDN

7

11

LBOUT

VOUT 10

6 IPGM

SW 3, 14

GND

2, 4, 13, 15

* AVX TPSD476K016

** COILTRONICS CTX50-4

+

0.1µF

47µF*

16V

×2

50µH**

+

47µF*

16V

×2

VOUT

– 5V

45mA

1574 • F04

Figure 4. Positive-to-Negative 5V Converter

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 ModeTM 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

5

VIN

LTC1574

12

7

LBIN

SHDN

11

LBOUT

3, 14

SW

6

IPGM

10

VFB

GND

2, 4, 13, 15

+

L1**

100µH

56k

33k

100µF*

10V

C2

6.8nF

+

VIN

5V

VOUT

3.3V

425mA

100µF*

10V

* AVX TPSD107K010

** COILTRONICS CTX100-4

Figure 5. Low Noise 5V to 3.3V Regulator

1574 • F05

5

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