Datasheet

LT3506/LT3506A
9
3506afc
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 effi ciency
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.
Table 1. Inductors
PART NUMBER
VALUE
(μH) ISAT (A) DCR (Ω)
HEIGHT
(mm)
Sumida
CR43-3R3 3.3 1.44 0.086 3.5
CR43-4R7 4.7 1.15 0.109 3.5
CDC5d23-2R2 2.2 2.16 0.030 2.5
CDRH5D28-2R6 2.6 2.60 0.013 3.0
CDRH6D26-5R6 5.6 2.00 0.027 2.8
CDH113-100 10 2.00 0.047 3.7
Coilcraft
DO1606T-152 1.5 2.10 0.060 2.0
DO1606T-222 2.2 1.70 0.070 2.0
DO1608C-332 3.3 2.00 0.080 2.9
DO1608C-472 4.7 1.50 0.090 2.9
DO1813P-682HC 6.8 2.20 0.080 5.0
Cooper
SD414-2R2 2.2 2.73 0.061 1.35
SD414-6R8 6.8 1.64 0.135 1.35
UP1B-100 10 1.90 0.111 5.0
Toko
(D62F)847FY-2R4M 2.4 2.5 0.037 2.7
(D73LF)817FY-2R2M 2.2 2.7 0.03 3.0
Input Capacitor Selection
Bypass the input of the LT3506 circuit with a 4.7F 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-
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