LTC1574CS datasheet(6/8 Pages) LINER

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LTC1574

LTC1574-3.3/LTC1574-5

APPLICATIONS INFORMATION

RIPPLE

OUT

=×







 ()

−

410

AP-P

Solving for L in the above equation and with VD = 0.5V,

L = 44

µH. The next higher standard value of L is 50µH

(example: Coiltronics CTX50-4). The operating frequency,

ignoring voltage across diode VD is:

kHz

OUT

≈×

−









2 5 10 1

111

With the value of L determined, the requirements for CIN

and COUT are calculated. For CIN, its RMS current rating

should be at least:

RMS

OUT

RMS

−

()

[] ()

174

For COUT, the RMS current rating should be at least:

RMS

PEAK

RMS

≈

()

300mA

TYPICAL APPLICATIONS

Low Noise, High Efficiency 3.3V Regulator

VIN

100pF

2, 4, 13, 15

3, 14

µH†

0.1

µF

6.8nF

µF*

25V

× 2

1574 TA03

VOUT

3.3V

450mA

56k

33k

VIN

4V TO 12.5V

GND

LTC1574

IPGM

LBIN

LBOUT

SHDN

VFB

* AVX TPSD226K025

** AVX TPSD107K010

† COILTRONICS CTX50-4

100

µF**

10V

× 2

inductor L1 is chosen for its excellent self-shielding prop-

erties. Open magnetic structures such as drum and rod

cores are to be avoided since they inject high flux levels

into their surroundings. This can become a major source

of noise in any converter circuit.

Design Example

As a design example, assume VIN = 9V (nominal),

VOUT = 5V and IOUT = 350mA maximum. The LTC1574-5

is used for this application with IPGM (Pin 6) connected to

VIN. The minimum value of L is determined by assuming

the LTC1574-5 is operating in continuous mode.

TIME

IPEAK

AVG CURRENT = IOUT

= 350mA

IPEAK + IV

1574 • F06

Figure 6. Continuous Inductor Current

With IOUT = 350mA and IPEAK = 0.6A (IPGM = VIN), IV = 0.1A.

The peak-to-peak ripple inductor current, IRIPPLE, is 0.5A

and is also equal to:

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