LT3976
16
3976f
For more information www.linear.com/3976
applicaTions inForMaTion
Table 2. Inductor Vendors
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.tokoam.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Murata www.murata.com
The inductor value must be sufficient to supply the desired
maximum output current (I
OUT(MAX)
), which is a function
of the switch current limit (I
LIM
) and the ripple current.
I
OUT(MAX)
= I
LIM
–
Δ
L
2
The LT3976 limits its peak switch current in order to protect
itself and the system from overload and short-circuit faults.
The LT3976’s switch current limit (I
LIM
) is typically 10A at
low duty cycles and decreases linearly to 8A at DC = 0.8.
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:
ΔI
L
=
1– DC
( )
• V
OUT
+ V
D
( )
L • f
SW
where f
SW
is the switching frequency of the LT3976, DC is
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3976 will deliver
depends on the switch current limit, the inductor value,
and the input and output voltages. The inductor value may
have to be increased if the inductor ripple current does
not allow sufficient maximum output current (I
OUT(MAX)
)
given the switching frequency, and maximum input voltage
used in the desired application.
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, than you can relax 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. Be aware that if
the inductance differs from the simple rule above, then the
maximum load current will depend on the 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
operation, see Linear Technology’s Application Note 44.
Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillations, see Application Note 19.
One approach to choosing the inductor is to start with
the simple rule given above, look at the available induc-
tors, and choose one to meet cost or space goals. Then
use the equations above to check that the LT3976 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.
Current Limit Foldback and Thermal Protection
The LT3976 has a large peak current limit to ensure a 5A
max output current across duty cycle and current limit
distribution, as well as allowing a reasonable inductor
ripple current. During a short-circuit fault, having a large
current limit can lead to excessive power dissipation and
temperature rise in the LT3976, as well as the inductor and
catch diode. To limit this power dissipation, the LT3976
starts to fold back the current limit when the FB pin falls
below 0.8V. The
LT3976 typically
lowers the peak current
limit about 50% from 10A to 5A.
During start-up, when the output voltage and FB pin are low,
current limit foldback could hinder the LT3976’s ability to
start up into a large load. To avoid this potential problem,
the LT3976’s current limit foldback will be disabled until
the SS pin has charged above 2V. Therefore, the use of
a soft-start capacitor will keep the current limit foldback
feature out of the way while the LT3976 is starting up.
The LT3976 has thermal shutdown to further protect the
part during periods of high power dissipation, particularly
in high ambient temperature environments. The thermal
shutdown feature detects when the LT3976 is too hot
and shuts the part down, preventing switching. When the
thermal event passes and the LT3976 cools, the part will
restart and resume switching. A thermal shutdown event
actively discharges the soft-start capacitor.
