LTC3554-2_15 Datasheet, PDF (23/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554/LTC3554-1/
LTC3554-2/LTC3554-3
OPERATION
When FSEL is low, the frequency of the switching regulators
is reduced to 1.125Mhz. The slower switching frequency
reduces switching losses and raises efficiency as shown
in Figures 4 and 5. Switch node slew rate is also increased
to minimize transition losses. As the programmed output
voltage decreases, the difference in efficiency is more
appreciable.
switching regulators from operating at low supply voltages
where loss of regulation or other undesirable operation
may occur. If driving the step-down switching regulator
input supply from a voltage other than the V_OUT pin, the
regulators should not be operated outside the specified
operating range as operation is not guaranteed beyond
this range.
Low Supply Operation
An undervoltage lockout circuit on the V_OUT pin (V_OUT
UVLO) shuts down the step-down switching regulators
when V_OUT drops below about 2.6V. It is thus recom-
mended that the step-down switching regulator input
supply (BVIN) be connected directly to the power path
output (V_OUT). The UVLO prevents the step-down
Inductor Selection
Many different sizes and shapes of inductors are avail-
able from numerous manufacturers. Choosing the right
inductor from such a large selection of devices can be
overwhelming, but following a few basic guidelines will
make the selection process much simpler.
100
BAT = 3.8V
90
1000
80 _EFFICIENCY
100
70
60
10
50
POWER LOSS
40
1
30
20
10
0
0.01
0.1
FSEL = L
FSEL = H
0
0.1
1
10 100 1000
LOAD CURRENT (mA)
3554 F04
Figure 4. 1.2V Output Efficiency and Power Loss vs
Load Current
100
BAT = 3.8V
90
1000
80
100
EFFICIENCY
70
60
10
50
POWER LOSS
40
1
30
20
10
0
0.01
0.1
FSEL = L
FSEL = H
0
0.1
1
10 100 1000
LOAD CURRENT (mA)
3554 F05
Figure 5. 3.3V Output Efficiency and Power Loss
vs Load Current
3554123ff
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