LT3688 LINER [Linear Technology], LT3688 Datasheet - Page 15

• Manufacturer Part Number: LT3688
• Description: Dual 800mA Step-Down Switching Regulator with Power-On Reset
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Table 2. Inductor Vendors
VENDOR
Murata
TDK
Toko
Sumida
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, then 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 effi ciency. Be
aware that if the inductance differs from the simple rule
above, then the maximum load current will depend on
input voltage. In addition, low inductance may result in
discontinuous mode operation, which further reduces
maximum load current. Discontinuous operation occurs
when I
output current and discontinuous mode operation, see
Linear Technology’s Application Note AN44. Finally, for
duty cycles greater than 50% (V
inductance is required to avoid sub-harmonic oscillations:
where V
f
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 LT3688 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3688 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
SW
L
MIN
is in MHz, and L
OUT
F
= V
is the voltage drop of the catch diode (~0.4V),
PART SERIES
(
is less than ΔI
CDRH6D38
SLF10145
OUT
SLF7045
CDRH74
LQH55D
DC62CB
D63CB
D75C
CR54
CR75
D75F
+ V
MIN
F
)
•
is in μH.
1.2MHz
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
L
TYPE
Open
Open
Open
Open
f
/ 2. For details of maximum
SW
OUT
/V
www.murata.com
www.component.tdk.com
www.toko.com
www.sumida.com
IN
> 0.5), a minimum
URL
When the switch is off, the potential across the induc-
tor is the output voltage plus the catch diode drop. This
gives the peak-to-peak ripple current in the inductor
where f is the switching frequency of the LT3688 and L is the
value of the inductor. The peak inductor and switch current is
To maintain output regulation, this peak current must be
less than the LT3688’s switch current limit I
least 1.25A for at low duty cycles and decreases linearly
to 0.9A at DC = 0.9. The maximum output current is a
function of the chosen inductor value:
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 given above, look at the available inductors, and
choose one to meet cost or space goals. Then use these
equations to check that the LT3688 will be able to deliver
the required output current. Note again that these equations
assume that the inductor current is continuous.
Input Capacitor
Bypass the input of the LT3688 circuit with a ceramic
capacitor of an X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 2.2μF to 4.7μF ceramic capacitor
is adequate to bypass the LT3688 and will easily handle
the ripple current. Note that larger input capacitance
is required when a lower switching frequency is used.
If the input power source has high impedance, or there
is signifi cant inductance due to long wires or cables,
I
I
ΔI
= 1.25A • 1– 0.3DC
SW(PK)
OUT(MAX)
L
=
(
1– DC
= I
= I
(
L(PK)
LIM
)
L • f
(
V
= I
–
OUT
OUT
ΔI
2
L
)
+ V
–
+
ΔI
F
ΔI
2
)
2
L
L
LT3688
LIM
. I
LIM
15
is at
3688f