LTC3400B_15 Datasheet, PDF (6/12 Pages) Linear Technology – 600mA, 1.2MHz Micropower Synchronous Boost Converter in ThinSOT

LTC3400/LTC3400B
U
OPERATIO
burst threshold, the LTC3400 will resume continuous
PWM operation seamlessly. Referring to the Block Dia-
gram, an optional capacitor (C_FF) between V_OUT and FB in
some circumstances can reduce the peak-to-peak V_OUT
ripple and input quiescent current during Burst Mode
operation. Typical values for C_FF range from 15pF to
220pF. The LTC3400B does not use Burst Mode operation
and features continous operation at light loads, eliminat-
ing low frequency output voltage ripple at the expense of
light load efficiency.
APPLICATIO S I FOR ATIO
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.
(OPTIONAL)
V_IN
1 SW V_IN 6
2 GND V_OUT 5
3 FB SHDN 4
SHDN
V_OUT
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RECOMMENDED COMPONENT PLACEMENT. TRACES
CARRYING HIGH CURRENT ARE DIRECT. TRACE AREA AT
FB PIN IS SMALL. LEAD LENGTH TO BATTERY IS SHORT
of inductance will allow greater output current capability
by reducing the inductor ripple current. Increasing the
inductance above 10µH 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.
180 ^{VIN =1.2V}
160
140
V_OUT = 3V
V_OUT = 3.3V
V_OUT = 3.6V
120
V_OUT = 5V
110
80
60
3 5 7 9 11 13 15 17 19 21 23
INDUCTANCE (µH)
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Figure 3. Maximum Output Current vs
Inductance Based On 90% Efficiency
Figure 2. Recommended Component Placement
for Single Layer Board
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 3.3µH is
necessary for 3.6V and lower voltage applications and
4.7µH for output voltages greater than 3.6V. Larger values
6
I_OUT(MAX) = η • ^{}I_P – _fV•^IN_{L••D2}^{} • ₍1– D₎
where:
η = estimated efficiency
I_P = peak current limit value (0.6A)
V_IN = input (battery) voltage
D = steady-state duty ratio = (V_OUT – V_IN)/V_OUT
f = switching frequency (1.2MHz typical)
L = inductance value
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