LT3436 LINER [Linear Technology], LT3436 Datasheet - Page 7

• Manufacturer Part Number: LT3436
• Description: 3A, 800kHz Step-Up Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
INDUCTOR CHOICE AND MAXIMUM OUTPUT
CURRENT
When choosing an inductor, there are 2 conditions that
limit the minimum inductance; required output current,
and avoidance of subharmonic oscillation. The maximum
output current for the LT3436 in a standard boost con-
verter configuration with an infinitely large inductor is:
Where η = converter efficiency (typically 0.87 at high
current).
As the value of inductance is reduced, ripple current
increases and I
tance for a required output current is given by:
The second condition, avoidance of subharmonic oscilla-
tion, must be met if the operating duty cycle is greater than
50%. The slope compensation circuit within the LT3436
prevents subharmonic oscillation for inductor ripple cur-
rents of up to 1.4A
value to be:
These conditions define the absolute minimum induc-
tance. However, it is generally recommended that to
prevent excessive output noise, and difficulty in obtaining
stability, the ripple current is no more than 40% of the
average inductor current. Since inductor ripple is:
I
I
L
L
OUT MAX
P P RIPPLE
MIN
MIN
−
(
=
=
2
V V
)
IN OUT
V
1 4
OUT
= 3
(
.
OUT(MAX)
=
V
A
V V
V V
( )
OUT
f
IN OUT
IN OUT
V
U
V
⎛
⎜
⎝
V
P-P
IN
(
(
–
OUT
3
OUT
( )
•
V
f
–
, defining the minimum inductor
IN
η
is reduced. The minimum induc-
INFORMATION
(
( )( )
V
)
L f
U
OUT
–
–
V
V
V
IN
IN
IN
)(
•
)
)
I
OUT
η
W
)
⎞
⎟
⎠
U
The recommended minimum inductance is:
The inductor value may need further adjustment for other
factors such as output voltage ripple and filtering require-
ments. Remember also, inductance can drop significantly
with DC current and manufacturing tolerance.
The inductor must have a rating greater than its peak
operating current to prevent saturation resulting in effi-
ciency loss. Peak inductor current is given by:
Also, consideration should be given to the DC resistance
of the inductor. Inductor resistance contributes directly to
the efficiency losses in the overall converter.
Suitable inductors are available from Coilcraft, Coiltronics,
Dale, Sumida, Toko, Murata, Panasonic and other manu-
factures.
Table 2
PART
NUMBER
Coilcraft
DO1608C-222
Sumida
CDRH3D16-1R5
CDRH4D18-1R0
CDC5D23-2R2
CR43-1R4
CDRH5D28-2R6
CDRH6D38-3R3
CDRH6D28-3R0
Toko
(D62F)847FY-2R4M
(D73LF)817FY-2R2M
L
I
LPEAK
MIN
=
=
0 4
(
. (
V
(
V
IN
OUT
V
) (
V
2
OUT
IN
V
)(
•
OUT
VALUE
I
) (
OUT
η
(µH)
2.2
1.5
1.0
2.2
1.4
2.6
3.3
3.0
2.4
2.2
2
I
OUT
–
)
V
+
IN
)( )
V V
I
(Amps)
)
f
IN OUT
SAT(DC)
2
2.4
1.6
1.7
2.2
2.5
2.6
3.5
3.0
2.5
2.7
(
V
OUT
( )( )
L f
−
0.043
0.035
0.056
0.013
0.024
0.037
V
DCR
0.07
0.03
0.02
0.03
(Ω)
IN
)
LT3436
HEIGHT
(mm)
2.9
1.8
2.0
2.5
3.5
3.0
4.0
3.0
2.7
3.0
3436fa
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