LT3505 LINER [Linear Technology], LT3505 Datasheet - Page 14

• Manufacturer Part Number: LT3505
• Description: Monolithic Dual Tracking 3A Step-Down Switching
• Manufacturer: LINER [Linear Technology]

and its saturation current should be about 30% higher. To
keep efficiency high, the series resistance (DCR) should
be less than 0.05Ω.
For applications with a duty cycle of about 50%, the induc-
tor value should be chosen to obtain an inductor ripple
current less than 40% of peak switch current.
Of course, such a simple design guide will not always result
in the optimum inductor for your application. A larger value
provides a slightly higher maximum load current, and will
reduce the output voltage ripple. If your load is lower than
2.5A, then you can decrease the value of the inductor and
operate with higher ripple current. This allows you to use
a physically smaller inductor, or one with a lower DCR
resulting in higher efficiency.
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 LT3501 limits its switch
current in order to protect itself and the system from
overload faults. Therefore, the maximum output current
that the LT3501 will deliver depends on the current limit,
the inductor value, switch frequency, and the input and
output voltages. The inductor 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 volt-
age plus the catch diode drop. This gives the peak-to-peak
ripple current in the inductor:
where f is the switching frequency of the LT3501 and L
is the value of the inductor. The peak inductor and switch
current is
To maintain output regulation, this peak current must be
less than the LT3501’s switch current limit I
LT3501
applicaTions inForMaTion
14
I
∆I
SW PK
L
( ) = I
=
(
1– Dc
LPK
)
L • f
(
= I
V
OUT
OUT
+ V
+
∆I
D
2
)
L
LIM
. I
LIM
is
3.5A over the entire duty cycle range. The maximum output
current is a function of the chosen inductor value:
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 LT3501 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
I
Figure 4 illustrates the inductance value needed for a 3.3V
output with a maximum load capability of 3A. Referring
to Figure 4, an inductor value between 3.3µH and 4.7µH
will be sufficient for a 15V input voltage and a switch
frequency of 750kHz. There are several graphs in the
Typical Performance Characteristics section of this data
sheet that show inductor selection as a function of input
voltage and switch frequency for several popular output
voltages and output ripple currents. Also, low inductance
L
/2 as calculated above.
I
Figure 4. Inductor Values for 3A Maximum Load Current
vs Frequency and Input Voltage
OUT(MAX)
1500
1250
1000
750
250
500
= I
7
V
I
LIM
RIPPLE
OUT
9
= 3.3V
–
= 1A
11
∆I
2
INPUT VOLTAGE (V)
13
L
L = 2.2µH
L = 3.3µH
= 3.5 –
15
L = 4.7µH
17
L = 6.8µH
19
∆I
2
L
21
OUT
23
3501 F04
25
is less than
3501fd