Datasheet

LTC3529
9
3529fb
APPLICATIONS INFORMATION
Inductor Selection
The LTC3529 can utilize small surface-mount chip induc-
tors due to its fast 1.5MHz switching frequency. Larger
values of inductance will allow slightly greater output
current capability by reducing the inductor ripple current.
Increasing the inductance above 10μH will increase com-
ponent size while providing little improvement in output
current capability.
USB On-The-Go specifi cations limit output capacitance to
6.5μF. When using a 6.5μF output capacitance, a 4.7μH
inductor must be used to maintain stability. Larger induc-
tors may be used with larger output capacitors.
The minimum inductance value for a given allowable induc-
tor ripple ΔI (in Amps peak-to-peak) is given by:
L
VVV
IfV
µH
IN MIN OUT IN MIN
OUT
>
()
() ()
•–
••Δ
where V
IN(MIN)
is the minimum input voltage, f is the
operating frequency in MHz (1.5MHz Typ), and V
OUT
is
the output voltage (5V).
The inductor current ripple is typically set for 20% to 40%
of the maximum inductor current (I
P
). High frequency
ferrite core inductor materials reduce frequency depen-
dent power losses compared to cheaper powdered iron
cores, improving effi ciency. To achieve high effi ciency, a
low ESR inductor should be utilized. The inductor must
have a saturation current rating greater than the worst
case average inductor current plus half the ripple current.
Molded chokes and some chip inductors usually do not have
enough core to support peak LTC3529 inductor currents.
To minimize radiated noise, use a shielded inductor. See
Table 2 for suggested components and suppliers.
Table 2. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER VALUE
(μH)
MAX
CURRENT
(A)
DCR
(Ω)
HEIGHT
(mm)
Sumida CDRH5D16NP 4.7 2.15 0.064 1.8
TDK VLF5014S 4.7 2 0.098 1.4
Coilcraft MSS6122 4.7 1.82 0.065 2.2
Cooper Bussmann SD25-4R7 4.7 2.3 0.043 2.5
PCB Layout Guidelines
The LTC3529 switches large currents at high frequencies.
Special care should be given to the PCB layout to ensure
stable, noise-free operation. Figure 1 depicts the recom-
mended PCB layout to be utilized for the LTC3529. A few
key guidelines follow:
1. All circulating current paths should be kept as short
as possible. This can be accomplished by keeping the
copper traces to all components in Figure 1 short and
wide. Capacitor ground connections should via down
to the ground plane in the shortest route possible. The
bypass capacitors on V
IN
and V
OUT
should be placed
close to the IC and should have the shortest possible
paths to ground.
2. The PGND pin should be shorted directly to the ex-
posed pad, as shown in Figure 1. This provides a single
point connection between the small signal ground and
the power ground, as well as a wide trace for power
ground.
3. All the external components shown in Figure 1 and their
connections should be placed over a complete ground
plane.
4. Use of multiple vias in the die attach pad will enhance
the thermal environment of the converter, especially if
the vias extend to a ground plane region on the exposed
bottom surface or inner layers of the PCB.
Figure 1. LTC3529 Recommended PCB Layout
3
4
2
1
6
5
7
8
MULTIPLE VIAs
TO GROUND PLANE
V
IN
RST
SNSGND
FAULT
V
OUT
SW
SHDN
PGND