Datasheet

LTC3499/LTC3499B
10
3499fc
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3499/LTC3499B
demand careful attention to board layout. Advertised per-
formance will not be achieved with careless layout. Figure 2
shows the recommended component placement. A large
copper area will help to lower the chip temperature. Traces
carrying high current (SW, V
OUT
, GND) are kept short.
The lead length to the battery should be kept as short as
possible. The V
IN
and V
OUT
ceramic capacitors should be
placed as close to the IC pins as possible.
APPLICATIONS INFORMATION
The inductor current ripple is typically set to 20% to 40%
of the maximum inductor current. For high efficiency,
choose an inductor with high frequency core material,
such as ferrite, to reduce core losses. The inductor should
have low ESR (equivalent series resistance) to reduce the
I
2
R power losses, and must be able to handle the peak
inductor current without saturating. To minimize radiated
noise, use a toroidal or shielded inductor. See Table 1 for
some suggested inductor suppliers.
Table 1. Inductor Vendor Information
PART NUMBER SUPPLIER WEB SITE
MSS5131 and
MOS6020 Series
Coilcraft www.coilcraft.com
SLF7028 and
SLF7045 Series
TDK www.component.tdk.com
LQH55D Series Murata www.murata.com
CDRH4D28 Series Sumida www.sumida.com
D53LC and
D62CB Series
Toko www.tokoam.com
DT0703 Series CoEV www.coev.net
MJPF2520 Series FDK www.fdk.com
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The maximum
ripple voltage due to charge is given by:
V
RBULK
=I
P
•
V
IN
C
OUT
• V
OUT
• f
( )
where I
P
= peak inductor current and f = switching
frequency.
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is simply given by:
V
RCESR
= I
P
• C
ESR
where C
ESR
= capacitor equivalent series resistance.
Figure 2: Recommended Component Placement
3499 F02
EXPOSED PAD FOR DD8
SW
C
C1
C
C2
R2
RZ
R1
L
V
IN
V
BATT
C
IN
C
OUT
GND
FB
SS
VC
9
5
7
8
4
3
2
1
6
V
OUT
+
C
SS
SHDN
COMPONENT SELECTION
Inductor Selection
The LTC3499/LTC3499B allow the use of small surface
mount inductors and chip inductors due to the fast 1.2MHz
switching frequency. A minimum inductance value of
2.2µH is required. Larger values of inductance will allow
greater output current capability by reducing the induc-
tor ripple current. Increasing the inductance above 10µH
will increase total solution area while providing minimal
improvement in output current capability.