LT3689 LINER [Linear Technology], LT3689 Datasheet - Page 15

• Manufacturer Part Number: LT3689
• Description: 700mA Step-Down Regulator with Power-On Reset and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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
can be calculated using the following equation:
Example: V
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
where V
f
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
use 10μH or larger) with a saturation rating of 2.5A, or
SW
L
V
V
is the switching frequency in MHz, and L is in μH. The
IN MIN
IN MIN
=
(
(
(
V
F
OUT
is the voltage drop of the catch diode (~0.4V),
)
)
OUT
=
=
MAX
V
+
3 3
OUT
= 3.3V
.
Table 2. Inductor Vendors
VENDOR
Sumida
Toko
Würth Elektronik
V
0 90
number, the minimum operating voltage
F
V
.
0 90
) •
+
.
+
V
0 4
D
. 2 2
.
−
V
f
SW
MHz
V
D
– .
0 4
+
V
SW
V
www.sumida.com
www.toko.com
www.we-online.com
+
0 4
.
URL
V
=
4 1
.
J
V
> 125°C,
PART SERIES
WE-PD2(M)
WE-TPC(M)
CDRH4D28
CDRH5D28
A916CY
D585LC
higher. To keep the effi ciency 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
then the DA current comparator will regulate the bottom
peak to I
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 fi xed at 1.2A. The AC
component depends on the output voltage, inductor size
and a fi xed time delay between the DA comparator turn-
ing off and switch turning on. Therefore, the DA current
limit I
under overload conditions.
I
LIM(DA)
LIM(DA)
INDUCTANCE RANGE
LIM(DA)
= 1.2A −
1.2 to 4.7
will increase as the output voltage collapses
2.5 to 10
1.1 to 39
2.2 to 22
2 to 12
1 to 10
(μH)
. This will result in higher inductor ripple
(
V
LT3689/LT3689-5
LIM(DA)
OUT
L
+ V
4.5 × 4.5
5.5 × 5.5
6.3 × 6.2
4.8 × 4.8
5.2 × 5.8
) at high output currents
8.1 × 8
(mm)
D
SIZE
)
• 0.25µs
15
3689fa