LT3975
APPLICATIONS INFORMATION
TheLT3975limitsitspeakswitchcurrentinordertoprotect
continuous. Discontinuous operation occurs when I
OUT
itselfandthesystemfromoverloadandshort-circuitfaults.
is less than ΔI /2.
L
The LT3975’s switch current limit (I ) is typically 5.4A at
LIM
Current Limit Foldback and Thermal Protection
low duty cycles and decreases linearly to 4.4A at DC = 0.8.
The LT3975 has a large peak current limit to ensure a 2.5A
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 LT3975, as well as the inductor and
catch diode. To limit this power dissipation, the LT3975
starts to fold back the current limit when the FB pin falls
below 0.8V. The LT3975 typically lowers the peak current
limit about 40% from 5.4A to 3.3A.
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:
1–DC • VOUT + VD
(
)
(
)
∆IL =
L • fSW
where f is the switching frequency of the LT3975, DC is
SW
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3975 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
Duringstart-up,whentheoutputvoltageandFBpinarelow,
current limit foldback could hinder the LT3975’s ability to
start up into a large load. To avoid this potential problem,
the LT3975’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 LT3975 is starting up.
not allow sufficient maximum output current (I
)
OUT(MAX)
giventheswitchingfrequency,andmaximuminputvoltage
used in the desired application.
The optimum inductor for a given application may differ
fromtheoneindicatedbythissimpledesignguide.Alarger
valueinductorprovidesahighermaximumloadcurrentand
reducestheoutputvoltageripple.Ifyourloadislowerthan
the maximum load current, than you can relax the value of
the inductor and operate with higher ripple current. This
allowsyoutouseaphysicallysmallerinductor, oronewith
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,lowinductancemayresultindiscontinuousmode
operation, which further reduces maximum load current.
For details of maximum output current and discontinuous
operation, see Linear Technology’s Application Note 44.
The LT3975 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 LT3975 is too hot
and shuts the part down, preventing switching. When the
thermal event passes and the LT3975 cools, the part will
restart and resume switching. A thermal shutdown event
actively discharges the soft-start capacitor.
Input Capacitor
BypasstheinputoftheLT3975circuitwithaceramiccapaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3975 and will easily handle the ripple cur-
rent. Note that larger input capacitance is required when
a lower switching frequency is used (due to longer on
times). If the input power source has high impedance, or
there is significant inductance due to long wires or cables,
additional bulk capacitance may be necessary. This can
be provided with a low performance electrolytic capacitor.
Finally, for duty cycles greater than 50% (V /V > 0.5),
OUT IN
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 LT3975 will be
able to deliver the required output current. Note again
that these equations assume that the inductor current is
3975f
15
Linear [ Linear ]
