LTC1574-5 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1574-5' PDF : 8 Pages View PDF
LTC1574
LTC1574-3.3/LTC1574-5
APPLICATIONS INFORMATION
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.
IPEAK
AVG CURRENT = IOUT
IV
= IPEAK + IV
2
= 350mA
TIME
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:
( ) IRIPPLE
=
4
×
10−6
VOUT +
L
VD
AP-P
Solving for L in the above equation and with VD = 0.5V,
L = 44µH. The next higher standard value of L is 50µH
(example: Coiltronics CTX50-4). The operating frequency,
ignoring voltage across diode VD is:
f
≈
2.5
× 1051 −
VOUT
VIN
= 111kHz
With the value of L determined, the requirements for CIN
and COUT are calculated. For CIN, its RMS current rating
should be at least:
[ ( )] ( ) 1/2
IRMS
=
IOUT
VOUT
VIN − VOUT
VIN
ARMS
= 174mA
For COUT, the RMS current rating should be at least:
( ) IRMS
≈
IPEAK
2
ARMS
= 300mA
TYPICAL APPLICATIONS
Low Noise, High Efficiency 3.3V Regulator
VIN
4V TO 12.5V
5
6
IPGM
VIN
SHDN 7
12
10
LBIN LTC1574 VFB
11
LBOUT
SW 3, 14
GND
2, 4, 13, 15
* AVX TPSD226K025
** AVX TPSD107K010
† COILTRONICS CTX50-4
+ 22µF*
25V
×2
0.1µF
50µH†
100pF
6.8nF
+ 100µF** 56k
10V
×2
33k
VOUT
3.3V
450mA
1574 TA03
6
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