Datasheet

LT3509
14
3509fd
For more information www.linear.com/LT3509
When the switch is off, the potential across the inductor is
the output voltage plus the catch diode forward voltage. This
gives the peak-to-peak ripple current in the inductor:
∆I
L
=(1– DC)
V
OUT
+
V
F
L • f
SW
where:
DC = Duty Cycle
f
SW
= Switching Frequency
L = Inductor Value
V
F
= Diode Forward Voltage
The peak inductor and switch current is:
I
SWPK
=I
LPK
=I
OUT
+
∆
I
L
2
To maintain output regulation, this peak current must be
less than the LT3509’s switch current limit I
LIM
. This is
dependent on duty cycle due to the slope compensation.
For I
LIM
is at least 1.4A at low duty cycles and decreases
linearly to 1.0A at DC = 0.8.
The theoretical minimum inductance can now be calcu
-
lated as:
L
DC
f
VV
II
MIN
MIN
OUT F
LIM OUT
=
+
1–
•
–
where DC
MIN
is the minimum duty cycle called for by the
application i.e.:
DC
VV
VV
V
MIN
OUT MA
XF
IN MINS
WF
=
+
+
()
()
–
There is a limit to the actual minimum duty cycle imposed
by the minimum on-time of the switch. For a robust design
it is important that inductor that will not saturate when
the switch is at its minimum on-time, the input voltage
is at maximum and the output is short circuited. In this
case the full input voltage, less the drop in the switch, will
appear across the inductor. This doesn’t require an actual
short, just starting into a capacitive load will provide the
same conditions. The Diode current sensing scheme will
ensure that the switch will not turn-on if the inductor
current is above the DA current limit threshold, which has
a maximum of 1.1A. The peak current under short-circuit
conditions can then be calculated from:
I
Vt
L
A
PEAK
IN ON MIN
=+
•
.
()
11
The inductor should have a saturation current greater than
this value. For safe operation with high input voltages this
can often mean using a physically larger inductor as higher
value inductors often have lower saturation currents for
a given core size. As a general rule the saturation current
should be at least 1.8A to be short-circuit proof. However, it’s
generally better to use an inductor larger than the minimum
value. For robust operation at input voltages greater than
30V, use an inductor with a value of 4.2µH or greater, and
a saturation current rating of 1.8A or higher. The minimum
inductor has large ripple currents which increase core
losses and require large output capacitors to keep output
voltage ripple low. Select an inductor greater than L
MIN
that keeps the ripple current below 30% of I
LIM
.
applicaTions inForMaTion
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