LT3669-2_15 Datasheet, PDF (29/40 Pages) Linear Technology – IO-Link Transceiver with Integrated Step-Down Regulator and LDO

LT3669/LT3669-2
APPLICATIONS INFORMATION
Table 3. Inductor Vendors
VENDOR
Murata
TDK
Toko
Coilcraft
Sumida
Würth Elektronik
Coiltronics
URL
www.murata.com
www.componenttdk.com
www.toko.com
www.coilcraft.com
www.sumida.com
www.we-online.com
www.cooperet.com
For robust operation in fault conditions (start-up or
short-circuit) and high input voltage (>30V), choose the
saturation current high enough to ensure that the inductor
peak current does not exceed 0.6A and 1.3A for LT3669
and LT3669-2, respectively. For example, an LT3669-2
application running from an input voltage of 36V using a
33μH inductor with a saturation current of 0.8A will toler-
ate 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 cur-
rent 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
current. This allows the use of a physically smaller induc-
tor, or one with a lower DCR resulting in higher efficiency.
Be aware that if the inductance differs from the simple
rule, then the maximum load current will depend on input
voltage. In addition, low inductance may result in discon-
tinuous 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_OUT/ V_L+ > 0.5), a minimum inductance is required
to avoid subharmonic oscillations:
L_MIN
= (V_OUT
+
V_D )
•
k
f_SW
(k = 6.5 in LT3669; k = 2.6 in LT3669-2)
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:
I_{SW (PEAK )}


=
I_{L(PEAK )}
=
I_{OUT(MAX )}
+ ^϶I
L
2
where I_L(PEAK) is the peak inductor current, I_OUT(MAX) is
the maximum output load current, and ΔI_L is the induc-
tor ripple current. The LT3669 limits its switch current in
order to protect itself and the system from overload faults.
Therefore, the maximum output current that the LT3669
will deliver depends on the switch current limit, the induc-
tor value and the input and output voltages.
When the switch is off, the voltage 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
L • f_SW
+
V_D )
where f_SW is the switching frequency of the LT3669, DC
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 switch current limit I_LIM which is
0.325A (LT3669) and 0.65A (LT3669-2) at low duty cycles
and decreases to 0.24A (LT3669) and 0.48A (LT3669-2).
The maximum output current is also a function of the
chosen inductor value and can be approximated by the
following expression:
I_OUT(MAX)
=
I_LIM
−
ΔI_L
2
=
I_LIM(DC
=
0)
•
(1 −
0.26
• DC)
−
ΔI_L
2
(I_{LIM(DC = 0)} = 0.325A in LT3669;
I_{LIM(DC = 0)} = 0.65A in LT3669-2)
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule—look at the available inductors, and choose
one to meet cost or space goals. Then use these equations
For more information www.linear.com/LT3669
3669fa
29