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LTC3554 Datasheet, PDF (23/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators | |||
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LTC3554
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 efï¬ciency as shown
in Figures 4 and 5. Switch node slew rate is also increased
to minimize transition losses. As the programmed output
voltage goes down, the difference in efï¬ciency is more
appreciable.
ing 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 VOUT pin, the
regulators should not be operated outside the speciï¬ed
operating range as operation is not guaranteed beyond
this range.
Low Supply Operation
An undervoltage lockout circuit on the VOUT pin (VOUT
UVLO) shuts down the step-down switching regulators
when VOUT 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 (VOUT). The UVLO prevents the step-down switch-
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
80 EFFICIENCY
70
60
1000
100
10
50
40
30
20
10
0
0.01
POWER LOSS
1
0.1
FSEL = L
FSEL = H
0
0.1
1
10 100 1000
LOAD CURRENT (mA)
LTXXXX GXX
Figure 4. 1.2V Output Efï¬ciency 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)
LTXXXX GXX
Figure 5. 3.3V Output Efï¬ciency and Power Loss
vs Load Current
3554p
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