LTC3429_15 Datasheet, PDF (8/12 Pages) Linear Technology – 600mA, 500kHz Micropower Synchronous Boost Converter with Output Disconnect

LTC3429/LTC3429B
U
OPERATIO
is achieved by terminating the switching of the synchro-
nous PMOS and applying V_IN statically on its gate. This
ensures that the slope of the inductor current will reverse
during the time current is flowing to the output. Since the
PMOS no longer acts as a low impedance switch in this
mode, there will be more power dissipation within the IC.
This will cause a sharp drop in the efficiency (see Typical
Performance Characteristics, Efficiency vs V_IN). The maxi-
mum output current should be limited in order to maintain
an acceptable junction temperature.
APPLICATIO S I FOR ATIO
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3429/LTC3429B
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.
V_IN
1 SW V_IN 6
2 GND V_OUT 5
3 FB SHDN 4
SHDN
V_OUT
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 LTC3429
versus inductance value is given in the equation below and
illustrated graphically in Figure 3.
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 (500kHz typical)
L = inductance value
3429 F02
RECOMMENDED COMPONENT PLACEMENT. TRACES
CARRYING HIGH CURRENT ARE DIRECT. TRACE AREA AT
FB PIN IS SMALL. LEAD LENGTH TO BATTERY IS SHORT
Figure 2. Recommended Component Placement
for Single Layer Board
COMPONENT SELECTION
Inductor Selection
The LTC3429/LTC3429B can utilize small surface mount
and chip inductors due to its fast 500kHz switching
frequency. Typically, a 4.7µH inductor is recommended
for most applications. Larger values of inductance will
allow greater output current capability by reducing the
8
200
V_IN = 1.2V
180
160
V_OUT = 3.3V
140
120
100
V_OUT = 5V
80
60
40
20
0
3 5 7 9 11 13 15 17 19 21 23
INDUCTANCE (µH)
3429 F03
Figure 3. Maximum Output Current vs
Inductance Based on 90% Efficiency
3429fa