LTC3400 Linear Technology, LTC3400 Datasheet - Page 6

• Manufacturer Part Number: LTC3400
• Description: 600mA/ 1.2MHz Micropower Synchronous Boost Converter in ThinSOT
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3400/LTC3400B
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3400/LTC3400B
demands careful attention to board layout. You will not get
advertised performance with careless layout. Figure 2
shows the recommended component placement. A large
ground pin copper area will help to lower the chip tempera-
ture. A multilayer board with a separate ground plane is
ideal, but not absolutely necessary.
COMPONENT SELECTION
Inductor Selection
The LTC3400/LTC3400B can utilize small surface mount
and chip inductors due to their fast 1.2MHz switching
frequency. A minimum inductance value of 3.3 H is
necessary for 3.6V and lower voltage applications and
4.7 H for output voltages greater than 3.6V. Larger values
of inductance will allow greater output current capability
by reducing the inductor ripple current. Increasing the
inductance above 10 H will increase size while providing
little improvement in output current capability.
The approximate output current capability of the LTC3400/
LTC3400B versus inductance value is given in the equa-
tion below and illustrated graphically in Figure 3.
6
Figure 2. Recommended Component Placement
for Single Layer Board
RECOMMENDED COMPONENT PLACEMENT. TRACES
CARRYING HIGH CURRENT ARE DIRECT. TRACE AREA AT
FB PIN IS SMALL. LEAD LENGTH TO BATTERY IS SHORT
V
IN
U
U
1
2
3
SW
GND
FB
SHDN
3400 F02
V
OUT
V
W
IN
V
OUT
6
5
4
(OPTIONAL)
SHDN
U
where:
The inductor current ripple is typically set for 20% to 40%
of the maximum inductor current (I
ferrite core inductor materials reduce frequency depen-
dent power losses compared to cheaper powdered iron
types, improving efficiency. The inductor should have low
ESR (series resistance of the windings) to reduce the I
power losses, and must be able to handle the peak
inductor current without saturating. Molded chokes and
some chip inductors usually do not have enough core to
support the peak inductor currents of 850mA seen on the
LTC3400/LTC3400B. To minimize radiated noise, use a
toroid, pot core or shielded bobbin inductor. See Table 1
for some suggested components and suppliers.
I
V
D = steady-state duty ratio = (V
f = switching frequency (1.2MHz typical)
L = inductance value
I
P
OUT MAX
IN
= estimated efficiency
= peak current limit value (0.6A)
= input (battery) voltage
(
180
160
140
120
110
80
60
Figure 3. Maximum Output Current vs
Inductance Based On 90% Efficiency
)
3
V
IN
5
=1.2V
•
7
I
P
9
INDUCTANCE ( H)
–
11 13
f L
V
• •
IN
•
D
15 17 19
2
• –
OUT
V
V
V
V
OUT
OUT
1
OUT
OUT
P
= 3.3V
= 3.6V
– V
). High frequency
= 3V
= 5V
D
21
3400 F03
IN
23
)/V
OUT
3400f
2
R