Datasheet
LT3689/LT3689-5
15
3689fe
For more information www.linear.com/LT3689
this condition and forces the switch to turn off, allowing
the inductor current to charge up the boost capacitor.
This places a limitation on the maximum duty cycle. The
maximum duty cycle that the LT3689 can sustain is 90%.
From this DC
MAX
number, the minimum operating voltage
can be calculated using the following equation:
V
IN(MIN)
=
V
OUT
+ V
D
0.90
− V
D
+ V
SW
Example: V
OUT
= 3.3V
V
IN(MIN)
=
3.3V + 0.4V
0.90
– 0.4V + 0.4V = 4.1V
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = (V
OUT
+ V
F
) •
2.2MHz
f
SW
where V
F
is the voltage drop of the catch diode (~0.4V),
f
SW
is the switching frequency in MHz, and L is in µH. The
inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should
be at least 30% higher. For robust operation in fault con-
ditions (start-up or short-circuit) and high input voltage
(>30V), use an 8.2µH or greater inductor (for T
J
> 125°C,
use 10µH or larger) with a saturation rating of 2.5A, or
higher. To keep the efficiency high, the series resistance
(DCR) should be less than 0.15Ω and the core mate-
rial should be intended for high frequency applications.
Table 2 lists several vendors and suitable types.
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 LT3689 limits its switch
current in order to protect itself and the system from
overload faults. Therefore, the maximum output current
that the LT3689 will deliver depends on the switch current
limit, the inductor value, and the input and output volt-
ages. Also, if the inductor current’s bottom peak exceeds
the DA current limit (I
LIM(DA)
) at high output currents
then the DA current comparator will regulate the bottom
peak to I
LIM(DA)
. This will result in higher inductor ripple
current and will further limit the max output current. The
DA current limit consists of a DC and an AC component.
The nominal DC component is fixed at 1.2A. The AC
component depends on the output voltage, inductor size
and a fixed time delay between the DA comparator turn-
ing off and switch turning on. Therefore, the DA current
limit I
LIM(DA)
will increase as the output voltage collapses
under overload conditions.
I
LIM(DA)
= 1.2A −
V
OUT
+
V
D
( )
L
• 0.25µs
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
INDUCTANCE RANGE
(µH)
SIZE
(mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
1.2 to 4.7
2.5 to 10
4.5 × 4.5
5.5 × 5.5
Toko www.toko.com A916CY
D585LC
2 to 12
1.1 to 39
6.3 × 6.2
8.1 × 8
Würth Elektronik www.we-online.com WE-TPC(M)
WE-PD2(M)
1 to 10
2.2 to 22
4.8 × 4.8
5.2 × 5.8
APPLICATIONS INFORMATION