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LT1959 Datasheet(PDF) 11 Page - Linear Technology

Part No. LT1959
Description  4.5A, 500kHz Step-Down Switching Regulator
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT1959 Datasheet(HTML) 11 Page - Linear Technology

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11
LT1959
APPLICATIONS INFORMATION
Table 2
SERIES
CORE
VENDOR/
VALUE
DC
CORE
RESIS-
MATER-
HEIGHT
PART NO.
(
µH) (Amps) TYPE TANCE()
IAL
(mm)
Coiltronics
CTX2-1
2
4.1
Tor
0.011
KM
µ
4.2
CTX5-4
5
4.4
Tor
0.019
KM
µ
6.4
CTX8-4
8
3.5
Tor
0.020
KM
µ
6.4
CTX2-1P
2
3.4
Tor
0.014
52
4.2
CTX2-3P
2
4.6
Tor
0.012
52
4.8
CTX5-4P
5
3.3
Tor
0.027
52
6.4
Sumida
CDRH125
10
4.0
SC
0.025
Fer
6
CDRH125
12
3.5
SC
0.027
Fer
6
CDRH125
15
3.3
SC
0.030
Fer
6
CDRH125
18
3.0
SC
0.034
Fer
6
Coilcraft
DT3316-222
2.2
5
SC
0.035
Fer
5.1
DT3316-332
3.3
5
SC
0.040
Fer
5.1
DT3316-472
4.7
3
SC
0.045
Fer
5.1
Pulse
PE-53650
4
4.8
Tor
0.017
52
9.1
PE-53651
5
5.4
Tor
0.018
52
9.1
PE-53652
9
5.5
Tor
0.022
52
10
PE-53653
16
5.1
Tor
0.032
52
10
Dale
IHSM-4825
2.7
5.1
Open
0.034
Fer
5.6
IHSM-4825
4.7
4.0
Open
0.047
Fer
5.6
IHSM-5832
10
4.3
Open
0.053
Fer
7.1
IHSM-5832
15
3.5
Open
0.078
Fer
7.1
IHSM-7832
22
3.8
Open
0.054
Fer
7.1
Tor = Toroid
SC = Semiclosed geometry
Fer = Ferrite core material
52 = Type 52 powdered iron core material
KM
µ = Kool Mµ®
Output Capacitor
The output capacitor is normally chosen by its Effective
Series Resistance (ESR), because this is what determines
output ripple voltage. At 500kHz, any polarized capacitor
is essentially resistive. To get low ESR takes
volume, so
physically smaller capacitors have high ESR. The ESR
range for typical LT1959 applications is 0.05
Ω to 0.2Ω. A
typical output capacitor is an AVX type TPS, 100
µF at 10V,
with a guaranteed ESR less than 0.1
Ω. This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical, and values from
22
µF to greater than 500µF work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22
µF solid
tantalum capacitor, it will have high ESR, and output ripple
voltage will be terrible. Table 3 shows some typical solid
tantalum surface mount capacitors.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size
ESR (Max.,
)
Ripple Current (A)
AVX TPS, Sprague 593D
0.1 to 0.3
0.7 to 1.1
AVX TAJ
0.7 to 0.9
0.4
D Case Size
AVX TPS, Sprague 593D
0.1 to 0.3
0.7 to 1.1
C Case Size
AVX TPS
0.2 (typ)
0.5 (typ)
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true, and type TPS capacitors are
specially tested for surge capability, but surge ruggedness
is not a critical issue with the
output capacitor. Solid
tantalum capacitors fail during very high
turn-on surges,
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
triangular with a typical value of 200mARMS. The formula
to calculate this is:
Output Capacitor Ripple Current (RMS):
I
VV
V
Lf V
RIPPLE RMS
OUT
IN
OUT
IN
( ) =
()
()
()( )( )
029
.
Kool M
µ is a registered trademark of Magnetics, Inc.


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