LT3506/LT3506A
The current in the inductor is a triangle wave with an
average value equal to the load current. The peak switch
current is equal to the output current plus half the peak-to-
peak inductor ripple current. The LT3506 limits its switch
current in order to protect itself and the system from
overload faults. Therefore, the maximum output current
that the LT3506 will deliver depends on the current limit,
the inductor value and the input and output voltages. L
is chosen based on output current requirements, output
voltage ripple requirements, size restrictions and efficiency
goals. When the switch is off, the inductor sees the output
voltage plus the catch diode drop. This gives the peak-to-
peak ripple current in the inductor:
ΔI
L
= (1 – DC)(V
OUT
+ V
D
)/(L • f)
where f is the switching frequency of the LT3506 and L
is the value of the inductor. The peak inductor and switch
current is
I
SWPK
= I
LPK
= I
OUT
+
ΔI
L
/2.
To maintain output regulation, this peak current must be
less than the LT3506’s switch current limit I
LIM
. I
LIM
is at
least 2A at low duty cycle and decreases linearly to 1.7A
at DC = 0.8. The maximum output current is a function of
the chosen inductor value:
I
OUT(MAX)
= I
LIM
–
ΔI
L
/2 = 2A • (1 – 0.21 • DC) –
ΔI
L
/2
If the inductor value is chosen so that the ripple current
is small, then the available output current will be near
the switch current limit. One approach to choosing the
inductor is to start with the simple rule given above, look
at the available inductors, and choose one to meet cost or
space goals. Then use these equations to check that the
LT3506 will be able to deliver the required output current.
Note again that these equations assume that the inductor
current is continuous. Discontinuous operation occurs
when I
OUT
is less than
ΔI
L
/2 as calculated above.
U
Table 1. Inductors
Part Number
Sumida
CR43-3R3
CR43-4R7
CDC5d23-2R2
CDRH5D28-2R6
CDRH6D26-5R6
CDH113-100
Coilcraft
DO1606T-152
DO1606T-222
DO1608C-332
DO1608C-472
DO1813P-682HC
Cooper
SD414-2R2
SD414-6R8
UP1B-100
Toko
(D62F)847FY-2R4M
(D73LF)817FY-
2R2M
2.4
2.2
2.5
2.7
0.037
0.03
2.7
3.0
2.2
6.8
10
2.73
1.64
1.90
0.061
0.135
0.111
1.35
1.35
5.0
1.5
2.2
3.3
4.7
6.8
2.10
1.70
2.00
1.50
2.20
0.060
0.070
0.080
0.090
0.080
2.0
2.0
2.9
2.9
5.0
3.3
4.7
2.2
2.6
5.6
10
1.44
1.15
2.16
2.60
2.00
2.00
0.086
0.109
0.030
0.013
0.027
0.047
3.5
3.5
2.5
3.0
2.8
3.7
Value
(μH)
ISAT (A)
DCR (W)
Height
(mm)
APPLICATIO S I FOR ATIO
W
U
U
Input Capacitor Selection
Bypass the input of the LT3506 circuit with a 4.7μF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type can be used if there is
additional bypassing provided by bulk electrolytic or tan-
talum capacitors. The following paragraphs describe the
input capacitor considerations in more detail. Step-down
regulators draw current from the input supply in pulses
with very fast rise and fall times. The input capacitor is
required to reduce the resulting voltage ripple at the LT3506
and to force this very high frequency switching current
into a tight local loop, minimizing EMI. The input capaci-
3506afb
LINER [ LINEAR TECHNOLOGY ]
