lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 14

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3695
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
where f
output voltage, V
is the inductor value in μH.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. To keep the effi ciency high, the series
resistance (DCR) should be less than 0.1Ω, and the core
material should be intended for high frequency applications.
Table 2 lists several vendors and suitable types.
For robust operation in fault conditions (start-up or short-
circuit) and high input voltage (>30V), the saturation
current should be chosen high enough to ensure that the
inductor peak current does not exceed 3.5A. For example,
an application running from an input voltage of 36V
using a 10μH inductor with a saturation current of 2.5A
will tolerate the mentioned fault conditions.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. If your load is lower
than the maximum load current, then you can relax the
value of the inductor and operate with higher ripple cur-
rent. This allows you to use a physically smaller inductor,
or one with a lower DCR resulting in higher effi ciency.
Be aware that if the inductance differs from the simple
rule above, then the maximum load current will depend
on input voltage. In addition, low inductance may result
in discontinuous mode operation, which further reduces
maximum load current. For details of maximum output
current and discontinuous mode operation, see Linear
Technology’s Application Note 44. Finally, for duty cycles
greater than 50% (V
is required to avoid sub-harmonic oscillations:
14
L
L
MIN
=
(
SW
V
=
OUT
(
is the switching frequency in MHz, V
V
OUT
+
V
D
+
D
) •
V
is the catch diode drop (~0.5V) and L
OUT
D
f
. 1 8
SW
) •
/V
f
. 1 2
SW
IN
> 0.5), a minimum inductance
OUT
is the
The current in the inductor is a triangle wave with an av-
erage value equal to the load current. The peak inductor
and switch current is:
where I
the maximum output load current and ΔI
tor ripple current. The LT3695 limits its switch current in
order to protect itself and the system from overload faults.
Therefore, the maximum output current that the LT3695 will
deliver depends on the switch current limit, the inductor
value and the input and output voltages.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
where f
is the duty cycle and L is the value of the inductor.
To maintain output regulation, the inductor peak current
must be less than the LT3695’s switch current limit, I
If the SYNC pin is grounded, I
duty cycles and decreases to 1.1A at DC = 90%. If the
SYNC pin is tied to 0.8V or more or if it is tied to a clock
source for synchronization, I
duty cycles and decreases to 0.85A at DC = 90%. The
maximum output current is also a function of the chosen
inductor value and can be approximated by the following
expressions depending on the SYNC pin confi guration:
For the SYNC pin grounded:
For the SYNC pin tied to 0.8V or more, or tied to a clock
source for synchronization:
I
I
I
ΔI
OUT MAX
OUT MAX
SW PEAK
L
(
=
(
(
L(PEAK)
SW
(
1
−
is the switching frequency of the LT3695, DC
)
)
)
DC
=
=
=
I
I
I
is the peak inductor current, I
L PEAK
LIM
LIM
) •(
L f
(
•
V
−
−
SW
OUT
Δ
Δ
)
2
2
=
I
I
L
L
+
I
OUT MAX
=
=
V
1 45
1 18
D
.
.
(
LIM
)
LIM
A
A
is at least 1.18A at low
)
is at least 1.45A at low
•(
•(
+
1 0 24
1 0 29
Δ
−
−
2
I
L
.
.
L
is the induc-
•
•
OUT(MAX)
DC
DC
)
)
−
−
Δ
Δ
LIM
2
2
3695f
I
I
L
L
is
.